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 \(3 \times 10^8 \text{ m s}^{-1}\) in vacuum, \(2.25 \times 10^8 \text{ m s}^{-1}\) in water and \(2 \times 10^8 \text{ m s}^{-1}\) 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.

Teacher's Note

When you look at a straw in a glass of water, it appears bent at the water surface - this is refraction in action, as light bends when moving between water and air.

Refraction From Denser To Rarer Medium

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.

Teacher's Note

A swimming pool always appears shallower than it actually is because light from the bottom bends away as it exits the water, making objects appear closer than they really are.

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 'u'. The second law is also known as Snell's law. Here, the constant (u) 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

Snell's Law explains why objects underwater appear magnified and closer - the bending of light at the water-air interface creates an optical illusion that changes how we perceive distances and sizes.

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 P₄ 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.

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).

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