Question

prism produces spectrum but not the glass slab . explain!

Answer 1

ANSWER:

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The Dispersion phenomenon using a glass prism and not a glass slab because when light passed through glass slab, it encounters TWO interfaces--one entering and the other leaving.
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All of the bending (and dispersion) that takes place at the first interfaces is exactly reversed at the second, 'undoing' the effect of the first interface; so although the emerging ray of light is displaced slightly from the entering ray, it travels in the same direction as the incoming ray and all wavelengths that separated at the first interface are re-combined. 
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And in case of prism the second interface is NOT parallel to the first, the effects of the first interface are NOT reversed and the colors separated at that interface continue along different paths upon leaving the prism.
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Hence, the prism produces spectrum but not the glass slab.
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