PSEB Class 10 Biology Solutions For Chapter 3 Electricity

Electric Current (I)

The amount of electric charge (electrons or ions) flowing through a particular area in a unit time is called electric current. Since in metals, electric charge flows through electrons, therefore, electric current can also be defined as the rate of flow of electrons in a metal (or conductor).

Points To Remember About Electric Current

1. In an electric circuit, the direction of flow of electric current is taken as opposite to the direction of flow of electrons. This is because electrons were not discovered when the phenomenon of electricity was first observed. The only sub-atomic particles known at that time were protons (Recall that protons are positively charged particles). So, electric current was considered to be the flow of positive charges, and the direction of flow of positive charges was taken to be the direction of electric current.
2. SI unit of electric current is ampere (A). One ampere is the amount of electric current flowing through an electric circuit when an electric charge (Q) of 1 coulomb flows through it in1 second.
3. Amount of electric current in an electric circuit is measured using Ammeter.
4. Ammeter is always connected in series in an electric circuit.

Electric Charge (Q) – It is the property of matter due to which matter experiences an attractive or a repulsive force when it is placed in an electromagnetic field. SI unit of electric charge is coulomb (C).

1. One coulomb is the charge contained in 1.6 x 10¹⁸ electrons (Note: Recall that charge on 1 electron = 1.6 x 10^-19C)

• Some examples of electric charge in daily life are as follows:
• Chattering sound produced by synthetic clothes – When the clothes made up of nylon are rubbed against some other fabric or against the wearer’s skin, static electricity is formed. This electrostatic force developed between the skin and the clothing particles is accountable for the chattering sound caused while removing such clothes.
• Development of electric charge on a rod – When a metal rod or glass rod is rubbed with a cloth, positive or negative charge develops on it. The type of charge developed depends on the material of the rod, For Example. If a glass rod is rubbed with a cloth, positive charge will develop, whereas if a metal rod is rubbed with a cloth, negative charge will develop.
• Feeling a short-term electric shock on touching some metal surface – When a person randomly touches a metallic surface (For Example, a doorknob), he or she is prone to feel a short-term electric shock. This is due to the existence of an electrostatic force between the doorknob and the person’s hand. Since the doorknob is made up of metal, it is capable of transferring the electrons to every object that comes in contact with it. This leads to the development of electrostatic interaction between the doorknob and the skin.
• Photocopy Machine – In a photocopy machine, the original paper is placed on a glass screen. The image of this original paper is transferred to a drum that is positively charged. The ink powder or the toner used is usually charged with a negative polarity. The drum rolls against the paper on which the impression is required to be created. The ink then gets transferred to the paper due to the electrostatic force of attraction, creating a photocopied image of the original document.

PSEB Class 10 Physics Solutions Chapter 3

Relation Between Electric Current (I) And Electric Charge (Q)

If a charge Q flows through a conductor of any cross-section in time (t), then the electric current (I) through the cross-section is given by the equation

I = Q/t

Potential Difference Or Voltage (V)

It is defined as the work involved or the energy released in the transfer of a unit quantity of electricity (.e, charge) from one point to the other, i.e.,

Potential difference = Work done/ Charge

Or, V = W/Q

• SI unit of potential difference is volt (V).
• One volt is the potential difference between two points in a current-carrying conductor when 1 joule of work is done to move a charge of1 coulomb from one point to another in an electric circuit, i.e.,
  • 1 volt = 1 joule/ lcoulomb
  • Or, 1V = 1JC^-1
• Potential difference in an electric circuit is measured using an instrument called Voltmeter. Voltmeter is always connected in parallel in the circuit.

Cause Of Potential Difference

The generation of potential difference across the terminals of the cell occurs due to the chemical action within a cell or battery.

Important Points About Electric Circuit

1. An electric circuit converts electrical energy into other forms of energy, For Example. in a bulb, electrical energy is converted into light energy and thermal energy.
2. It has at least two parts – a voltage source (e.g. a cell or a battery) and a conductor (i.e., wires).
3. Switches in the electrical circuits control the flow of current. When the switch is turned on, the circuit is closed and current can flow through it. When the switch is turned off, the circuit is open and current cannot flow through it.
4. An electric circuit can function only if it forms a closed loop from the battery and back again as shown:

Circuit Diagram

It is a graphical display of an electric circuit using either basic images of parts or industry-standard symbols. A simple circuit diagram consists of multiple parts such as cell or battery, switches, wires, electric components such as an ammeter or a voltmeter, a bulb, etc as shown in the figure below:

Important Points About Circuit Diagrams

1. In a circuit diagram, the direction of current is always from positive to negative terminal of the battery or cell (Recall that electrons will follow in opposite direction).

2. The longer end of the battery is the positive terminal and the shorter end of the battery is the negative terminal.

3. The positive end of the ammeter or voltmeter is connected to the positive end of the battery or cell and their negative end is connected to the negative end of the battery.

Circuit Diagram Symbols

Series And Parallel Electric Circuits

These are the two types of electric circuits. They differ in the number of loops through which current can flow.

Ohm’s Law

It gives the relationship between potential difference (V) and the current (I).

Ohm’s law states that the current (I) through a conductor between two points is directly proportional to the potential difference (V) across the two points, provided that the temperature remains the same, i.e,

V aI

or, V = IR (1)

where R is constant for the given metallic wire at a given temperature, and is called its resistance.

Resistance (R) – It is the property of a conductor to resist the flow of charge through it.

• SI unit of resistance is ohm (0).
• 1 Ohm is the resistance of a conductor when a potential difference of1 volt is applied across its ends, producing a current of 1 ampere.
• Resistance (R) of a conductor depends on the following factors:
  • Length (I) of the conductor, i.e., R oc I
  • Area of cross-section or thickness (A) of the conductor, i.e., R oc 1/A, and
  • Nature of the material of the conductor.

Combining factors (1) and (2), we get

∴ \(R \propto \frac{l}{A} \quad \text { or, } \quad R=\rho \frac{l}{A}\)

where p (rho) is a constant of proportionality and is called the ELECTRICAL RESISTIVITY of the material of the conductor. SI unit of resistivity is Q m. Resistivity is a characteristic property of the material.

• Metals and alloys have very low resistivity. They are good conductors of electricity.
• Insulators (For Example, rubber and glass) have intermediate resistivity.
• Resistivity of an alloy is generally higher than that of its constituent metals. Alloys do not oxidise (burn) readily at high temperatures. For this reason, they are commonly used in electrical heating devices, like electric iron, toasters etc.
• Both resistance and resistivity of a material vary with temperature.

Electricity PSEB Class 10 Notes

Difference Between Resistance And Resistivity

Effect Of Change In Resistance On Current

Resistance of a conductor is inversely proportional to the current (as can be deduced from equation i), i.e. R = V/I

Therefore, if the resistance is doubled, the current gets halved.

Resistor – It is a device having electrical resistance.

• Resistors are used in an electric circuit for protection, operation, or current control.
• Resistors can be connected in a series connection, in a parallel connection or in combinations of both series and parallel.
• Resistors always obey Ohm’s Law irrespective of the combination or complexity of resistor network.

Resistors In Series – Resistors are said to be connected in “Series”, when they are chained together in a single line as shown:

Resistors In Parallel – Resistors are said to be connected together in parallel when both of their terminals are respectively connected to each terminal of the other resistor or resistors, as shown:

Conductance (G) – The reciprocal of resistance is called conductance, i.e., G = 1/R

SI units of conductance are siemens (S).

Heating Effect Of Electric Current – When an electric current is passed through a conductor, the conductor becomes hot after some time due to the conversion of some electric energy into heat energy.

Cause Of Heating Effect Of Electric Current – Heating effect of electric current occurs due to collision of electrons in the conductor, as explained below:

• When a potential difference is applied across the ends of a conductor, its free electrons get accelerated in a direction opposite to the applied electric field.
• But the speed of the electrons does not increase beyond a certain speed because during the course of their motion they collide frequently with the positive metal ions.
• During these collisions between the free electrons and the metal ions, the kinetic energy of the moving electrons is transferred to the metal ions. As a result of this, the metal ions begin to vibrate about their mean position.
• Due to the vibrations of the metal ions about their mean positions, the kinetic energy of the metal ions increases. This in turn results in an increase in temperature, i.e., heating of the conductor.

Factors Affecting Heating Effect Of Current – The Heating effect of electric current depends on three factors:

1. Resistance Of The Conductor– The Higher the resistance more the heating effect.
2. Amount Of Current Flowing Through The Conductor– Higher the amount of current, more will be the heat generated.
3. Time For Which The Current Flows Through The Conductor – Longer the time for which current flows through the conductor more will be the heat generated.

Derivation of formula for heat produced by electric current flowing through a conductor

Consider the following circuit diagram:

Let I = current flowing through the circuit

R = Resistance of the resistor

V = Potential difference across the two terminals of the resistor

t = Time during which a charge Q flows across the circuit.

