ray-optics-and-optical-instruments Question 20
Question: Q. 5. 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 (a) blue. (b) green. (c) violet. (d) red.
[NCERT Exemplar]
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Solution:
Ans. Correct option : (d)
Explanation : As we know that the velocity of wave is :
$$ v=v \lambda $$
When light ray goes from one medium to other medium, the frequency of light remains unchanged. So, $v \propto \lambda$ or greater the wavelength, greater the speed. And, the light of red colour is of highest wavelength and therefore of highest speed. So, after travelling through the slab, the red colour emerges first.
