Rays and beams

Imaging with a lens

Rays from each point of an object spread out, pass a lens, and meet again at one point of the image. With the object 200 mm in front of a 100 mm lens the image forms 200 mm behind it, the same size and upside down.

What you are looking at

Rays fan out from the arrow’s tip and foot. Behind the lens they converge where the screen sits.

How the bench is set up

An arrow object 20 mm tall, a thin lens of focal length 100 mm, a screen 200 mm behind the lens.

The theory

For a thin lens with object distance $u$ and image distance $v$ ,

\frac{1}{f} = \frac{1}{u} + \frac{1}{v}, \qquad m = -\frac{v}{u}.

What to measure

1/f = 1/u + 1/v; magnification −v/u

Drag the object closer to the lens and find where the rays meet now.
Check the image distance against the lens equation.

What the model leaves out

The lens is ideal: thin, with no aberrations. Swap in a thick singlet to see spherical aberration.

Things to try

Put the object inside the focal length: the rays diverge and the image is virtual.
Use two lenses to make a microscope.

References

E. Hecht (2017), Optics, 5th edition, Pearson.

More experiments, and how the lab works, are in the laser lab’s overview.