An image of a flower is produced on a screen by a lens 1.60 m from the screen. T

Question

An image of a flower is produced on a screen by a lens 1.60 m from the screen. The image of the flower has a linear magnification of 2.5. What is the focal length of the lens? (JC 22. 37) A converging lens is held near a flashlight images the bulb on a wall 1.60 m from the lens. With the flashlight fixed the lens is moved closer to the wall and a sharp image is again formed when the lens is 15 cm from the wall. (a) What is the focal length of the? lens? (b) How far is the flashlight bulb from the wall? (JC 22.41)

Explanation / Answer

7.

do = object distance

m = magnification = 2.5

di = image distance = - 1.60

f = focal length

using the magnification equation

m = - di/do

2.5 = - (-1.60)/do

do = 0.64 m

using the lens equation

1/f = 1/di + 1/do

1/f = 1/(- 1.60) + 1/0.64

f = 1.067 m

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