JEE Main 2023 · 12 April · morning · 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 : EM waves used for optical communication have longer wavelengths than that of microwave, employed in Radar technology.
Reason R : Infrared EM waves are more energetic than microwaves, (used in Radar)
In the light of given statements, choose the correct answer from the options given below.
- (a) Both A and R are true but R is NOT the correct explanation of A
- (b) A is true but R is false
- (c) Both A and R are true and r is the correct explanation of A
- (d) A is false but R is true
Answer and solution
Answer (d) A is false but R is true
Solution
Physics sir · 3 steps
Optical fibres carry near-infrared light, around a micrometre long; radar uses centimetre waves, so A fails.
- Check Assertion A. Optical communication sends near-infrared light down fibres, with wavelengths like 0.85 μm and 1.55 μm. Radar microwaves have wavelengths of a few centimetres. Optical waves are about 10⁴ times shorter, so A is false.
- Check Reason R. Photon energy is E = hν = hc/λ. Infrared has a shorter wavelength than microwaves, so each infrared photon carries more energy. R is true.
Wave Typical wavelength Photon energy Infrared (optical fibre) about 1 μm higher Microwave (radar) about 1 cm lower - So. A is false and R is true, which is option (d).
The idea to keep. Learn the order of the spectrum: radio, microwave, infrared, visible, UV, X-ray, gamma. Along it, wavelength falls and photon energy hc/λ rises.
Answer from the NTA final answer key. Solution by PrepWiser.