PRISM


A prism has two plane surfaces AB and AC inclined to each other as shown in following  figure. A is called the angle of prism or refracting angle. 
(i)    Refraction Through A

 2437_prism.jpg

 

A light ray striking at one face of a triangular glass prism gets refracted twice and emerges out from the other face as shown above. The angle between the emergent and the incident rays is called the angle of deviation (D). The angle between the two refracting faces involved is called the refracting angle (A) of the prism.

From AXY, we have:           A + (90° – r1) + (90° – r2) = 180°

As     r1 + r2 = A                                               (i)

Deviation D = (i – r1) + (e – r2)

            D = (i + e) – (r1 + r2)

       D = i + e – A

       A + D = i + e                                      (ii)

We also have two equations from Snell's Law at X & Y.

2200_snells law.jpg

(i)     Angle of Deviation

It can be easily seen that if we reverse the emergent ray, it goes back along the same path. The angles of incidence and emergence get interchanged but the angle of deviation remains same.

578_angle of incidence.jpg

Hence the same angle of deviation D is possible for two different angles of incidence :

θ1 and θ2, where θ1 + θ2 = A + D.

(ii)   Minimum Deviation

The angle of deviation is minimum when the path of light ray through the prism is symmetrical. i.e., angle of incidence = angle of emergence

1954_minimum deviation.jpg

 

(iii)       Grazing Incidence

When i = 90°, the incident ray grazes along the surface of the prism and the angle of refraction inside the prism becomes equal to the critical angle for glass - air. This is known as grazing incidence

 

(iv)       Grazing Emergence
When e = 90°, the emergent ray grazes along the prism surface. This happens when the light ray strikes the second face of the prism at the critical angle for glass - air. This is known as grazing emergence.

 

(v)       Maximum Deviation

The angle of deviation is same for both the above cases (grazing incidence & grazing emergence) and it is also the maximum possible deviation if the light ray is to emerge out from the other face without any total internal reflection.

(vi)     Dispersion of Light

When a ray of light passes through a prism, it splits up into rays of constituent colours or wavelengths. This phenomenon is called dispersion of light.

The refractive index of a medium is different for light rays of different wavelengths. Larger the wavelengths, the lesser is the refractive index.

i.e.    mred < mviolet because 1red > 1violet

 

A ray of white light passing through a prism gets splits into different colours because the deviation is more for violet and less for red. This phenomenon is known as dispersion.

1753_dispersion of light.jpg

 

For a prism with very small refracting angle A, the deviation D is given by:

d >> (m-1)A

Hence deviaiton of violet = Dv = (μv - 1)A,

and for red = Dr = (μr – 1)A

The angle between the red and violet rays is known as the angle of dispersion.
The angle of dispersion = f = Dv
 - Dr =  (mv - mr)A

1454_dispersive power.jpg

This phenomenon arises due to the fact that refractive index varies with wavelength. It has been observed for a prism that μ decreases with the increase of wavelength, i.e.

mviolet > mred

2246_dispersive power.jpg

(viii)    Deviation without Dispersion

This mean an achromatic combination of two prisms in which net or resultant dispersion is zero and deviation is produced. For the two prisms,

1554_deviation without dispersion.jpg

 

Where w and w' are the dispersive powers of the two prisms and d and d' their mean deviations.

(ix)      Dispersion without deviation

A combination of two prisms in which deviation produced for the mean ray by the first prism is equal and opposite to that produced by the second prism is called a direct vision prism. This combination produces dispersion without deviation.

For deviation to be zero, (d + d') = 0

1343_dispersion without deviation.jpg

 
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