Question

A laser beam having a wavelength of 8000 Å and aperture 0.5 cm is sent to moon.
Calculate, (a) Angular spread of the beam (b) A real spread of the beam when it
reached the moon. Given distance of moon from the earth = 4 x10 8 m.

Answer 1

Given:

The wavelength of the beam is 8000 A= 8×10⁻⁷ m

The aperture is 0.005 m

To find:

The angular speed and real spread of beam

Solution:

The formula of angular speed is

Ф=1.22×λ/a

Putting the above values

Ф=1.22×8×10⁻⁷/0.005

Ф=1.952×10⁻⁴ rad

Now the real spread of beam is

=(DФ)²

=(4×10⁸×1.952×10⁻⁷)

=7.808×10³ m²