Problem 25.60 You are standing 2.6m from a 110-W lightbulb. Part A If the pupil

Question

Problem 25.60

You are standing 2.6m from a 110-W lightbulb.

Part A

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

Express your answer using two significant figures.

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Part B

Repeat part A for the case of a 1.0-mm-diameter laser beam with a power of 0.30mW .

Express your answer using two significant figures.

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Problem 25.60

You are standing 2.6m from a 110-W lightbulb.

Part A

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

Express your answer using two significant figures.

E =   ?J  

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Part B

Repeat part A for the case of a 1.0-mm-diameter laser beam with a power of 0.30mW .

Express your answer using two significant figures.

E =   ?J  

SubmitMy AnswersGive Up

Explanation / Answer

part a: the area of the pupil of the eye is ?r2 = ? * (2.5 * 10-3m)2 = 1.96 * 10-5 m2

the area of a 2.4 m sphere is 4?r2 = 4? * (2.4 m)2 = 72.38 m2

So the eye intercepts 2.71 * 10-7 of the light from the bulb. The 140 W bulb at 5% efficiency puts out 7 W of light.

(7 W)(2.71 * 10-7) = 1.90 * 10-6 W = 1.90 * 10-6 J/s = 1.9 ?J in one second.

part b: all of the laser goes into the eye :   0.30 mW = .30 mJ/s = 300 ?J in one second.

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