Lenses bend light. Bring rays together, catch a picture on a screen, make a magnifier, then build a camera, a projector and a telescope!
☀️ Never look at the Sun through a lens, a telescope or binoculars. They focus sunlight and can hurt your eyes in moments.
LENS FACTS
Convex lenses (fat in the middle) bring light rays together. Parallel rays meet at the focal point. The distance from the lens to that point is the focal length, f.
Concave lenses (thin in the middle) spread light rays out, as if they came from a focal point on the near side.
The lens rule: 1/f = 1/(object distance) + 1/(image distance).
A real image forms where the rays really meet. You can catch it on a screen, and it is upside down.
Put an object closer than the focal length and you get a virtual image: upright and bigger, but you can't catch it on a screen. That is a magnifying glass.
Lens power is measured in dioptres: 1 divided by the focal length in metres. A lens with f = 25 cm is +4 D. Concave lenses have minus powers.
Glasses for short sight (myopia) use concave, minus lenses. Glasses for long sight and reading glasses use convex, plus lenses.
Your eye has lenses too: the clear cornea and the lens inside add up to about +60 D. They make an upside-down picture on the retina, and your brain turns it the right way up.
A simple telescope has two convex lenses, fo + fe apart. It magnifies fo ÷ fe times.
☀️ Never look at the Sun through a lens, a telescope or binoculars.