JEE Main 2025 · 22 January · evening · Single correct · +4 / −1
Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : In Young's double slit experiment, the fringes produced by red light are closer as compared to those produced by blue light.
Reason (R) : The fringe width is directly proportional to the wavelength of light.
In the light of the above statements, choose the correct answer from the options given below :
- (a) (A) is true but (R) is false
- (b) Both (A) and (R) are true and (R) is the correct explanation of (A)
- (c) (A) is false but (R) is true
- (d) Both (A) and (R) are true but (R) is NOT the correct explanation of (A)
Answer and solution
Answer (c) (A) is false but (R) is true
Solution
Physics sir · 3 steps
Fringe width grows with wavelength, and red has the longer wavelength, so red fringes are wider apart.
- Check the Reason. In Young's double slit experiment the fringe width is β = (λ D)/d. With the same D and d, β is directly proportional to λ, so (R) is true.
β = (λ D)/d ⇒ β ∝ λ
- Compare the colours. Red light has a longer wavelength (about 700 nm) than blue light (about 450 nm). So red fringes are wider, that is, farther apart, not closer.
- So. (A) is false and (R) is true, which is option (c).
The idea to keep. In YDSE, β = λ D/d: longer wavelength means wider fringes. Remember the order VIBGYOR, where violet has the narrowest fringes and red the widest.
Answer from the NTA final answer key. Solution by PrepWiser.