Question

To determine focal lengths of converging lenses we can focus an image of a distant object (ex: the trees outside) on a white card. Explain how this yields the focal length. Can this method be used for diverging lenses?

Answer 1

For distance object light comes from infinite to converging lens and focus on the focal point of lens. Now we can determine the distance of this point from lens which is known as focal length of converging lens. This point is in front of converging lens, hence can be observe.

This method is not helpful for diverging lens. Light coming from far object will diverge. It will not converge as in case of converging lens at focal point. But the diverge rays can be considered that they are coming from a point which is focal point. This point is behind the diverging lens, hence difficult to observe.

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