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I came across this question in mastering physics and simply could not solve it. I asked for the answer to the question and decided to move on.

1. The problem statement, all variables and given/known data

You are standing 1.7m from a 100-W lightbulb.

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.)

2. Relevant equations

Energy of a beam of light =(Energy Density)(Volume):

U = U_{EM}(Area X Speed Of Light X Change in time)

U = U_{EM}(A_{c}Δt)

Intensity:

I = U / (AΔt) = U_{EM}c

3. The attempt at a solution

I decided that the energy of the beam must be equal to 5% of the wattage of the light bulb.

U = (5/100)*100W

= 5 J/s

I then found the energy density (U_{EM}) by dividing U by volume (Area x distance from light source).

U_{EM}= U / (A_{c}Δt)

= (5) / (Pi((2.5*10^-3)^2) x 1.7)

= 149792.88 J/M^3

From this I derived the light intensity.

I = U_{EM}c

= (149792.88) x (3 x 10^8)

= 4.49 x 10^13 J/s

At this point I was completely lost. The intensity is simply too large and has exceeded the amount of energy the light bulb can provide. I tried following through to the answer by multiplying intensity by area of the pupil, but the number was still far larger than the energy provided by the bulb. I tried using 5J as the value of U_{EM}, but that didn't give the answer either.

The answer I received from mastering physics was 2.7 microjoules. Please explain to me the concept, I think i've misunderstood it and my notes aren't helpful at all. I don't need a worked answer, just an explanation of how I've misinterpreted the question or concept. Thank you, I'm starting to think I'm not cut out for this, spent three hours on such a simple question.

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# Homework Help: Intensity of light entering eye

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