Problem 25.60 You are standing 2.9m from a 120-W lightbulb. Part A If the pupil

Question

Problem 25.60

You are standing 2.9m 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 0.70mW .

Express your answer using two significant figures.

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Explanation / Answer

PART A

Light Intensity falling on the pupil = (5% of 120 )/ (4 * 3.14 * r2) W/m2, here r = 2.9m

Amount of energy entering the eye per sec = Light Intensity falling on the pupil * Area of pupil = (5% of 120 )/ (4 * 3.14 * 2.92) * 3.14 * (5/2000)2 J = 1.115 uJ

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PART B

Assuming that the laser beam is not diverging, diameter of the beam < diameter of pupils. Hence, the entire energy enters the eyes.

Amount of energy entering the eye per sec = 0.70mJ

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