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so I was studying for my Yr11 Physics test tomorrow and I came across a question that I got the wrong answer on, all of my friends are getting the same answer.

Question: You wish to project the image of a lamp, magnified three times, onto a screen 5.0m from the lamp. How far from the lamp should the mirror be placed?

I though that we would just use $M=\frac{-d_i}{d_o}$ where $M=3$ and $d_i=5$, which will work out that $d_o\approx -1.7$ which has a magnitude of 1.7m from the lamp

However, our answers book says that the answer is 2.5m

What have I done wrong? Also, please keep in mind that I am in year 11 and may not understand anything beyond that level

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closed as off-topic by John Rennie, jinawee, tpg2114, Qmechanic Dec 1 '13 at 13:49

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  • $\begingroup$ How far from the lamp should it be placed? What is the "It" $\endgroup$ – DJohnM Dec 1 '13 at 7:55
  • $\begingroup$ There was a question before it saying What kind of spherical mirror is needed? which I know is concave, so the "it" is a concave mirror $\endgroup$ – VikeStep Dec 1 '13 at 7:56
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Your approach is correct, but you really need to draw even a crude sketch.

The lamp is $x$ m from the mirror; the image is $5$ m from the lamp, which puts it $(5+x)$ from the mirror. These are the d values for your magnification formula, which is correct...

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