Upright and much smaller, on the same side as the object, seen by looking through the lens. Like the view through a peephole in a door.
- Object distance
- 26 cm
- Image distance
- 7.2 cm in front
- Size
- 0.28 × upright
What’s happening
A concave lens is thinner in the middle than at its edges, so it does the opposite of a magnifying glass: it spreads light out. After the lens, the light from the object’s tip never comes together again. Look through the lens and your eye follows that spreading light backwards to where it seems to start: a small, upright image on the object’s own side. Move the object wherever you like; the image only grows or shrinks a little, and it never goes further from the lens than the focus F.
Try this
- Guess first: if the object were a mountain far away, where would its image be? Then drag the object as far as it goes.
- Push the object right up against the lens. Is the image now as big as the object?
- Make the lens more strongly curved (a shorter focal length). Does the image shrink or grow?
- Put the object at the same spot in the Convex Lens demo. How are the two images different?
- At home: look through the glasses of someone who is short-sighted. Do things look bigger or smaller?
Learn more: the three rays, negative focal lengths, and glasses for short sight
The same three rays find the image, but after the lens they spread apart, so the image is where they seem to come from:
- Parallel ray: comes in parallel to the axis, and leaves as if it came from the near focus.
- Centre ray: goes straight through the middle of the lens without bending.
- Focal ray: heads for the far focus, and leaves parallel to the axis.
The same lens equation as for a convex lens works here:
but a concave lens’s focal length counts as negative. Then the image distance always comes out negative too: the image is on the object’s side, a virtual one. And its size is always between 0 and 1, upright and smaller:
Eye doctors measure a lens’s strength in diopters: one divided by the focal length in meters. A concave lens has a minus number: a −2 diopter lens has a focal length of −50 cm.
- Glasses for short sight: a short-sighted eye focuses far things in front of the back of the eye. A concave lens spreads the light a little first, so it focuses in the right place.
- Door peepholes: a concave lens shrinks the whole view outside to fit through a tiny hole.
- Opera glasses: a convex lens at the front and a concave one at the eye, as in Galileo’s first telescope, give an upright, magnified view.