ICSE Class 8 Physics Chapter 05 Light Energy

5 Light Energy

Theme: An object lying at the bottom of a vessel filled with water usually appear to be at different depth than it actually is. This is due to bending of light rays when it travels from water to air. This phenomenon is called refraction. Light bends when it passes obliquely from one medium to the other. Due to refraction, a mirage is observed on a hot sandy desert. Atmosphere also refract the rays coming from the sun. This causes advanced sunrise and delayed sunset. Previous learning emphasized on reflection of light by a plane mirror. How images are formed by a curved (concave) mirror is now dealt upon along with rules used to construct ray diagrams.

In This Chapter You Will Learn To

Define refraction

Discuss examples of refraction

Describe a spherical mirror

Describe a concave and a convex mirror

Define the terms, principal axis, centre and radius of curvature, focus and focal length for a spherical mirror

Describe rules for making ray diagrams for spherical mirror

Distinguish between real and virtual images

Use a ray diagram to show formation of a real image by a spherical mirror

Describe the characteristics of a real image formed by a spherical mirror

Describe dispersion of white light by a prism into constituent colours

Display a scientific attitude while making models

Show a creative mind set while studying real world optical phenomena

Communicate logical reasoning and explanations effectively using scientific terms

Learning Objectives

Revising previous concepts learnt by children

Building on children's previous learning

Demonstrating the phenomenon of refraction

Engaging children in pairs, individually or small groups in activities related to refraction

Explaining refraction with suitable examples

Demonstrating how concave and convex mirrors work

Representing of concave and convex mirrors through diagrams

Explaining the terms i.e. Focus, principal axis, centre of curvature, radius of curvature with the help of diagrams to children

Engaging children in activities related to image formation by a concave mirror using ray diagram

Explaining real and virtual images

Demonstrating the dispersion of white light into component colours

Knowing Concepts

Refraction: Definition, Examples of refraction

Curved mirrors: Convex, Concave, Reflecting surface (Convex and concave), Use of curved mirrors, Terms related to curved mirrors - Focus, principal axis, centre of curvature, radius of curvature

Rules for making ray diagrams of spherical mirrors

Real and virtual images

Ray diagrams with curved mirrors where real images are formed

Dispersion of white light into constituent colours

Speed Of Light In Different Media

In class VII, we have read that light travels faster in air than in water or glass. The speed of light in air is \(3 \times 10^8\) m s-1, in water it is 2.25 \times 10^8\) m s-1 and in glass it is only 2 \times 10^8\) m s-1. In the language of Physics, we say that glass is optically denser than water and water is optically denser than air or air is optically rarer than both water and glass.

Thus, a medium is said to be denser if the speed of light in it decreases, while it is said to be rarer if the speed of light in it increases. But in no medium, it can be more than \(3 \times 10^8\) m s-1.

Refraction Of Light

Light travels in a straight line path in a medium. But when a ray of light travelling in one transparent medium falls obliquely on the surface of another transparent medium, it travels in the other medium in a direction different from its initial path.

The change in direction of path of light when it passes from one optically transparent medium to another, is called refraction of light.

It has been experimentally observed that

(1) When a ray of light travels from a rarer to a denser medium (say, from air to water or from air to glass), it bends towards the normal as shown in Fig. 5.1.

(2) When a ray of light travels from a denser to a rarer medium (say, from water to air or from glass to air), it bends away from the normal as shown in Fig. 5.2.

(3) When a ray of light falls normally on the surface separating the two media, it passes undeviated (i.e., along the same path) as shown in Fig. 5.3.

Some Terms Related To Refraction Of Light

(1) Incident ray: The ray of light falling on the surface separating the two media, is called the incident ray.

(2) Refracted ray: The ray of light travelling in the other medium in the changed direction, is called the refracted ray.

(3) Normal: The perpendicular drawn on the surface separating the two media, at the point where the incident ray strikes it, i.e. at the point of incidence, is called the normal.

(4) Angle of incidence: The angle between the incident ray and the normal is called the angle of incidence 'i'

(5) Angle of refraction: The angle between the refracted ray and the normal is called the angle of refraction 'r'.

