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  4. A short pulse of white light is incident from air to a glass slab at normal incidence. After travelling through the slab, the first colour to emerge is

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Q. A short pulse of white light is incident from air to a glass slab at normal incidence. After travelling through the slab, the first colour to emerge is

Ray Optics and Optical Instruments

Solution:

Since $\lambda_{r} > \lambda_{g} > \lambda_{b} >\lambda_{v} , \mu_{r} < \mu_{g} <\mu_{b} <\mu_{r} < \mu_{v}$
$(\because \mu = a + b /\lambda ^2)$
$\therefore v_{r} > v_{g} > v_{b} > v_{v} (\because v = c/\mu)$.
After travelling through the slab, red colour will emerge first because it has largest velocity in slab.