Problem 25.60 You are standing 2.1m from a 120-W lightbulb. Part A If the pupil

Question

Problem 25.60

You are standing 2.1m from a 120-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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Explanation / Answer

5%of 120 W = 6 W

The surface area of the sphere of radius 2.1 m is 4pir^2 = 4pi*2.1^2 =55.417m^2

Power per unit area on the surface of this sphere is P = 6 /45.36 = 0.10826 W/m^2

Power on the circular area of radius 2.5 mm is pi*r^2* P = pi* 0.0025^2 *0.10826 = 12.126 uJ

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