Question

Laser light from a 7mW source of aperture diameter 1.5 cm and wavelength 5000 Å is focused by a lens of focal length 10 cm. The intensity of the image is __________

(a) 3 X 10^7 Wm^-2

(b) 4 X 10^7 Wm^-2

(c) 5 X 10^7 Wm^-2

(d) 6 X 10^7 Wm^-2

The question was posed to me during an interview.

I'd like to ask this question from Interaction of External Energy with the Atomic Energy States topic in chapter Laser of Engineering Physics II

Answer 1

Correct choice is (c) 5 X 10^7 Wm^-2

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

λ = 5000 Å = 5.0 X 10^-7 m, f = 0.1 m

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

Putting in the formula we get, Area = 1.4 X 10^-10 m^2

Intensity = Power/Area

= 7 X 10^-3 W/ 1.4 X 10^-10 m^2

= 5 X 10^7 Wm^-2.