We know that

PSEB Class 10 Physics Important Questions Chapter 3

Potential difference (V) between two points = Work done (W)/Charge (Q)

Or, V = W/Q ….(1)

But Q = I x t, and V = I x R …..(2)

Substituting the values of Q and V from equation (2) in equation (1), we get

I x R = W/ I x t

Or, I x R x I x t = W

Or, W = I²Rt

Assuming that all the work done is converted into heat energy we can replace “W” with “H” in the above equation, to get

H = I²Rt

Uses Of the Heating Effect Of Electric Current

1. In Heating Devices such as electric iron, electric toaster, electric oven, electric kettle, electric heater, etc.
2. Electric heating is used to produce light in an electric bulb. Here, the filament must retain as much of the heat generated as is possible, so that it gets very hot and emits light. It must not melt at such high temperature. A strong metal with high melting point such as tungsten (melting point 3380°C) is used for making bulb filaments.
3. Electric fuse used in electric circuits – It protects circuits and appliances by stopping the flow of any unduly high electric current. FUSE IS ALWAYS PLACED IN SERIES WITH THE DEVICE. A fuse consists of a piece of wire made of a metal or an alloy of appropriate melting point, For Example., aluminium, copper, iron, or lead. If a current larger than the specified value flows through the circuit, the temperature of the fuse wire increases. This melts the fuse wire and breaks the circuit. The fuse wire is usually encased in a cartridge of porcelain or similar material with metal ends.

Disadvantages Of Heating Effect Of Electric Current

1. Undesirable as it converts useful electrical energy into heat. Thus, some energy gets wasted.
2. In electric circuits, the unavoidable heating can increase the temperature of the components and alter their properties.

Electric Power (P) – The rate at which electric energy is dissipated or consumed in an electric circuit is called electric power (P).

The formula for calculating electric power is:

P = V x I …..(1)

But according to Ohm’s law, V = I x R…..(2)

Substituting value of “V” from equation (2) in equation (1), we get

P = I x R x I

Or, P = I²R

But, I = V/R (from Ohm’s law), therefore

P = V²/R

SI unit of electric power is watt (W).

One watt is the power consumed by a device that carries 1 A of current when operated at a potential difference of 1 V.

Thus, 1 W = 1 volt x l ampere = 1 V A

Since, unit ‘watt’ is very small, we use a much larger unit called ‘kilowatt’ (1 kW = 1000 W).

Further, since electrical energy is the product of power and time, the unit of electric energy is, therefore, watt hour (W h).

One watt hour is the energy consumed when 1 watt of power is used for 1 hour.

Commercial Unit Of Electric Energy is Kilowatt hour (kWh), commonly known as ‘unit’.

1 kW h = 1000 watt x 3600 second = 3.6 x 106 watt second = 3.6 x 106 joule (J).

PSEB Class 10 Science Chapter 3 Solutions

Chapter 3 Electricity Reason- Assertion Questions And Answers

The following questions consist of two statements -Assertion (A) and Reason (R). Answer these questions by selecting the appropriate option given below:

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Question 1. Assertion (A): Tungsten metal is used for making filaments of incandescent lamps.

Reason (R): The melting point of tungsten is very low.

Answer: 3. A is true but R is false.

Question 2. Assertion (A): When the resistances are connected end-to-end consecutively, they are said to be in series.

Reason (R): In case the total resistance is to be increased, then the individual resistances are connected in series.

Answer: 2. Both A and R are true but R is not the correct explanation of A.

Question 3. Assertion (A): When the resistances are connected between the same two points, they are said to be connected in parallel.

Reason (R): In case the total resistance is to be decreased, then the individual resistances are connected in parallel.

Answer: 2. Both A and R are true but R is not the correct explanation of A.

Question 4. Assertion (A): The fuse is placed in series with the device.

Reason (R): Fuse consists of a piece of wire made of a metal or an alloy of appropriate melting point.

Answer: 2. Both A and R are true but R is not the correct explanation of A.

Question 5. Assertion (A): The coil of a heater is cut into two equal halves and only one of them is used into heater. The heater will now require half the time to produce the same amount of heat.

Reason (R): The heat produced is directly proportional to square of current.

PSEB Class 10 Science Chapter 3 Solutions

Answer: 2. Both A and R are true but R is not the correct explanation of A.

Question 6. Assertion (A): A current carrying wire should be charged.

Reason (R): The current in a wire is due to flow of free electrons in a definite direction.

Answer: 4. A is false but R is true.

Question 7. Assertion (A): An Electron has a negative charge.

Reason (R): Electrons move always from a region of higher potential to a region of lower potential.

Answer: 3. A is true but R is false.

Question 8. Assertion (A): Heater wire must have high resistance and high melting point.

Reason (R): If resistance is high, the electric conductivity will be less.

Answer: 2. Both A and R are true but R is not the correct explanation of A.

Question 9. Assertion (A): In a chain of bulbs, 50 bulbs are joined in series. One bulb is removed now and circuit is completed again. If the remaining 49 bulbs are again connected in series across the same supply, then light gets decreased in the room.

Reason (R): Net resistance of 49 bulbs will be less than 50 bulbs.

Answer: 4. A is false but R is true.

Question 10. Assertion (A): The connecting wires are made of copper.

Reason (R): The electrical conductivity of copper is high.

Answer: 1. Both A and R are true and R is the correct explanation of A.

Chapter 3 Electricity Case Or Source-Based Questions And Answers

Question 1. The table shows the current in three electrical appliances when connected to a 240 V main supply.

1) Identify the electrical appliance having the highest electrical resistance

1. Electric lamp
2. Electric toaster
3. Electric kettle
4. Electric iron

Answer: 1. Electric lamp

2) Calculate the power rating of the kettle when connected to a 240 V main supply.

1. 2000 W
2. 2160 W
3. 3010 W
4. 200 W

Answer: 2. 2160 W

3) How much current would be drawn by an electric toaster when it is connected to a 120 V supply?

1. 4 A
2. 1 A
3. 10 A
4. 2A

Answer: 4. 2A

4) Calculate the power rating of the lamp when it is connected to a 240 V main supply.

1. 300 W
2. 120 W
3. 200 W
4. 240 W

Answer: 2. 120 W

5) 92 Watt = _____ J/S

1. 920
2. 9.2
3. 0.92
4. 92

Answer: 4. 92

PSEB Class 10 Science Chapter 3 Solutions

Question 2. In the below circuit, a nichrome wire of length ‘L’ is connected between points X and Y, and note the ammeter reading. The experiment is performed and repeated by inserting another nichrome wire of the same thickness but twice the length i.e., ‘2L’.

1) What are the changes observed in the ammeter readings?

1. Ammeter readings decreases, becomes half
2. Ammeter readings increases, becomes two times
3. Ammeter readings increases becomes quadrupled
4. Ammeter reading decreases becomes one- fourth

Answer: 1. Ammeter readings decreases, becomes half

2) What change is occurred in ammeter reading if instead of changing the length the area of cross- section is doubled?

1. Ammeter readings decreases, becomes half
2. Ammeter readings increases, becomes two times
3. Ammeter readings increases becomes quadrupled
4. Ammeter reading decreases becomes one -fourth

Answer: 2. Ammeter readings increases, becomes two times

3) If the resistors of 5 ohms and 10 ohms are connected in series in the above circuit. What is the ratio of the current passing through the two resistors?

1. 2:1
2. 3:1
3. 1:2
4. 1:1

Answer: 4. 1:1

4) If the resistors are connected in parallel

1. Current across each resistor is same and voltage changes
2. Current and voltage across each resistor is same
3. Current across each resistor varies and voltage remains same
4. Current changes, voltage changes

Answer: 3. Current across each resistor varies and voltage remains same

5) SI unit of current is denoted as

1. A
2. C
3. I
4. J

Answer: 1. A

Question 3. Sahil has two wires. Both wires are of the same material but are of different lengths and cross-sections. Sahil has to find the difference in their resistivities for various changes.

1) What is the difference in their resistivity’s for the above-mentioned condition?

1. Data given is insufficient to identify the change
2. Change in resistivity depends on change in length
3. Change in resistivity depends on change in area
4. No difference in resistivity’s as both wires are of same material

Answer: 4. No difference in resistivity’s as both wires are of same material

Explanation:

Resistivity’s depends on material of the conductor and not on its dimensions. So, both the wires will have same resistivity’s.

2) If Sahil stretches one of the wires, it becomes double the original length then is there any change in its resistances? If yes, what is the change?

1. Area reduces to half and resistance becomes four times the original value
2. Area becomes twice and resistance becomes four times the original value
3. Area becomes twice and resistance becomes 2 times the original value
4. Area decreased by half and resistance becomes 2 times the original value

Answer: 1. Area reduces to half and resistance becomes four times the original value.

3) If he stretches the other wire, it becomes triple its original length then how much is the change?

1. Area reduces to one- third and resistance becomes nine times the original value
2. Area becomes twice and resistance becomes four times the original value
3. Area becomes thrice and resistance becomes 3 times the original value
4. Area decreased by one- third and resistance becomes 9 times the original value

Answer: 4. Area decreased by one- third and resistance becomes 9 times the original value.

4) Sahil connect the two wires in series and observe change. What is the change? Why?

1. No change
2. Area increases
3. Area decreased by one- third and resistance becomes 9 times the original value
4. Resistance increases as area increases

Answer: 3. Area decreased by one- third and resistance becomes 9 times the original value.

Explanation: When wires are connected in series, there is increase in length. As resistance is directly proportional to length, resistance increases on increasing the length.

PSEB Class 10 Science Chapter 3 Solutions

5) Sahil connected the two wires in parallel and observe the change. What is the change? Give reason.

1. Resistance increases as length decreases
2. Resistance increases as length increases
3. Resistance decreases as area increases
4. Resistance increases as area increases

Answer: 3. Resistance decreases as area increases

Explanation: When wires are connected in parallel, the area increases and as we know that resistance is inversely proportional to cross-sectional area, resistance decreases.

