Upside down and smaller, on the far side, where a screen could catch it. Like a camera or your eye, looking at something far away.
- Object distance
- 26 cm
- Image distance
- 16.3 cm behind the lens
- Size
- 0.6 × upside down
What’s happening
A convex lens is thicker in the middle. It bends light inwards, so rays from the tip of the object meet again on the far side and make an upside-down picture you could catch on a screen. Bring the object closer than the focus F and the rays spread apart instead: they never meet, but your eye traces them back to a bigger, upright image. That is how a magnifying glass works.
Try this
- Start far away, then slowly bring the object closer. When does the image get bigger than the object?
- Put the object exactly on 2F. How big is the image?
- Cross the focus F. What happens to the image?
- Make the lens fatter (a shorter focal length). Where does the image go?
- At home: put a drop of water on a clear plastic sheet lying on a newspaper. The drop is a tiny convex lens. What does it do to the letters?
Learn more: the three rays and the lens equation
Any two of these three rays are enough to find the image:
- Parallel ray: comes in parallel to the axis, and leaves through the far focus.
- Centre ray: goes straight through the middle of the lens without bending.
- Focal ray: goes through the near focus, and leaves parallel to the axis.
The lens equation links the three distances, where is the object distance, the image distance and the focal length:
The image is times as big as the object; a minus sign means upside down:
- Beyond 2F: smaller and upside down (a camera).
- At 2F: the same size, upside down (a photocopier at 1:1).
- Between F and 2F: bigger and upside down (a projector).
- At F: no image; the light leaves in a parallel beam (a spotlight).
- Inside F: bigger and upright, on the same side (a magnifying glass).