Fig. 5.4 shows a light ray AO passing from a rarer medium (air) into a denser medium (glass). XY is the surface separating the two media. AO is the incident ray, OB is the refracted ray. NOM is the normal at the point of incidence O. angle AON is the angle of incidence i and angle BOM is the angle of refraction r.

refraction angle r. It is clear from the diagram that when the ray of light travels from air to glass, it bends towards the normal i.e., instead of moving along its earlier direction shown by the dotted line OC, the ray bends towards the normal and moves along OB (i.e. angle r is less than angle i).

Fig. 5.5 shows a light ray AO passing from a denser medium (glass) to a rarer medium (air). XY is the surface separating the two media. It is clear from the diagram that the incident ray AO bends away from the normal and is refracted as OB (i.e. angle r is greater than angle i).

Laws Of Refraction (Snell's Law)

Refraction of light obeys the following two laws also known as Snell's laws of refraction.

1. The incident ray, the normal at the point of incidence and the refracted ray, all lie in the same plane.

2. For a given pair of media and given colour of light, the ratio of the sine of angle of incidence i to the sine of angle of refraction r is a constant i.e.,

\[\frac{\sin i}{\sin r} = \text{constant}\]

This constant is denoted by the symbol \(\mu\) (read as mew).

It is known as the refractive index of the second medium with respect to the first medium. It is given as

\[\mu = \frac{\text{Speed of light in first medium}}{\text{Speed of light in second medium}}\]

For example, if a ray of light travels from air to water, then the constant \(\mu = \frac{\sin i}{\sin r}\) is the refractive index of water with respect to air. It is given as

\[\mu = \frac{\sin i}{\sin r} = \frac{3 \times 10^8 \text{ m s}^{-1}}{2.25 \times 10^8 \text{ m s}^{-1}} = \frac{4}{3} \text{ (or 1.33)}\]

Similarly, if a ray of light travels from air to glass, then \(\mu = \frac{\sin i}{\sin r} = \frac{3 \times 10^8 \text{ m s}^{-1}}{2 \times 10^8 \text{ m s}^{-1}} = 1.5\).

Note: The refractive index of air is 1. No medium can have refractive index less than 1.

Effects Of Refraction

(1) The depth of water in a vessel when seen from air appears to be less

Consider a vessel containing water as shown in Fig. 5.6. Its real depth is AO. But when seen obliquely i.e. at an angle above O from air, its depth appears to be AI which is less than AO.

Reason: A ray of light OA from the point O at the bottom of vessel is incident normally on the water-air surface XY. It travels straight along AD in air. Another ray OB incident from water on the surface XY, when passes to air, bends away from the normal BN, and goes along the path BC. The two refracted rays AO and BC when produced back, meet at I. Thus I is the image of O.

Thus to the observer in air, the depth of vessel appears to be AI instead of AO, due to refraction of light from water to air. The apparent depth AI is less than the real depth AO.

Do You Know?: Real depth / Apparent depth = Refractive Index. Since, refractive index of water is 4/3, so the apparent depth is 3/4 th the real depth.

The change in depth due to refraction can be demonstrated by the following activities.

Activity 1

(1) Take a coin and an empty glass vessel. Place the coin in the vessel. Put the vessel on a table and step back till the coin is just out of your view. It is hidden from your eye by the edge of the vessel as shown in Fig. 5.7(a).

(2) Keep your eye in this position and ask your friend to pour water gradually in the vessel. You will find that when there is sufficient water in the vessel, the coin becomes visible because then it appears to be slightly raised from position A to position B as shown in Fig. 5.7(b).

Explanation: In Fig. 5.7(a), when there is no water in the vessel, the coin is not visible because the ray of light from the coin travelling in a straight line does not reach the eye.

In Fig. 5.7(b), when water is poured in the vessel, the coin becomes visible because the ray of light from the point A of the coin, travelling in a straight line in water changes its direction (i.e. it bends) at the surface of water and reaches the eye. Thus, the light ray bends as it leaves water and enters air. The ray now appears to come from a point B instead of A. In other words, the coin appears to be raised from position A to position B.

Activity 2

(1) Take an empty beaker and a pencil. Place the pencil ABC obliquely in the beaker and look at it from the side. It appears straight as shown in Fig. 5.8(a).

(2) Now pour water in the beaker up to its brim. You will notice that the pencil now appears to be bent as ABD at the surface of water as shown in Fig. 5.8(b).

Explanation: The ray of light coming from the tip C of the pencil bends at the surface of water as it enters in air and it appears to be coming from the point D instead of C. In other words, it is due to refraction of light from water to air that the pencil ABC appears as ABD.

