# Why a prism can disperse white light but a glass slab cannot??

2
by vhm

2015-02-12T12:21:40+05:30

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When light passes through a prism it undergoes deviation and refraction as there are many ways in which light can enter the prism.
This causes light to bend in different range and such ranges cause light to scatter into its component colors.

In case of glass slab light enter and exit in a single straight line in two ways.
so there is no possibility of light to undergo multiple refraction and so no dispersion takes place.
2015-02-12T13:52:38+05:30

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Certified answers contain reliable, trustworthy information vouched for by a hand-picked team of experts. Brainly has millions of high quality answers, all of them carefully moderated by our most trusted community members, but certified answers are the finest of the finest.
Glass prism (triangular)

The angle of incidence of light rays on one face of the triangular prism and the angle of incidence of the second face of the prism (after the light travels through the prism) are such that the deviations of the colors at each face  add up.  So the dispersion more enhanced.
This is because of the way the two faces of the prism are inclined to each other (< 90 deg).  The individual colors have different angles of refraction and hence different angles of deviation Dm,  given by
μ = Sin (A+Dm)/2    ÷   Sin A/2
Hence, they are dispersed and separated by a small angle, which is the difference in their deviations.

Dm = angle of minimum deviation for ONE color = (μ - 1) A
Since μ is different for different colors,  Dm is different.  hence, we see them separately.  Larger the angle A, the larger is the separation between the colors.

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Glass slab

The angle between the two faces of a glass slab is 0⁰ (or 180⁰ or 360⁰).  Due to this reason,
As the two deviations cancel,  the incident and emergent rays are both parallel.

We can show this mathematically too.

μ = Sin (A + Dm)/2    ÷    Sin A/2    for the prism.  Normally  A < 90⁰

Sin (A + Dm)/2 = μ * Sin A/2

A glass slab is a prism with A = 0⁰,

Sin Dm/2 = μ * 0 = 0      =>Dm = 0

Angle of deviation by the slab is 0.  Thus the incident ray and emergent ray are parallel for each color.  Now, each color in the incident white ray are collinear.  Hence, the colors in the emergent ray are collinear.