JEE Main 2021 · 16 March · evening · Single correct · +4 / −1
Red light differs from blue light as they have :
- (a) Different frequencies and same wavelengths
- (b) Different frequencies and different wavelengths
- (c) Same frequencies and different wavelengths
- (d) Same frequencies and same wavelengths
Answer and solution
Answer (b) Different frequencies and different wavelengths
Solution
Physics sir · 3 steps
In vacuum every colour travels at the same c, so if λ changes, f must change with it.
- One fixed speed. All light travels in vacuum at the same speed c = 3 × 10⁸ m s⁻¹. Frequency and wavelength are tied together by that speed.
c = fλ
- Colour sets both. Since c is fixed, a larger λ means a smaller f. Red light has λ ≈ 700 nm and blue light has λ ≈ 450 nm, so red has the lower frequency and blue the higher.
- So. Red and blue light differ in both frequency and wavelength, which is option (b).
The idea to keep. For light in vacuum, c = fλ with c fixed. You cannot change one of f or λ alone; colour fixes both together.
Answer from the NTA final answer key. Solution by PrepWiser.