From the above, we conclude that when a light ray passes from one transparent medium to another, it bends. The direction in which light ray bends, depends upon whether light travels from a rarer medium to a denser medium or from a denser medium to a rarer medium.

Teacher's Note

When you look at objects underwater, they always appear closer and larger than they actually are due to refraction - this is why swimming pools look shallower than they really are.

Early Sunrise And Late Sunset

Before sunrise and after sunset the upper atmospheric layers are warmer than the lower layers near the earth's surface. So the atmospheric layers near the earth's surface are denser than those above. When the sun is just below the horizon, the light from sun, while coming towards the earth, suffers refraction from a rarer to a denser layer and so it bends towards the normal at each refraction. Due to continuous bending of light rays at different successive layers, the sun can

be seen even when its actual position is just below the horizon as shown in Fig. 5.9. As a result, sun is seen in advance, a few minutes before it rises above the horizon in the morning. Similarly, in the evening, sun is seen delayed by 3 to 4 minute longer above the horizon after the sun set.

Mirage In A Desert

Sometimes, in deserts, an inverted image of a tree is seen which gives a false impression of water under the tree. This is called a mirage.

The cause of mirage is the refraction of light. In a desert, the sand becomes very hot during day-time and it rapidly heats the layers of air in contact with it. Therefore, the layers of air near the ground are warmer (and hence rarer) than the upper layers. In other words, the successive upper layers are denser than those below them.

When a ray of light from sun after reflection from the top of a tree travels from a denser to a rarer layer, it bends away from the normal. As a result, in refraction at the surface of separation of successive layers, each time the angle of refraction increases and the angle of incidence of ray going from denser medium to rarer medium also increases, till a stage is reached when the angle of refraction becomes 90 degree. On further increase in angle of incidence, the ray of light travelling from a denser to a rarer layer, is not refracted, but it suffers reflection. This reflected ray now travels from the rarer to the denser layer, so it bends towards the normal, at each refraction. On reaching the eye of the observer, an inverted image of the tree is seen. Thus it gives a false impression of a pool of water in front of the tree (Fig. 5.10).

Refraction Of Light Through A Rectangular Glass Block

Fig. 5.11 shows a rectangular glass block PQRS. A light ray AB falls on the surface PQ. NBM is the normal at the point of incidence B to the surface PQ. At the surface PQ, the ray AB enters from air to glass, so it bends towards the normal NBM and travels along BC. At the surface RS, another refraction

occurs. N1CM, is the normal at the point of incidence C to the surface RS.

The ray BC now enters from glass to air (i.e., from a denser medium to a rarer medium), so it bends away from the normal N1CM, and travels along CD. The ray AB is called the incident ray, BC the refracted ray, and CD the emergent ray.

The emergent ray CD is parallel to the incident ray AB. Thus, both the incident and emergent rays are in the same direction, but the emergent ray is laterally displaced from the incident ray. (In Fig. 5.11 lateral displacement is shown by x).

Teacher's Note

When light passes through a window pane or glass slab, the image you see appears slightly displaced - this is why objects look shifted when viewed through thick glass.

Prism

A prism is a transparent medium bounded by five plane surfaces with a triangular cross section. Two opposite surfaces of prism are identical and parallel triangles, while the other three surfaces are rectangular and inclined on each other as shown in Fig. 5.12.

In symbol form, it is represented by the triangle ABC.

Refraction Of Light Through A Prism

Fig. 5.13 shows a prism ABC. A ray of light PQ of single colour falls obliquely on the face AB of the prism. This ray enters from air to glass (i.e., from a rarer medium to a denser medium), so it bends towards the normal NQM to the face AB and travels along QR. At the face AC of the prism, another refraction occurs. The ray QR now enters from glass to air (i.e., from a denser medium to a rarer medium), so it bends away from the normal N'RM' to the face AC and travels along RS. Thus, for the incident ray PQ, the refracted ray inside the prism is QR and the emergent ray outside the prism is RS. Thus, on passing through a prism, the light ray bends towards the base of the prism.

Do You Know?: The emergent ray through a prism is not in direction of the incident ray, but it bends towards the base of the prism because in a prism, refraction occurs at two inclined surfaces. On the other hand, in a rectangular glass block, refraction of light occurs at two parallel surfaces, so the emergent ray is in direction of the incident ray, but laterally displaced.

