Question

A monochromatic plane wave of speed $c$ and wavelength $\lambda $ is diffracted at a small aperture. The diagram illustrates successive wavefronts. After what time will some portion of the wavefront $XY$ reach $P$ ?

• A. $\frac{3 \lambda }{2 c}$
• B. $\frac{2 \lambda }{c}$
• C. $\frac{3 \lambda }{c}$
• D. $\frac{4 \lambda }{c}$

Answer 1

Answer: (C)

Distance between successive wave fringes is a wavelength $\lambda $ .
So path difference from $XY$ to $P$ is $3\lambda $
Time taken : $\frac{3 \lambda }{c}$