Question

Laser light from a 2mW source of aperture diameter 1.5 cm and wavelength 5000 Å is focused by a lens of focal length 20 cm. The intensity of the image is __________

(a) 1.57 X 10^6 Wm^-2

(b) 2.57 X 10^6 Wm^-2

(c) 3.57 X 10^6 Wm^-2

(d) 4.57 X 10^6 Wm^-2

This question was posed to me in my homework.

The question is from Introduction and Applications of Laser topic in portion Laser of Engineering Physics II

Answer 1

Right choice is (c) 3.57 X 10^6 Wm^-2

Best explanation: Area of the image =  \(\frac{\pi\lambda^2f^2}{a^2}\)

λ = 5000 Å = 5.0 X 10^-7m, f = 0.2 m

a = 1.5/2 cm = 0.75 X 10^-2 m

Putting in the formula we get, Are = 5.6 X 10^-10 m^2

Intensity = Power/Area

= 2 X 10^-3W/ 5.6 X 10^-10 m^2

= 3.57 X 10^6 Wm^-2.