Question 4. Sonia has a set of five substances. She has a chart stating resistivities of all the substances.

She has to choose an appropriate substance for performing electrical tasks. Which of the above substance according to you

1) Can be used as an insulator

1. A
2. B
3. B as well as C
4. E

Answer: 4. E

2) Can be used for domestic wiring

1. A
2. B
3. A as well as C
4. D

Answer: 3. A as well as C

3) Can be utilised in making solar cells and transistors

1. A
2. B
3. C
4. D

Answer: 4. D

4) Is an alloy

1. A
2. B
3. C
4. E

Answer: 2. B

5) Behaves as a semiconductor

1. A
2. D
3. C
4. E

Answer: 2. D

PSEB Class 10 Physics Solutions For Chapter 2 The Human Eye And The Colourful World

The Human Eye

Points To Remember About the Human Eye

1. The human eyeball is approximately spherical in shape. Its diameter is about 2.3 cm.
2. Eyes are located in eye sockets in the skull.
3. The human eye consists of the following parts:

Functioning Of The Human Eye

1. Light reflected by an object enters the eye through the cornea, and then passes the pupil.
2. The convex lens in the eye contracts or relaxes to focus the light onto the retina. Light is refracted as it passes through the convex lens. This causes the image that hits the retina to be reduced in size and flipped upside down.
3. Light-sensitive cells (rod and cone cells) present in the retina gather information and send electrical signals to the brain via the optic nerve.
4. Cells in the visual cortex of the brain flip the image from the retina, turning it the right way round.

PSEB Class 10 Physics Solutions Chapter 2

Pupil Reflex

The change in size of the pupil to control the amount of light entering the eye is called the pupil reflex.

Pupil reflex is initiated with the detection of the brightness of light entering the eye by the retina. It then passes an impulse to the brain through sensory neurones, the brain interprets the impulse.

and send back a message to the muscles of the iris through motor neurones, triggering a response, the change in size of the pupil due to contraction of radial or circular muscles. This is done to protect the eyes from damage.

Thus, in bright light, pupil size is reduced as too much light falling on the retina could damage it.

In dim light, pupil size is increased to allow as much light as possible to enter the eye.

Important Note for Students – Do not confuse the function of circular muscles and ciliary muscles. Remember that:

1. Circular muscles affect the size of the iris
2. Ciliary muscles affect the shape of the lens.

Power Of Accomodation

The ability of the eye lens to adjust its focal length to some extent by changing its curvature with the help of ciliary muscles is called accommodation. The amount of focusing needed by the lens depends on the distance of the object being viewed. Light from near objects requires a more convex lens than light from distant objects.

• For seeing Close Objects, ciliary muscles contract (Remember this using mnemonic CCC, meaning Ciliary muscles Contract for Close Vision). Due to this, the curvature of the eye lens increases, and it becomes thicker. Consequently, the focal length of the eye lens decreases. This enables us to see nearby objects clearly.
• For seeing Distant Objects, the ciliary muscles relax. Due to this, the curvature of the eye lens decreases and it becomes thin. Consequently, the focal length of the eye lens increases. This enables us to see distant objects clearly.

Near And Far Point

Thus, a normal eye can see objects clearly that are between 25 cm and infinity.

Why Do We Have Two Eyes For Vision And Not Just One?

• It gives a wider field of view. A human being has a horizontal field of view of about 150° with one eye and of about 180° with two eyes.
• Two eyes also provide a better detection of objects.

Note:

• Our two eyes are positioned on the front of our heads. This reduces our field of view in comparison to animals having their two eyes positioned on opposite sides of their heads.
• Each of our eyes sees a slightly different image because our eyes are separated by a few centimetres. The two images are combined into one by our brain, using the extra information, to tell us how close or far away things are.

Refractive Defects Of Vision And Their Correction

Refractive defects of vision occur due to partial loss of the power of accommodation of the eye.

Three common refractive defects of vision are – Myopia (Near-Sightedness), Hypermetropia (Far Sightedness) and Presbyopia.

The Human Eye And The Colourful World PSEB Class 10 Notes

Alternative ways to correct refractive defects of vision are:

• Use of contact lenses
• Eye surgery.

Note:

A person suffering from both myopia and hypermetropia requires bifocal lenses for correction. A common type of bifocal lenses consists of both concave and convex lenses, the upper portion consists of a concave lens to facilitate distant vision, and the lower part is a convex lens to facilitate near vision.

Cataract – It is an eye defect that occurs due to ageing. In this defect, the eye lens becomes milky and cloudy due to ageing, resulting in partial or complete loss of vision. This defect can be corrected only through cataract surgery.

Visual Impairment – Visual impairment means damage or malfunction of any part of the eye. It may occur due to damage to the cornea, pupil, eye lens, aqueous humour, vitreous humour, retina or optic nerve.

