State the laws of refraction of light. Explain the term ‘absolute refractive index of a medium’ and write an expression to relate it with the speed of light in vacuum.

OR

What is meant by power of a lens? Write its SI unit. A student uses a lens of focal length 40 cm and another of –20 cm. Write the nature and power of each lens.

The laws of refraction, also known as Snell’s law of refraction describes the behaviour of light as it goes across an interface from a one medium to another.

The following two statements are the laws of refraction of light:

(i) The incident ray, refracted ray and the normal to the interface of the two separate media at the point of incidence, all lies in the same plane.

(ii) The ratio of the sine of angle of incidence to the sine of angle of incidence for a given pair of media and the colour of light is always constant.

If i and r are the angles made by the incident ray and the refracted ray, respectively, at the point of incidence, then we can write;

The term absolute refractive index is defined as the ratio of the speed of light in the vacuum to the speed of light in a certain medium. It can also be expressed as,

n_m =

OR

The ability of a lens to converge or diverge the light rays, depending on the nature of the lens is measured in terms of the power of the lens. It is defined as the inverse of focal length, described as;

The SI unit of power of a lens is called Dioptre. It is often denoted by letter D. Since, the focal length of a lens is expressed in meters, then, power is expressed in Dioptre. Thus, the power of a lens of a focal length of 1m is 1D, and its unit is m^-1.

The question has given two lenses with focal length values as following;

Let’s call it f₁ and f₂.

f₁ = 40 cm

f₂ = -20cm

Now, the corresponding values of power of these lenses are, say P₁ and P₂.

Power of a lens is defined as,

Therefore, for the lens 1,

P₁ = =

The value of power in this case is positive in nature that signifies that the lens is converging in nature.

Similarly, for the lens 2,

P₁ = =

The power of this lens is negative, that speaks about the nature of the lens. A lens with a negative power is diverging in nature In conclusion, a lens with negative power is always diverging in nature and for positive power the converging capacity of the lens is significant.