Lens Lab

Slide a candle towards a lens and watch where its image appears. Is it upside down? Is it bigger? Can you catch it on a screen?

Lens
Image distance (v)
16.7 cm
Image is
Real, inverted
Magnification
×0.67
Image height
2.7 cm

The maths, live

Lens formula 1/f = 1/u + 1/v1/10.0 = 1/25.0 + 1/v → v = 16.7 cm
Magnification m = v ÷ um = 16.7 ÷ 25.0 = 0.67
Real is positivef < 0 for a diverging lens · v < 0 means a virtual image

Start far away and slide the candle towards the lens. Where is the image the same size? What happens once you pass F?

How it works

A converging (convex) lens bends rays of light towards each other. Rays coming in parallel to the axis all pass through one point, the focal point F. Its distance from the lens is the focal length, f. Three easy rays (parallel, through the centre and through F) show where the image forms.

The lens formula links the object distance u, the image distance v and the focal length: 1/f = 1/u + 1/v. Here we use the 'real is positive' rule, so a virtual image has a negative v. The magnification m = v ÷ u tells you how many times bigger the image is. Inside the focal length the rays never meet, but they seem to come from a bigger, upright, virtual image behind the lens. That is a magnifying glass.

Try this

  1. Find where the candle must be for the image to be the same size as the candle.
  2. Move the candle inside F. What kind of image do you get now?
  3. Switch to a diverging lens. Can you ever make a real image with it?