Eye Donation

Corneal blindness can be cured through corneal transplantation of donated eyes. Important points about eye donation are:

1. Anyone can donate eyes, independent of age or sex.
2. People using spectacles, or those who have undergone cataract surgery, can also donate eyes.
3. People having diabetes, hypertension, asthma and those without communicable diseases can also donate eyes.
4. Eyes must be removed within 4-6 hours after death. This is done by trained persons from eye bank at the home of the deceased or at a hospital.
5. Eye removal takes only 10-15 minutes and does not lead to any disfigurement.
6. The identities of both the donor and the recipient remain confidential.
7. One pair of eyes gives vision to up to FOUR CORNEAL BLIND PEOPLE. This is because one pair of eyes has various components, such as retina, cornea, and muscles, that can be transplanted to the corneal blind people to help in retaining their vision.
8. Persons having AIDS, Hepatitis B or C, rabies, acute leukaemia, tetanus, cholera, meningitis or encephalitis cannot donate blood.

Refraction Of Light Through A Prism

1. When light passes from air to the glass of the prism, due to the differences in the refractive index between the air and the glass, light bends once entering the prism.
2. Since the sides of the prism are angled, the light bends again when it exits the prism.
3. Different wavelengths of light refract differently.
4. Longer wavelengths refract less while shorter wavelengths refract more. Therefore, when a beam of white light is passed through a prism, different wavelengths of light get refracted in different angularly in order, resulting in the splitting of white light into a spectrum of seven colours (VIBGYOR starting from bottom) as shown:

Spectrum – It is defined as an orderly continuum of colours (VIBGYOR) formed due to the dispersion of white light through a prism.

The phenomenon due to which this splitting of white light into a spectrum of seven colours (VIBGYOR) occurs on passing through a glass prism is called DISPERSION.

Rainbow is an example of dispersion of sunlight by tiny water droplets (which act as small prisms) present in the atmosphere.

More About Rainbow

Rainbow is always formed in a direction opposite to that of the Sun.

A rainbow is formed due to three phenomenon – Refraction, Dispersion and Total Internal Reflection of light as shown in figure:

Total Internal Reflection – It is the phenomenon which occurs when the light rays travel from an optically denser medium to an optically rarer medium.

Scattering Of Light – The phenomenon of the spreading of light in different directions by colloidal particles is called scattering of light.

∴ \(\text { Scattering } \propto \frac{1}{\text { wavelength }}\)

Some phenomena that occur due to scattering of light are – the blue colour of the sky, the colour of water in the deep sea, the reddening of the sun at sunrise and the sunset.

Atmospheric Refraction

It is defined as the refraction of light caused by the bending of light rays through different angles on passing through different layers of the Earth’s atmosphere. Some effects of atmospheric refraction are:

1. Apparent flickering of objects placed behind hot air or fire- Air above a hot surface becomes hot and rises up. Its space is occupied by cool air. Since the refractive index of hot and cool air is different, atmospheric refraction occurs, and the object appears to flicker.

2. Stars near the horizon appear slightly higher than their actual position. The density of air decreases with height, i.e., at higher altitudes air particles can spread more, so air is optically rarer at higher altitudes.

PSEB Class 10 Physics Important Questions Chapter 2

Since light from a star coming from the horizon has to travel from an optically rarer to an optically denser medium, it will bend towards normal. As a result, stars appear higher.

Note: Planets do not twinkle because they are much closer to the Earth. Therefore, they nullify the twinkling effect.

3. Apparent flattening of the Sun’s disc at sunrise and sunset.

4. Advanced Sunrise and Delayed Sunset

The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction.

During Sunrise, the sun is rising above the horizon. The rays from the sun get refracted as they travel from an optically denser medium to an optically rarer medium.

This results in the bending of rays of light towards the normal. Since the human eye sees the sun rays as a straight line, it appears as the sun has risen, though it has actually not yet risen.

Due to the same reason, the Sun is visible to us about 2 minutes after the actual sunset.

Difference Between Dispersion, Scattering And Refraction

Tyndall Effect

It is the phenomenon of scattering of a beam of light by colloidal particles suspended in Earth’s atmosphere (For Example., smoke, tiny water droplets, dust particles and molecules of air). When a beam of light strikes such fine particles, the path of the beam becomes visible.

Some examples or applications of Tyndall effect in daily life are:

• Scattering of sunlight by tiny water droplets in the mist on passing through a canopy of a dense forest.
• Path of light becomes visible when light enters a dark and dusty room through a slit or ventilator.

The colour of the scattered light depends on the size of the colloidal particles.

1. Very fine particles scatter mainly blue light, i.e., shorter wavelength. Since the upper layers of the atmosphere contain very fine particles of water vapours and gases, these particles are effective in scattering of light of shorter wavelengths, i.e, blue. Due to this reason, if we look at the sky from the Earth, it appears blue to us.

