# A couple question on illumination.

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.

## Answers and Replies

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rl.bhat
Homework Helper
Illumination on a screen is inversely proportional to the square of the distance and directly proportional to the luminous intensity of the lamp.
So L1/d1^2 = L2/d2^2.
You can use this relation both the problems.

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Last edited:
#1: 110 cd
#2: 6.9 m
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My answers and work:
#1:

12.5 cd xxxxxx cd
-------- = --------
3.0m^2 9.0m^2

My answer: 112.5cd OR 113.0 cd.
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My answers and work:
#2:

45 cd 15 cd
------ = ------
12 m xxxx m

My Answer: 4.00 m
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As you can see, I'm just slightly off from what my instructor's answers were.

So, are my answers correct?

Bump. Am I correct?

Bump. All I need is clarification...

rl.bhat
Homework Helper
First answer is correct.
In the second problem
45/12^2 = 15/d^2.
Solve for d.

First answer is correct.
In the second problem
45/12^2 = 15/d^2.
Solve for d.
Thank you, sir.