Question 36
A monochromatic plane wave of speed c and wavelength λ is diffracted at a small aperture.
The diagram illustrates successive wavefronts.
After what time will some portion of the wavefront GH reach point P?
A 3λ / 2c B 2λ / c C 3λ / c D 4λ / c
Reference: Past Exam Paper – March 2017 Paper 12 Q28
Solution:
Answer: C.
Speed of wave = c
The distance between two successive wavefronts is equal to a wavelength.
So, the distance between wavefront GH and the wavefront containing point P is equal to 3 wavelengths (= 3λ).
Speed = distance / time
Time = distance / speed
Time = 3λ / c
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