# Energy/Light-Some problem set questions

jefgreen
Q1:

Two lamps illuminate a screen equally. The first lamp has an intensity of 12.5cd and is 3.0m from the screen. The second lamp is 9.0m from the screen. What is its intensity?

I know I need to use P=4*pi*(I), but there is no variable included for distance (d). Another formula given is: Eill=P/(4*pi*d^2). How should I go about solving this problem now?
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Q2:

A 15cd point source lamp provide equal illuminations on a wall. if the 45cd lamp is 12m away from the wall, how far from the wall is the 15cd lamp.

Again, there is no variable for distance in the equation: P=4*pi*I.

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willem2
d = v/t is wrong. It should be d = vt. Where did you get 343? you should be using the speed of light

jefgreen
For #1: d=vt.

d=(3x10^8m/s)(__t__)
What value would I use for t? I've tried everything...

jefgreen

phyzzydud
for #1, after you do the d = v * t using the speed of light, don't forget to halve the distance, since that's round trip.

jefgreen
What would my value for t be?

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If the signal takes 0.003 seconds for the round trip, how long does it take to go up and how long does it take to come back down? jefgreen
half of 0.003?

jefgreen
The answer I have is: 450,000 or 4.5x10^5m

But, my instructor tells me the answer is 4.5x10^8 jefgreen
And is my answer for #2 correct?

Homework Helper
The answer I have is: 450,000 or 4.5x10^5m

But, my instructor tells me the answer is 4.5x10^8 It's useful to have a sense of scale. Earth is 40 000 km in circumference. Low Earth orbit (where the Space Shuttle orbits) is around 300 km from the ground. Geosynchronous orbit, the highest useful orbit, is 36 000 km from Earth's center.

jefgreen

Homework Helper
Find the illumination 8.0m below a 405lm lamp. The answer I received was: .504lx. Is this answer correct?
Yes.

Two lamps illuminate a screen equally. The first lamp has an intensity of 12.5cd and is 3.0m from the screen. The second lamp is 9.0m from the screen. What is its intensity?

I know I need to use P=4*pi*(I), but there is no variable included for distance (d). Another formula given is: Eill=P/(4*pi*d^2). How should I go about solving this problem now? Lastly, I am cognizant that 1lm=1cd.

First, a lumen is not equal to a candela. A candela measures power per steradian, whereas a lumen measures total power.

Are you aware of the formula I=x/r^2, where x is power/steradian?

jefgreen
No, I'm not familiar with the latter formula. Is my answer to the Cape Canaveral question correct?

Homework Helper
Yes.

jefgreen
Could you help me with my NEW #1 and 2 questions? Those are the last in the problem set our Physics teacher gave us.

jefgreen
Yes.

Thank you so much .

jefgreen