However, skv appears dark to people flying at high altitudes and astronauts due to the absence of particles at such high altitudes, due to which scattering of light does not occur.

2. Particles of medium size mainly scatter red light, i.e., longer wavelength. Therefore, red is the least scattered light.

Since it is not scattered by dust particles and fog, the same colour can be seen up to a long distance. For this reason, danger signs are made in red colour.

3. Very large particles scatter all the wavelengths of light, due to which the scattered light appears white in colour. Clouds appear white because clouds are made up of water vapours which condense to form large water droplets.

Due to the large size of the water droplets, all wavelengths of light are scattered, and the clouds appear white.

Chapter 2 The Human Eye And The Colourful World Reason- Assertion Questions And Answers

For questions given below, two statements are given—one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (1), (2), (3) and (4) as given below.

1. Both A and R are true, and R is the correct explanation of the assertion.
2. Both A and R are true, but R is not the correct explanation of the assertion.
3. A is true, but R is false.
4. A is false, but R is true

Question 1. Assertion (A): Sunlight reaches us without dispersion in the form of white light and not as its components.

Reason (R): Dispersion takes place due to variation of refractive index for different wavelengths but in vacuu,m the speed of light is independent of wavelength and hence vacuum is non-dispersive.

Answer: 1. Both A and R are true, and R is the correct explanation of the assertion.

Question 2. Assertion (A): Stars appear to be twinkling and planets do not twinkle.

Reason (R): The blue colour scatters much more than the red colour by air particles.

Answer: 2. Both A and R are true, but R is not the correct explanation of the assertion.

Question 3. Assertion (A): A stick partly immersed in water appears to be bent.

Reason (R): Refraction of light when it passes from water into air.

Answer: 1. Both A and R are true, and R is the correct explanation of the assertion.

Question 4. Assertion (A): In a rainbow, light at the inner surface of the water drop gets internally reflected.

Reason (R): The angle between the refracted ray and the normal to the drop surface is greater than the critical angle.

Answer: 2. Both A and R are true, but R is not the correct explanation of the assertion.

PSEB Class 10 Science Chapter 2 Solutions

Question 5. Assertion (A): The Refractive index of glass with respect to air is different for red light and violet light.

Reason: The Refractive index of a pair of media depends on the wavelength of light used.

Answer: 1. Both A and R are true, and R is the correct explanation of the assertion.

Question 6. Assertion (A): When objects are observed through hot air, they appear to be moving slightly.

Reason (R): The hotter air is optically denser and the colder air is optically rarer.

Answer: 3. A is true, but R is false.

Question 7. Assertion (A): When white light passes through a glass prism, red colour is deviated the least.

Reason (R): Red colour has the minimum speed in the glass prism.

Answer: 3. A is true, but R is false.

Question 8. Assertion (A): The Sun appears white when it is overhead in the sky.

Reason (R): Light coming from the Sun has to travel a relatively shorter distance through the atmosphere to reach us.

Answer: 1. Both A and R are true, and R is the correct explanation of the assertion.

Question 9. Assertion (A): The blue colour of the sky appears due to the scattering of blue colour.

Reason (R): Blue colour has the shortest wavelength in the visible spectrum.

Answer: 1. Both A and R are true, and R is the correct explanation of the assertion.

Chapter 2 The Human Eye And The Colourful World Case Or Source-Based Questions

Question 1. Hypermetropia is a common eye condition where nearby objects appear blurred, but your vision is clearer when looking at things further away. If you feel your eyes are often tired and you have problems focusing on objects close to your eyes, you may have hypermetropia. A person is suffering from hypermetropia (long-sightedness). The near point of the person is 1.5 m. Assume that the near point of the normal eye is 25 cm.

1) The type of lens to be used in his spectacles is

1. Concave
2. Convex
3. Plano concave
4. Cylindrical

Answer: 2. Convex

2) The focal length of the lens he should use is

1. 20 cm
2. 30 cm
3. 40 cm
4. 50 cm

Answer: 2. 30 cm

3) The power of the lens is

1. 2.2 D ;
2. 1.1 D ;
3. 4.4 D ;
4. 3.3 D

Answer: 4. 3.3 D

4) What is another name for Hypermetropia?

1. Short-sightedness
2. Long-sightedness
3. Night blindness
4. Presbyopia

Answer: 2. Long-sightedness

5) A The power of a lens is +2.5 D. What kind of lens is it, and what is its focal length?

