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Saturday, June 5, 2021

A monochromatic plane wave of speed c and wavelength λ is diffracted at a small aperture.

 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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