Question

A monochormatic 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 ?​

Answer 1

Given that,

Speed of wave = c

Wavelength = λ

We know that,

The distance between two consecutive lines is λ.

So, The wavefront XY is separated by a distance of 3λ from the wavefront that has reached point P.

The distance that need to be traveled = 3λ

We need to calculate the time

Using formula of velocity

$v=\dfrac{d}{t}$

$t=\dfrac{d}{v}$

Where, d = distance

v = velocity

Put the value into the formula

$t=\dfrac{3\lambda}{c}$

Hence, Some portion of the wavefront XY reach P in $\dfrac{3\lambda}{c}$.