.05 cm far must the lens be moved to change the focus of the camera from a person 25 m away to one that is 4.0 m away
The focal length of a thin lens in air is the separation between its major foci, also known as focal points, and its center. The focal length is the distance at which a beam of collimated light will be concentrated to a single spot for a converging lens (for instance, a convex lens).
If v is the image distance in the first scenario
1 / v - 1 / -25 = 1 / .05
1 / v = 1 / .05 - 1 / 25
= 20 - .04 = 19.96
v = .0501 m = 5.01 cm
The second instance
u = 4 ,
1 / v - 1 / - 4 = 1 / .05
1 / v = 20 - 1 / 4 = 19.75
v = .0506 = 5.06 cm
Therefore, the lens must be advanced by 5.06 - 5.01 =.05 cm.
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