CBSE Class 12 Physics Refraction at Plane Surface Theory and Examples

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CBSE Class 12 Physics Refraction at Plane Surface Theory and Examples. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

The phenomenon of change in path of light as it goes from one medium to another is called refraction. Refraction phenomenon can be categories, depending upon the type of separating surface which can be further subdivided depending upon the shape of the medium as follows


Let us study the refraction phenomenon for individual cases :

The refractive index (μ) of a medium is defined as the ratio of the speed of light in vacuum (c) to the speed of light in the medium (v), i.e., μ = c/v

1. It is a scalar quantity without any unit or dimension.

2. If ε0 and μ0 are electric permittivity and magnetic permeability of vacuum while ε and μ are that of a given medium then


3. For vacuum, μ = 1 as v = c

4. Absolute refractive index of any medium is greater than one.

5. Refractive index of a medium also depends on the wavelength of light used. As per Cauchy’s formula
μ = A + B/λ2 + ..... longer the wavelength smaller is the refractive index.


The relative refractive index of two media is equal to the ratio of their absolute refractive indices.


(a) Frequency (and hence colour) and phase do not change (while wavelength and velocity changes)
(b) Incident ray, refracted ray, and normal always lie in the same plane.



(a) When light passes from rarer to denser medium it bends toward the normal. Using Snell’s law


(b) When light passes from denser to rarer medium it bends away from the normal. From Snell’s law.


(c) When light propagates through a series of layers of different medium, then according to Snell’s law


(A) If light is incident normally on a boundary i.e.,
i = 0º, Then from Snell’s law, μ1 sin 0 = μ2 sin r, sin r = 0 i.e. r = 0 i.e., light passes undeviated from the boundary. (So boundary will be invisible)
(B) If the refractive indices of two media are equal i.e., if,
μ1 = μ2 = μ, Then from Snell’s law μ sin i = μ sin r i = r i.e., ray passes undeviated from the boundary with i = r ≠ 0 and boundary will not be visible. This is also why a transparent solid is invisible in a liquid if μS = μL

An object O placed in first medium (refractive index μ1) is viewed from the second medium (refractive index μ2). Then the image distance dAP and the object distance dAC are related as
dAP =(μ21)dAC

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