Which of the following statements are correct?
(A) According to wave theory, energy carried by a wave depends on the amplitude of the wave, not on the speed of wave propagation
(B) For a given frequency, intensity of light in the photon picture is determined by the number of photons crossing a unit area
(C) At a large distance from the source, a small portion of the spherical wave can be approximated by a plane wave
(D) the reflected and refracted light both have different frequencies than the incident frequency
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (A) and (C) only
Note: The given answer is as per the official NTA Answer Key. However, according to us the correct answer should be Option (1), because statement (B) is correct, and it seems that NTA has treated (B) as incorrect since it leaves out the words “per unit time”, as explained below.
(A) According to wave theory, energy carried by a wave depends on the amplitude of the wave, not on the speed of wave propagation. — Correct. In the wave picture the energy is proportional to the square of the amplitude.
(B) For a given frequency, intensity of light in the photon picture is determined by the number of photons crossing a unit area. — Correct. For a given frequency every photon has the same energy \( h\nu \), so the intensity depends only on the number of photons crossing unit area in unit time. The statement leaves out “per unit time”, but the idea is the same.
(C) At a large distance from the source, a small portion of the spherical wave can be approximated by a plane wave. — Correct. The radius of the wavefront is so large that a small part of it is almost flat.
(D) the reflected and refracted light both have different frequencies than the incident frequency. — Incorrect. The frequency stays the same on reflection and refraction; only the speed and wavelength change on refraction.
The correct statements are (A), (B) and (C), which is Option (1). The NTA key, Option (2), treats (B) as incorrect.
NCERT (2026-27):
Huygens Principle – “… At a large distance from the source, a small portion of the sphere can be considered as a plane and we have what is known as a plane wave [Fig. 10.1(b)].”
Coherent and Incoherent Addition of Waves – “… (a) Reflection and refraction arise through interaction of incident light with the atomic constituents of matter. … Thus, the frequency of scattered light equals the frequency of incident light.
(b) No. Energy carried by a wave depends on the amplitude of the wave, not on the speed of wave propagation.
(c) For a given frequency, intensity of light in the photon picture is determined by the number of photons crossing an unit area per unit time.”