Question

A 0.1 watt LASER beam with an aperture of 5.0 mm emits a light of wavelength 6943 Å. Calculate the areal spread and intensity of the image when the beam is focused with lens having a focal length of 100 mm.

Answer 1

Answer:

Given A laser light beam of power 20 m W is focused  on a target by a lens of focal length 0.05 m. If the aperture of  the laser be 1 mm and the wavelength of its light 7000 Å  calculate the angular spread of the laser, the area of the target  hit by it, and the intensity of the impact on the target.  

Given P = 20 mW = 20 x 10^-3 W

Focal length f = 0.05 m, d = 1 mm = 1 x 10^-3 m, λ = 7000 A = 7000 x 10^-10 m, theta = 7 x 10^-4 radians

Linear speed of the laser s = f x theta

                                           = 0.05 x 7 x 10^-4

                                           = 0.35 x 10^-4 m

Now area of target hit by laser = s^2

                                                  = (0.35 x 10^-4)^2

                                                  = 0.1225 x 10^-8

                                                  = 1.225 x 10^-9 sq m

So intensity of impact on the target = power of the laser beam / area hit by the laser beam.

                                                       = 20 x 10^-3 / 1.225 x 10^-9 W / m^2

                       = 16.326 x 10^6 W/m^2

                       = 1.632 x 10^7 W / m^2

Explanation: