Question

You are standing 2.2m from a 110-W lightbulb.

A. If the pupil of your eye is a circle 5.0mm in diameter, how much energy enters your eye per second? (Assume that 5.0% of the lightbulb's power is converted tolight.)

Express the answer using two significant figures please.

E=___uJ

B. Repeat part A for the case of a 1.0mm--diameter laser beam with a power of 0.40mW .

E=___uJ

Answer 1

The light from a 159 watt bulb radiates in all directions therefore;

you need to get the surface area of a sphere of 1.7 meters =As1

( 150 watts x .05 /As ) times eye opening area = watts that enter eye

(150 X.05/(4*pi*(1.7^2) ) X 2*pi *(.005^2) = watts entering the eye

a Joule = watt sec

for part b since the laser beam is smaller than the eye opening all the energy enters the eye.