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A student holds a concave lens of focal length 20 cm, 30 cm from a candle.

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

A student holds a concave lens of focal length $20\ \text{cm}$ , $30\ \text{cm}$ from a candle. No screen shows an image, so she looks through the lens at the candle instead. Where is the image she sees, and how tall is it compared with the flame?

Need a reminder? The key ideaShow

For a lens, $\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$ and $m = \dfrac{v}{u}$ . The object distance $u$ is negative; a convex lens has a positive $f$ , a concave lens a negative $f$ . A real image has a positive $v$ and a negative $m$ .

The worked solution

Have you tried it?

Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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