Dispersion Of White Light

Newton allowed white light from the sun to enter a dark room through a small aperture in a window and placed a glass prism in the path of light rays. The light coming out of the prism was received on a white screen. On the screen, a coloured patch like a rainbow was found as shown in Fig. 5.14. This patch was termed as spectrum.

Starting from the side of the base of the prism, the colours in the spectrum on the screen are in the following order:

Violet (V), Indigo (I), Blue (B), Green (G), Yellow (Y), Orange (O), and Red (R). The order of colours in the spectrum can easily be remembered by the word VIBGYOR. Thus, spectrum is the coloured band obtained on a screen on passing the white light through a prism.

From the above experiment, Newton concluded that white light is a mixture of seven colours.

Note that the prism does not produce colours, but it simply separates the colours which already exist in white light.

Thus, if white light is passed through a prism, it splits into different colours. This is called dispersion of light.

Cause Of Dispersion

In class VII, you have read that white light of sun is composed of seven prominent colours, namely, violet, indigo, blue, green, yellow, orange and red. The speed of light of all colours is same in air or vacuum, but it differs in a transparent medium such as glass or water. In a transparent medium (such as glass or water), the speed of violet light is minimum and of red light is maximum. Therefore, the refractive index \(\mu\) of a transparent medium is also different for lights of different colours.

Since, refractive index = speed of light in air / speed of light in medium, the refractive index of a medium is maximum for the violet light and minimum for the red light. Therefore, when white light enters a prism, it splits into its constituent colours while refraction at the first surface of the prism. These colours get farther separated from each other on refraction at the second surface of prism.

Do You Know?: In rainy season, sometimes after the rains, you see a rainbow in the sky, just opposite to the sun. It is due to dispersion of white light of sun by the rain drops which behave like small prisms.

The dispersion of white light can be demonstrated by the following activities.

Activity 3

To see dispersion of white light. Take a thick cardboard sheet. Make a small hole in it. Allow the sun light to pass through it in a dark room. Place a prism in the path of sun light coming through the hole and then a white screen behind the prism as shown in Fig. 5.15.

You will see that a band of colours is obtained on the screen with colours violet, indigo, blue, green, yellow, orange and red in order from the base of the prism upwards as shown in Fig. 5.15.

Activity 4

Take a circular disc of cardboard and divide it into seven sectors. Then paint the sectors with the seven colours (violet, indigo, blue, green, yellow, orange and red) in order, as shown in Fig. 5.16.

Rotate the disc rapidly. You will notice that the disc appears white.

This shows that seven colours violet, indigo, blue, green, yellow, orange and red being the constituent colours of white light, when combined produce the white effect.

Spherical Mirrors

Spherical mirrors are made by silvering the part AB of the hollow glass sphere as shown in Fig. 5.17.

The surface on which silvering is done, is called the silvered surface and the reflection of light takes place from the other surface which is called the reflecting surface.

Kinds of spherical mirrors

There are two kinds of spherical mirrors:

(i) Concave mirror and

(ii) Convex mirror.

(i) Concave mirror: A concave mirror is made by silvering on the outer surface of a part of the hollow sphere such that the reflection takes place from the inner (hollow or concave) surface as shown in Fig. 5.18 (a).

(ii) Convex mirror: A convex mirror is made by silvering on the inner surface such that the reflection takes place from the outer convexed (or bulged) surface as shown in Fig. 5.18(b).

Some Terms Related To A Spherical Mirror

(1) Pole: The geometric centre of the spherical surface of the mirror is called the pole of the mirror. It is the mid point of the aperture AB of the mirror. It is represented by the symbol P in Fig. 5.19.

(2) Centre of curvature: The centre of curvature of a mirror is the centre of the sphere of which the mirror is a part. It is represented by the symbol C in Fig. 5.19.

(3) Radius of curvature: The radius of curvature of a mirror is the radius of the sphere of which the mirror is a part. Thus, it is the distance of the centre of curvature C from any point of the surface of mirror. In Fig. 5.19, this is represented by the symbol R.

(4) Principal axis: It is a straight line joining the pole of the mirror to its centre of curvature. In Fig. 5.19, the line PC represents the principal axis. It may extend on either side of the pole.

Teacher's Note

A spoon is a perfect example of spherical mirrors - the inside is concave (makes you look bigger) and the outside is convex (makes you look smaller).

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