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.
Answers
The law of refraction states that :
1)The incident ray, reflected ray and the normal, to the interface of any two given mediums; all lie in the same plane.
2)The ratio of the sine of the angle of incidence and sine of the angle of refraction is constant.
Absolute refractive index means = speed of light in vaccum/speed of light in optical medium
[OR]
Power of a lens is the reciprocal of its focal length i.e P=1\f. Its SI unit is Dioptre.
f = 40cm
f₂ = -20cm
∴ P₁ = 1\40 = 0.025D (Lens is Convex)
P₂ = 1\-20 = -0.05D (lens is Concave)
The capacity of a lens to converge or diverge the incident light rays is called the power of a lens. It is the inverse of the focal length of the lens. Focal length is expressed in metres
It's SI unit is Dioptre
1 dioptre= 1/ Focal length (m)
