ICSE Class 8 Physics Chapter 02 Refraction of Light

Refraction Of Light

In vacuum, lights of different colours and wavelengths travel with the same speed but they travel with different speeds in different media. Light travels faster in air than in water or glass. The speed of light is 3 x 10^8 m s^-1 in vacuum, 2.25 x 10^8 m s^-1 in water and 2 x 10^8 m s^-1 in glass. A medium is said to be optically denser if it slows down the speed of light and it is said to be rarer if it increases the speed of light.

Concept Of Refraction

When light travels from one transparent medium to another transparent medium, it bends from its original path. This phenomenon of bending of light is called refraction. Refraction (or bending of light) takes place at the surface of separation of the two media.

Figure 2.1 (a) shows XY as the surface of separation between air and water media (water is denser than air). Let PQ be the incident ray which, at point Q, enters from air medium into water medium.

It is observed that as ray PQ enters from air to water, it does not follow the straight line path PQS but it bends along the path QR.

At point Q, a normal MN (i.e. perpendicular to surface XY) is drawn.

Thus, angle PQM, which is the angle between the incident ray PQ and normal MN, is called the angle of incidence (i) and angle RQN, which is the angle between the refracted ray QR and normal MN, is called the angle of refraction (r). Angle SQR is the angle of deviation (d) of light from its own path.

In Fig. 2.1(a) when the ray of light is travelling from a rarer medium (air) to a denser medium (water), we find that the angle of refraction -RQN is smaller than the angle of incidence -PQM; hence, we conclude:

Whenever light travels from a rarer medium to a denser medium, it bends towards the normal.

Figure 2.1(b) given above shows an incident ray of light PQ which, at point Q, enters from a denser medium (water) to a rarer medium (air). It is observed that the refracted ray does not cover a straight path PQS but bends away from the normal MN as ray QR. The angle SQR is the angle of deviation (d) of light from its own path.

This clearly shows that the angle of refraction -RQN is greater than the angle of incidence -PQM. So, we conclude:

Whenever light travels from a denser medium to a rarer medium, it bends away from the normal.

Terms Related To Refraction Of Light

1. Incident ray: The ray which falls on the surface of separation to enter into the other medium is known as the incident ray.

2. Refracted ray: The ray in the second medium obtained after refraction is known as the refracted ray. In Fig. 2.2, OB and QD are the refracted rays.

3. Normal: In Fig. 2.2, MN and PR are the normals. A normal is an imaginary straight line perpendicular to the refracting surface.

4. Angle of incidence: The angle between the incident ray and the normal at the point of incidence is known as the angle of incidence. It is generally represented by -i. In Fig. 2.2, -AOM and -CQR are the angles of incidence.

5. Angle of refraction: The angle between the refracted ray and the normal at the point of incidence is known as the angle of refraction. It is generally represented by -r. In Fig. 2.2, angles -BON and -POD are the angles of refraction.

Some Salient Points

1. When a ray of light passes obliquely from a rarer medium to a denser medium, it always bends towards the normal and the angle of refraction is smaller than the angle of incidence i.e. -r < -i (Fig. 2.3a).

2. When a ray of light passes obliquely from a denser medium to a rarer medium, it always bends away from the normal and the angle of refraction is greater than the angle of incidence i.e. -r > -i (Fig. 2.3b).

3. When a ray of light passes from one medium to another medium at right angle to the surface separating the two media, it does not bend i.e., it goes in its original direction only (Fig. 2.4).

In this case, the angle of incidence is zero and the angle of refraction is also zero i.e. -i = 0 and -r = 0.

Effects Of Refraction

1. When a stick is dipped partially in water, it appears to be bent and short.

Figure 2.5 shows a stick ABC with its part BC submerged in water. Consider two rays CD and CE starting from the point C. Ray CD being normal to the surface MN, separating water and air, is refracted without any deviation (bending), whereas the ray CE falling obliquely on the surface MN bends away from the normal as refracted ray EF. When ray EF is produced backwards, it meets ray CD at point C'. Hence, C' is the image (virtual image) of C which appears to be raised. Similarly, each point in part BC of the stick appears to be raised and finally, the part BC of the stick has its image as BC'. Clearly, BC' is bent at B and BC' is shorter than BC.

It must be noted here that only the part of the stick which is inside water appears to be bent and short.

2. A coin kept in a vessel filled with water appears to be raised (Fig. 2.6).

Laws Of Refraction

Refraction of light obeys the following two laws:

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 to the sine of angle of refraction is a constant i.e.,

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

This constant is denoted by the symbol μ'.

The second law is also known as Snell's law. Here, the constant (μ) is known as the refractive index of the second medium with respect to the first medium.

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

Teacher's Note

Refraction explains why objects underwater appear closer than they actually are - a practical observation when looking at fish in a pond or pool.

Refraction Through A Parallel Sided Glass Slab

You may trace the path of a refracted ray in a glass slab as shown in Fig. 2.7.

On a white sheet of paper, place a glass slab. Mark its boundary as ABCD and then remove it. Now mark a point Q on the side AB. Draw a normal M1N1 passing through point Q and perpendicular to AB. Draw another line PQ such that -PQM1 = -i = 60°. Fix two ordinary pins at P1 and P2 vertically on line PQ and place the glass slab back into its position ABCD. Look through the slab from the side CD. You will see the images of the pins P1 and P2. Now fix two more pins at P3 and P4 so that these pins and images of P1 and P2 are exactly in the same straight line. Remove the slab and all the pins. Encircle the dot marks left over by the pins. Join P1 with P3 and extend till it meets the side CD at point S. Join QS. -SQN1 is the angle of refraction -r corresponding to the angle of incidence -PQM1. Draw another normal M2N2 passing through S and perpendicular to CD. Measure angle -P3SM2 (marked as -e in the figure).

