(a) With the help of a labelled diagram explain how a convex lens converges a beam of parallel light rays. Mark the principal axis, optical centre, principal focus and focal length of the convex lens on the diagram.
(b) State whether the convex lens has a real focus or a virtual focus. (c) List some things that convex lens and concave mirror have in common.

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Solution

(a)

(b) The convex lens has a real focus because all the light rays pass through the focus.

(c) The equations we used for mirrors all work for lenses. A convex lens acts a lot like a concave mirror. Both converge parallel rays to a focal point, have positive focal lengths, and form images with similar characteristics. A concave lens acts a lot like a convex mirror.

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F

and

2 F

.
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0.5 m

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