The Focal Length of a Lens An object is located 27.0cm from a certain lens. The

Question

The Focal Length of a Lens An object is located 27.0cm from a certain lens. The lens forms a real image that is twice as high as the object. Part A What is the focal length of this lens? Part B Now replace the lens used in Part A with another lens. The new lens is a diverging lens whose focal points are at the same distance from the lens as the focal points of the first lens. If the object is 5.00cm high, what is the height of the image formed by the new lens? The object is still located 27.0cm from the lens.

Explanation / Answer

for diverging lens

f = -18 cm                     focal length for diverging lens is negative

do = object distance = 27 cm

di = image distance = ?

Using lens equation:

1 / di + 1 / do = 1 / f

1 / di + 1 / 27 = - 1 / 18

1 / di = - 1 / 27 - 1 / 18 = - 45 / 486

di = -10.8 cm

Using the magnification equation ::

hi / ho = - di / do

hi / 5 = - (-10.8) / 27

hi = 2 cm

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