In the Figure below a lens of focal length 7 cm is placed between a plane mirror
ID: 1450298 • Letter: I
Question
In the Figure below a lens of focal length 7 cm is placed between a plane mirror and a pin hole. The other side of the pin hole is illuminated by a light source (not shown), so that the light passes through the hole and is then collected by the lens. If the pin hole is at the focal point of the lens, then the light that passes through the lens will be parallel. When the parallel rays hit the plane mirror, the reflected rays will also be parallel, and when they pass through the lens again the rays will be focused at the pin hole. This is called the auto-collimation method for finding the focal length of a lens. For this part, the object is to find the location to place the lens and the pin hole to obtain auto-collimation.
Explanation / Answer
focal length= 7 cm. To be pinhole at the focul point, distance between the lens and the pinhole= 7
So, lens should be kept on the mark= 16-7 = 9 cm mark on the ruler