Question

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

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

Answer 1

Answer: (C)

This distance between the 2 successive lines (wavefronts) is $\lambda$ . This is for both the straight and curved lines.
So, wavefront XY is separated by a distance of $3\lambda$ from the wavefront that has reached point P.
Distance that needs to be traveled $=3\lambda$
Speed of light $=c$
Speed = Distance/Time
Time = distance/speed $=\frac{3\lambda }{c}$