Question

Suppose the focused image of a car passing by (100 m from your lens) crosses your 8.0 cm wide screen in 2.0 seconds. Your lens has a focal length of 20 cm. Making any reasonable assumptions necessary (and explaining them), find the speed of the car in m/s. If the car is driving eastward, in what direction does the image move?

Answer 1

The lens equation is

where d_o is the object distance, d_i is the image distance and f is focal length

Magnification (m) is given by

negative means the image is inverted.

So as the image is reduced by a factor of 499 the speed is also reduced by the same factor. Therefore speed of the car is given by

In a lens any movement of the object is shown in an opposite way(clear from the negative sign). Therefore if the car is driving eastward, the image will move westward.