1. Concave lens, 100 cm
2. Concave lens, 40 cm
3. Convex lens, 40 cm
4. Convex lens, 50 cm

Answer: 3. Convex lens, 40 cm

Question 2. The spreading of light by the air molecules is called scattering of light. The light with the shortest wavelength scatters more. The sun appears red at sunrise and sunset; the appearance of blue sky is due to the scattering of light. The colour of the scattered light depends on the size of the particles. The smaller the molecules in the atmosphere, scatter smaller the wavelengths of light. The amount of scattering of light depends on the wavelength of light. When light from the sun enters the Earth’s atmosphere, it gets scattered by the dust particles and air molecules present in the atmosphere. The path of sunlight entering the dark room through a fine hole is seen because of the scattering of the sunlight by the dust particles present in its path inside the room.

1) To an astronaut in a spaceship, the colour of Earth appears

1. Red
2. Blue
3. White
4. Black

Answer: 2. Blue

2) At the time of sunrise and sunset, the light from the sun has to travel.

1. Longest distance of the atmosphere.
2. Shortest distance to the atmosphere
3. Both (1) and (2);
4. Can’t say

Answer: 1. Longest distance of the atmosphere

3) The colour of the sky appears blue, which is due to the

1. Refraction of light through the atmosphere
2. Dispersion of light by air molecules
3. Scattering of light by air molecules
4. All of these.

Answer: 3. Scattering of light by air molecules

4) At the time of sunrise and sunset

1. Blue colour is scattered, and red colour reaches our eyes
2. Red colour is scattered, and blue colour reaches our eyes
3. Green and blue are scattered, and orange reaches our eyes
4. None of these

Answer: 1. Blue colour is scattered and red colour reaches our eye

5) The danger signs are made red in colour, because

1. The red light can be seen from the farthest distance
2. The scattering of red light is the least
3. Both (1) and (2)
4. None of these

Answer: 3. Both (1) and (2)

Question 3. Atmospheric refraction is the phenomenon of the bending of light as it passes through Earth’s atmosphere. As we move above the surface of Earth, the density of air goes on decreasing. Local conditions like temperature, etc., also affect the optical density of Earth’s atmosphere. On account of atmospheric refraction, stars appear higher than they actually are; advanced sunrise; delayed sunset, oval appearance of the sun at sunrise and sunset; stars twinkle, planets do not.

PSEB Class 10 Science Chapter 2 Solutions

1) Due to atmospheric refraction, the apparent length of the day

1. Increases
2. Decreases
3. Remains the same
4. All of these

Answer: 1. Increases

2) The apparent position of the star appears raised due to

1. Atmospheric refraction
2. Scattering of light
3. Both (1) and (2)
4. None of these

Answer: 1. Atmospheric refraction

3) The sun appears oval-shaped or flattened due to

1. Dispersion
2. Scattering
3. Atmospheric refraction
4. Cannot say

Answer: 3. Atmospheric refraction

4) The twinkling of stars and the non-twinkling of planets is accounted for by

1. Scattering of tight
2. Dispersion of light
3. Atmospheric refraction
4. None of these

Answer: 3. Atmospheric refraction

5) In the absence of an atmosphere, the colour of the sky appears

1. Blue
2. Black
3. Red
4. Yellow

Answer: 4. Yellow

Question 4. When white light is incident on one refracting surface of the prism, the light splits up into constituent colours violet, indigo, blue, green, yellow, orange and red. The process of splitting white light into its seven constituent colours is called dispersion. When the dispersed white light is made to fall on a screen, we get a band of seven colours is called the spectrum of white light. Red colour bends the least on passing through the prism, and violet colour bends through the maximum angle on passing through the prism.

image

1) The splitting of white light can be done by

1. Fens
2. Prism
3. Mirror
4. None of these

Answer: 2. Prism

2) Which property of light is used by a prism to form a spectrum?

1. Reflection
2. Refraction
3. Dispersion
4. Scattering

Answer: 2. Refraction

3) When a red light passes through a prism, it

1. Will not split
2. Will split into seven colours
3. Will split into white colour
4. Will split into many different colours

Answer: 1. Will not split

4) The spectrum produced by the white light by a prism is called

1. Pure spectrum
2. Impure spectrum
3. Monochromatic spectrum
4. None of these

Answer: 2. Impure spectrum

Question 5. Millions of people in the developing countries of the world are suffering from corneal blindness, These persons can be cured by replacing the defective cornea with the cornea of a donated eye. A charitable society in your city has organised a campaign in your neighbourhood in order to create awareness about this fact. If you are asked to participate in this mission, how would you contribute to this noble cause?

1) State the objective of organising such campaigns.

Answer: The objective of organising such a campaign is to make people aware and realize their duties towards society.

2) List two arguments that you would give to motivate people to donate their eyes after death.

Answer:

1. By donating our eyes after we die, we can light the life of a blind person.
2. One pair of eyes gives vision to two corneal blind people.

3) List two values which are developed in the persons who actively participate and contribute in such programmes.

Answer:

1. It shows the concern for others,
2. It also shows responsible behaviour towards society.