Question

A 0.64-mW laser produces a beam of light with a diameter of 2.9 mm. (a) What is the average intensity of this beam? W/m2 (b) At what distance does a 150-W light bulb have the same average intensity as that found for the laser beam in part (a)? (Assume that 5.0% of the bulb's power is converted to light.) cm

Answer 1

Part a)

here,

power output of laser , P = 0.64 mW = 6.4 * 10^-4 W

diameter , d = 2.9 mm = 0.0029 m

radius , r = 0.00145 m

a)

the average intensity of this beam , I = P /(pi * r^2)

I = 6.4 * 10^-4 /( pi * 0.00145^2)

I = 96.89 W/m^2

Hence, the average intensity of the beam is 96.89 W/m²

Part b)

power P = 150 W

150 /(pi * r^2) = 96.89

r = 0.7 m

r = 70 cm

Hence, the distance where the 150W bulb have the same average intensity as that for the laser beam is 70 cm