HardPhysicsHigh School (9–10)Optics › Lenses and Optical InstrumentsNumericalBoard Exam

A convex lens forms a real image that is 4 times as large as the object.

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

A convex lens forms a real image that is $4$ times as large as the object. The object is $15\ \text{cm}$ from the lens. Find the image distance and the focal length.

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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