I know this is a simple question, however, I have never been taught the practical side to Physics properly so here we go.

ray diagrams

I know I can use the lens equation to find the image distance, but if I wanted to find the image distance experimentally, what do I do?

Real images can be projected onto screens, but how do you do this? Should I move the screen back and forth until a clear image of the object forms on the screen? This seems logical but would this just find the focal length as that is where the image is clearest, or is that just for distant objects? I have included a diagram of the sort of experiment I have in mind, though I am looking for a general method rather than a solution to the two specific scenarios given.

  • 1
    $\begingroup$ The answer always depends on exactly what you are testing, but in photography, we do exactly what you say: we move the screen back and forth until the image pops into focus (well, more precicely, we move elements of our compound lens until the image focuses exactly on our fixed screen, but its a similar idea). We also have tools to aid this, such as test cards that make it easy to perceive how in focus the image is. $\endgroup$
    – Cort Ammon
    Jun 21, 2017 at 19:46

1 Answer 1


Right, you just find where the image is sharpest. I'll often image the florescent light in my lab onto a piece of paper on the floor to confirm the focal length of a lens. This is for something like a 50 mm lens, and I assume the distance from the lens to the source is infinite.. so the lens to image distance is a measure of the focal length.

  • $\begingroup$ You might mention the x10 rule of thumb for finding infinity. $\endgroup$ Jun 21, 2017 at 19:49
  • $\begingroup$ Sorry, I probably didn't make myself clear. I am trying to put the image on a screen, not finding the focal length. $\endgroup$
    – Inquirer
    Jun 21, 2017 at 19:54
  • $\begingroup$ @WhatRoughBeast, sorry I don't know the x10 rule, care to share? $\endgroup$ Jun 21, 2017 at 19:57
  • $\begingroup$ @HappyHippo Well you can move the screen, the source or the lens, and just find where the image is sharpest. $\endgroup$ Jun 21, 2017 at 19:59
  • $\begingroup$ @GeorgeHerold - If the source/lens distance is 10 times greater than the image/lens distance, you can (as a rough measure) treat the source distance as infinite. Just as, for instance, at more than 10 times its diameter you can generally treat an object as a point. (For analyses like gravitation, unresolved optical phenomena, etc.) $\endgroup$ Jun 21, 2017 at 20:01

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