Refraction and Dispersion of Light through a Prism

12th physics. NCERT notes


Prism is uniform transparent medium bounded between two refracting surfaces, inclined at an angle.

Refraction through a Prism:

A ray of light suffers two refractions on passing through a prism and hence deviates through a certain angle from its path.

Angle of Deviation

The angle subtended between the direction of incident light ray and emergent light fay from a prism is called angle of deviation (δ).

Prism Formula

The refractive index of material of prism

Dispersion of Light

The splitting of white light into its constituent colors in the sequence of VIBGYOR, on passing through a prism is called dispersion of light.

The refractive index μviolet > μRed, therefore, violet color deviates most and red color deviates least. i.e., δviolet > δRed.

Angular Dispersion

The angle subtended between the direction of emergent violet and red rays of light from a prism is called angular dispersion.

Angular dispersion: Angular Dispersion produced by a prism for white light is the difference in the angles of deviation of 2 extreme colors i.e., violet & red colors.

where δv and δR are the angle of deviation.

Dispersive Power:

The dispersive power of a prism is defined as the ratio of angular dispersion to the mean deviation produced y the prism. It is represented by W.

where μY = (μ v + μ R ) / 2, is mean refractive index.

W depends only on nature of the material of prism while both mean & angular deviation depend on the angle of prism & material of the prism.


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