Question

ASSERTION: In a glass medium red light travels faster than blue light.
REASON: Red light has a wavelength larger than blue.

A. Both assertion and reason are correct and reason is the correct explanation for assertion.
B. Both assertion and reason are correct but reason is not the correct explanation for assertion.
C. Assertion is correct but reason is incorrect.
D. Assertion is incorrect but reason is correct.

Answer 1

Hint: We know that according to the formula of wavelength in relation with velocity and frequency, that wavelength and velocity both are directly proportional to each other so velocity of light will be maximum for the light which has the highest wavelength.

Formula used:
Formula used:
$v=f\lambda $

Complete answer:
We know that
$v=f\lambda $,
Velocity of light is directly proportional to its wavelength.
We know that the wavelength in increasing order of visible light is as follows:
Violet < Indigo < Blue < Green < Yellow < Orange < Red.
Thus we can see that the red light has a much longer wavelength than the blue light.
So, we can say that the reason statement is true.
Now, let us consider that the velocity of light in air(blue or red) ${{v}_{a}}=c$
Now, let us consider that the velocity of light in glass(blue or red)
 ${{V}_{g}}=\dfrac{c}{{{n}_{g}}}=$ constant that is independent of light.
Hence we can say that both the light be it red or be it blue travels with the same speed in the glass.
Therefore we can say that assertion is false.

So, the correct answer is “Option D”.

Additional Information:
Wavelength of the light is the length of the wave from its starting position to its end position.

Note:
In the formula $v=f\lambda$, ‘v’ is the velocity of the light, ‘f’ is the frequency of the light, and $\lambda $(lamda) is the wavelength of the light. Students must know the wavelength for all the visible light rays to get the required result.