Examples for lenses

Example #1


a.) A converging lens (concave) has a focal length of 14 cm. Looking through the lens, one sees an image 20 cm behind the lens. Where is the object?


Since the image is behind the lens, it is virtual and the distance di is negative. Using the formula, , one can solve for do.

do = 8.24 cm

b.) If the height of the object is 1.5 cm, what is the height of the image.


Using the formula, to get

hi = 3.64 cm, upright

Example #2


A real image of a coin is observed 34 cm beyond a lens. The image height is 1.4 cm and it is known that the actual coin is 0.7 cm high. What is the focal length of the lens?


First, find the object distance using . Remember that since the image is real, that the image height is negative. ( do = 17 cm ). One can then use to find the focal length.

do = 11.33 cm

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