Question

One way to roughly measure the focal distance of an unknown converging lens is to use it to focus a far-away object. for example, you can focus an image of the overhead light fixture onto the floor. then the distance from the lens to the floor is approximately the focal length.

a. draw a ray diagram for this situation and discuss why it is a reasonable way to approximate the focal length.   

b. Do you expect this method to systematically over-estimate or under-estimate the focal length? c. why does this method not work for a diverging lens?

Answer 1

a) THe image from far away object(infinity in idieal case) converges into focus of convex lens as shown in diagram below

b) Since object is very far away so ideally image should be at focus but in real life image is formed somewhere right side(w.r.t. diagram above) of focus as infinity in real life is hard to be defined. So one can say that image formed is under estimated

c) This is because concave (diverging) lens forms virtual image and hence cant be taken in screen therefore making it difficult to measure it's focal length