## Refraction of light Science Notes

Reflection of light :

When light is incident on the surface of an object, in general, it is deflected in different directions. This process is called reflection of light.

Laws of reflection of light :

• The incident ray and the reflected ray of light are on the opposite sides of the normal to the reflecting surface at the point of incidence and all the three are in the same plane.
• The angle of incidence and the angle of reflection are equal in measure.

Refraction of light :

Refraction of light : The change in the direction of propagation of light as it passes obliquely from one transparent medium to another is called refraction of light. Refraction occurs as the velocity of light is different in different media.

Laws of refraction :

Laws of refraction of light:

→ The incident ray and the refracted ray are on the opposite sides of the normal to the surface at the point of incidence and all the three, i.e., the incident ray, the refracted ray and the normal are in the same plane.

→ For a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant (Snell’s law). This constant is called the refractive index of the second medium with respect to the first medium.
[Note: Here, a ray means a ray of light)

Refractive index :

→ Refractive index : If i is the angle of incidence and r is the angle of refraction, then
$$\frac{\sin i}{\sin r}$$ = constant (Snell’s law).
This constant is called the refractive index of the second medium with respect to the first medium, and is denoted byn. Thus, 2n1 = $$\frac{\sin i}{\sin r}$$

→ Also, if y, is the magnitude of) the velocity of light in the first medium and u, is the magnitude of) the velocity of light in the second medium, then
2n1 = $$\frac{v_{1}}{v_{2}}$$

→ Similarly, the refractive index of the first medium with respect to the second medium is given by
1n2 = $$\frac{v_{2}}{v_{1}}$$

→ If the first medium is vacuum, 2n1 is considered with respect to vacuum. It is called the absolute refractive index of the medium 2 and is denoted by n.

→ The refractive index of a medium depends upon the magnitude of the velocity of light in the medium. [Note: The absolute refractive index of air is 1.003. This shows that for almost all practice purposes, the speed of light in air is very nearly the same as that in vaccum.]

→ Behaviour of a ray of light in refraction : When a ray of light passes obliquely from an optically rarer medium (medium of lower refractive index) to an optically denser medium (medium of higher refractive index), it bends towards the normal at the point of incidence. Here, i is greater than r, and 2n1 is greater than 1. The greater the value of 2n1, the greater is the bending towards the normal.

→ When a ray of light passes obliquely from an optically denser medium to an optically rarer medium, it bends away from the normal at the point of incidence. Here, r is greater than i, and 2n1 is less than 1. The greater the value of 2n1, the less is the bending away from the normal.

→ If a ray of light is incident normal to the interface between any two media (whether passing from an optically rarer medium to an optically denser medium or from an optically denser medium to an optically rarer medium), the angle of incidence is zero and so also the angle of refraction. Here, the light goes ahead in the same direction.
$$\frac{\sin i}{\sin r}$$ = constant = n

→ n is called the refractive index of the second medium with respect to the first medium. This second law is also called, Snell’s law. A ray incident along the normal (i = 0) goes forward in the same direction (r = 0).

→ Absolute refractive indices of some media

 Substance Refractive index Air 1.0003 Ice 1.31 Water 1.33 Alcohol 1.36 Kerosene 1.39 Fused quartz 1.46 Turpentine oil 1.47 Benzene 1.50 Crown glass 1.52 Rock salt 1.54 Carbon disulphide 1.63 Dense flint glass 1.66 ‘ Ruby 1.76 Sapphire 1.76 Diamond 2.42

→ Local atmospheric conditions affect refraction of light : e.g., mirage, objects beyond and above a holi fire appear to be shaking.

→ Twinkling of a star and atmospheric refraction : As a star is far away from the earth, it appears as a point source of light. When starlight enters the earth’s atmosphere, it undergoes refraction continuously in the medium with gradually varying refractive index.

→ The bending of starlight occurs towards the normal as it passes from the optically rarer, part of the medium to the optically denser part. Hence, when a star is observed near the horizon, its apparent position is slightly higher than the actual position.

→ Further, the apparent position varies with time as the medium is not stationary. Also, there is fluctuation in the brightness of a star when observed from the earth. This is called twinkling of a star.

→ The planets are relatively closer to the earth. Hence they appear as a collection of a large number of point sources of light. The net fluctuation in the brightness of a planet, therefore, turns out to be zero. Also, there is no change in the average position of a planet. Hence, planets do not twinkle.

The advanced sunrise and delayed sunset are also the result of atmospheric refraction.

Dispersion of light :

→ Dispersion of light: The process of separation of light into its component colours while passing through a medium is called dispersion of light. The band of coloured components of a light beam is called its spectrum.

→ The formation of a rainbow is due to refraction, dispersion, internal reflection and again refraction of sunlight by water droplets under appropriate conditions.

→ Our eyes are sensitive to electromagnetic radiation of wavelength in the range 400 nm to 700 nm. [1 nm – 10-m; nm nanometer] Wavelength of red light is close to 700 nm and that of violet light is close to 400 nm.

→ Partial and total internal reflection: When light travels from one medium to another, part of incident light comes back into the first medium. This is called partial reflection. Remaining part is refracted.

→ When light travels obliquely from a denser medium to a rarer medium, the angle of refraction r is greater than the angle of incidence i. Also, the ratio sini/sinr is constant For a particular value of 1, r becomes 90°. This particular angle of incidence is called critical angle.

→ For i > critical angle, as r cannot be greater than 90°, light is totally reflected in the denser medium. This is called total internal reflection. Here, 2n1 = $$\frac{\sin i}{\sin 90^{\circ}}$$ = sin i, where i is sin 90° the critical angle.