Interference of light corresponds to conservation of energy because
A. All fringes are of equal width.
B. All bright fringes are of same intensity and the dark fringes are of same intensity.
C. All fringes are separated by equal distance.
D. Energy is transferred from dark fringe to bright fringe.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → B and D only
In interference, energy is neither created nor destroyed; it is only redistributed. For two waves of equal intensity \( I_0 \), the intensity is \( 4I_0 \) at every bright fringe and zero at every dark fringe, so its average is the sum of the two intensities:
\( \displaystyle I_{\mathrm{av}} = \frac{4I_0 + 0}{2} = 2I_0 = I_0 + I_0 \)
(A) All fringes are of equal width. — Incorrect. Equal fringe width (\( \beta = \lambda D/d \)) is a geometric feature; it says nothing about energy.
(B) All bright fringes are of same intensity and the dark fringes are of same intensity. — Correct. Every bright fringe has \( 4I_0 \) and every dark fringe zero, so the average stays \( 2I_0 \).
(C) All fringes are separated by equal distance. — Incorrect. This is again only a geometric property.
(D) Energy is transferred from dark fringe to bright fringe. — Correct. The energy missing at the dark fringes appears at the bright fringes, so no energy is lost.
NCERT (2026-27):
Seeing the Single Slit Diffraction Pattern – “… In interference and diffraction, light energy is redistributed. If it reduces in one region, producing a dark fringe, it increases in another region, producing a bright fringe. There is no gain or loss of energy, which is consistent with the principle of conservation of energy.”