Case: Read the passage and answer the question.
YDSE is the abbreviation used for Young’s double-slit experiment, which shows the interference of light. Interference of light is the phenomenon in which two light waves meet each other while travelling in the same medium. A light source is kept behind a wall in which two slits, S1 and S2, are made. On the screen there appears a pattern of alternate bright and dark fringes. The central fringe is always bright.
Which of the following is true about this phenomenon?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → The process conserves energy.
Option (1) The process conserves energy. — Correct. In interference the light energy is only redistributed on the screen. Less of it reaches the dark fringes and more reaches the bright fringes. There is no gain or loss of energy in total.
Option (2) It verifies the dual nature of light. — Incorrect. The superposition of waves explains interference, so it shows the wave nature of light. Other effects, such as photoelectric emission, show the particle nature of light.
Option (3) The bright fringes are not equally spaced. — Incorrect. The bright fringes lie at \( x_n=n\lambda D/d \). Neighbouring bright fringes are the same distance \( \lambda D/d \) apart, so the fringes are equally spaced.
Option (4) The fringes act as new coherent sources of light. — Incorrect. The fringes are only regions on the screen where the waves from \( S_1 \) and \( S_2 \) give maximum or zero intensity. The coherent sources are the two slits.
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.”