You will find -i = -e. Here, -e is known as the angle of emergence. Also, you will find that the incident ray PQ produced is parallel to the emergent ray ST (Fig. 2.7).

Thus, when a ray of light passes through a rectangular glass slab:

(i) the angle of emergence is equal to the angle of incidence i.e. -e = -i.

(ii) the emergent ray is parallel to the incident ray produced i.e., ST is parallel to QR.

Teacher's Note

A glass window pane demonstrates this principle - light passes straight through without changing direction, which is why windows don't distort the view.

Refraction Through A Prism

A prism is a glass block with each cross-section as a triangle and each face as a rectangle as shown in Fig. 2.8.

The refraction of a light ray PQ through a glass prism is shown in Fig. 2.9:

Here, PQ = incident ray, QR = refracted ray, RS = emergent ray, -i = incident angle, -r1 = refracted angle at first face, -r2 = incident angle at second face, -e = angle of emergence, -d = angle of deviation, -A = angle of prism, M1N1 = Normal to the face AB at point Q, M2N1 = Normal to the face AC at point R.

Experimentally, it is found that:

-i + -e = -A + -d.

It can easily be observed that when incident ray PQ gets refracted through face AB, it travels from a rarer medium (air) to a denser medium (glass). So, it bends towards the normal M1N1 as refracted ray QR. Now this ray QR works as the incident ray for face AC and is travelling from a denser medium (glass) to a rarer medium (air). So, it bends away from the normal M2N1 as emergent ray RS. The angle between the extended incident ray PQ and the emergent ray RS is the angle of deviation (-d).

Experimentally, it is found that corresponding values of -d. You will notice that as the angle of incidence (-i) increases, the value of -d first decreases, acquires a minimum value and then increases again. The minimum value it acquires is called the angle of minimum deviation represented by (d_m). At the condition of minimum deviation we find that -i = -e, -r1 = -r2, and QR is parallel to BC

The graph drawn between i and d is as follows:

The angle of minimum deviation depends on:

(1) Angle of prism

(2) Material of prism

(3) Angle of incidence

(4) Wavelength and colour of light used.

Teacher's Note

Prisms are used in binoculars and periscopes to bend light and create magnified images, showing how refraction is applied in optical instruments.

Dispersion Of Light Through A Prism

Sir Isaac Newton, in 1666, was the first Scientist to study about dispersion of light. He observed that, when a narrow beam of white light passes through a prism, it splits into its constituent colours. This happens because white light is made up of seven colours and when these colours pass through a prism, they deviate differently and so, move in different directions (Fig. 2.12).

It must be noted here that speed of light in a medium (other than air or vacuum) is different for the light of different colours. Thus, the refractive index of the medium is different for the light of different colours due to which, they deviate from their paths differently.

When a ray of white light passes through a prism, the red light travels the fastest and the violet light the slowest of all the seven colours. As a result the red light bends the least and the violet light bends the most, other colours lying in between.

The splitting (breaking-up) of white light into its constituent colours as it passes through a refracting medium (such as prism) is known as dispersion. The dispersion of white light into seven colours occurs because the lights of different colours bend through different angles while passing through a glass prism.

Experiment

Make a hole in a thick cardboard sheet to form a slit or use a hole in the window of your room. Through this hole, allow a narrow beam of light (sunlight) to pass through. When this narrow beam of sunlight passes through a prism and falls on a white screen, a band of colours is formed on the screen (see Fig. 2.12).

The band of colours on the screen resembles the seven colours of a rainbow and these seven colours from the base of the prism are in the order: Violet (V), Indigo (I), Blue (B), Green (G), Yellow (Y), Orange (O) and Red (R). This order of colours can be read as VIBGYOR and the corresponding band is called a spectrum.

Spectrum is the band of seven colours obtained on a white screen when white light passes through a prism and splits into its constituent colours.

The Rainbow

Rainbow is a spectacular example of dispersion of white light. Just after the rain, a large number of small droplets of water remain suspended in the air. Each drop acts like a small prism. When sunlight passes through these drops, it splits into seven colours. The dispersed light from a large number of drops forms a continuous band of seven colours; red on the outer side and violet on the inner side. This coloured band is called a rainbow. Thus, rainbow is produced due to the dispersion of white light by small raindrops suspended in the air after the rain.

It must be noted here that the prism does not produce or emit any colour. It only breaks the composite light into its constituent colours. Since white light is a mixture of seven colours (VIBGYOR), so a prism splits white light into these seven colours. This fact can easily be understood by the following experiment:

Take two identical prisms of the same material and place them as shown in Fig. 2.13.

Allow a narrow beam of light AB to pass through prism P and split into its constituent colours. On the other side of the prism P, the band of colours is allowed to fall on the identical inverted prism Q. The band of colours enter into prism Q and recombine to form a white light. This gives a narrow beam CD of white light. This experiment verifies that white light is a mixture of seven colours (VIBGYOR). It will also be observed that the rays AB and CD are parallel to each other i.e., the combination of two identical prisms (Fig. 2.13) work as a rectangular glass block.

Newton thus proved that recombination of seven colours gives white colour.

During sun rise and sunset, sun appears to be red in colour: During sunrise or sunset, light travels the largest distance through the atmosphere to reach the observer. As a result of scattering, the light loses violet, indigo, blue, green and yellow portions as their wavelengths are shorter than orange and red. Only red and orange reaches the observer's eye. Hence it appears to be red in colour.

Why do we use red colour light for the danger signal: Since the red colour light has the maximum wavelength therefore it scatters the least and the probability of reaching this light to the observer is the most. This is the reason why we use red colour light as a danger signal.

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