CBSE 2026 · Region 4 · Set 1 · Q18 · 2 marks
State Huygens principle. How did Huygens justify the absence of the backwave on a spherical wavefront ?In a single-slit diffraction experiment, light of wavelength $\displaystyle \lambda$ illuminates the slit of width 'a'. The diffraction pattern is observed on a screen kept at a distance D from the slits.(i)Depict variation of intensity in the fringe pattern with the angular position of the fringes.(ii)How is the linear width of central maximum affected when separation between the slit and the screen is decreased ?
State Huygens principle. How did Huygens justify the absence of the backwave on a spherical wavefront ?
In a single-slit diffraction experiment, light of wavelength $\displaystyle \lambda$ illuminates the slit of width 'a'. The diffraction pattern is observed on a screen kept at a distance D from the slits.
(i)
Depict variation of intensity in the fringe pattern with the angular position of the fringes.
(ii)
How is the linear width of central maximum affected when separation between the slit and the screen is decreased ?
Marking-scheme solution
According to Huygens' principle, each point of the wavefront is the source of a secondary disturbance and the wavelets emanating from these points spread out in all directions with the speed of the wave. These wavelets emanating from the wavefront are usually referred to as secondary wavelets and if we draw a common tangent to all these spheres, we obtain the new position of the wavefront at a later time.
The amplitude of the secondary wavelets is maximum in the forward direction and zero in the backward direction.
Alternatively:
Amplitude $\displaystyle (\mathrm{A}) \propto(1+\cos \theta)$; for back wave $\displaystyle \theta=180^{\circ} \Rightarrow$ Amplitude $\displaystyle =0$
Wave OpticsHuygens PrincipleUnderstandvery_short_answermedium
More from Wave Optics
- Assertion: In interference and diffraction of light, light energy reduces in one region producing a dark…2024 · asked 3×
- Assertion: In a Young's double-slit experiment, interference pattern is not observed when two coherent…2024 · asked 3×
- What are coherent sources? Why they are necessary for observing stable interference pattern? Draw a graph…2026 · asked 3×
- In a Young's double-slit experiment, the fringe width is found to be β. If the entire apparatus is immersed…2023 · asked 3×
- Assertion: In Young's double slit experiment all fringes are of equal width. Reason (R): The fringe width…2023 · asked 3×
- (i) A plane light wave propagating from a rarer into a denser medium, is incident at an angle i on the…2024 · asked 3×
- Assertion: Light added to light can produce darkness. Reason (R): When two coherent light waves interfere,…2026 · asked 3×
- In a Young's double-slit experiment, the screen is moved away from the plane of the slits. What will be its…2023 · asked 3×
CBSE Class 12 Physics past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.