A glass prism is able to produce a spectrum when white light passes through it but a glass slab does not produce any spectrum. Explain why is it so.

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In case of prism , after  final refraction , the incident ray and the emergent ray makes an angle with the incident ray calledthe angle of deviation . Every colour in the white has different  wavelength .The refractive index of prism is different for different colours and therefore different colours deviate through different  angles on passing through the prism, Thus , they are seen differently. So, there is dispersion in case of prism.

However, in case of rectangular glass slab , after refraction at the two parallel faces of the glass slab , a ray of light emerges in a direction parallel to the direction of the incidence of white light on the slab. As rays of all colours  emerge in the same direction  ( of incidence of white light  ) , hence there is no dispersion but only lateral displacement.

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The main reason "why a glass prism disperses white light into its component colours but a glass slab does not" is that, in a glass slab, the edges where the refraction of light takes place are PARALLEL to each other, thus all the seven colours of the light are refracted by the same amount and by the same angle at both the surfaces.

But in a glass prism, the two surfaces where refraction takes place are NOT PARALLEL to each other...so all the colours of the light are refracted by different amounts and by different angles...the violet colour being refracted the most and red colour being refracted the least(due to having different frequencies).

there is nothing to do with the power of the glass used...!!

hope it helps...!! :)

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