Lenses: How Convex and Concave Lenses Form Images

0:00How does a lens make a picture on a screen, or make things look bigger?

0:05A convex lens is thicker in the middle. The line through its centre is the principal axis.

0:12Rays parallel to the axis bend as they pass through the lens. This is called refraction.

0:19They meet at the focal point, F, so this is a converging lens.

0:25The distance from the lens centre to F is the focal length. There is an F on each side.

0:32Now place an object, drawn as an arrow, beyond F.

0:37A ray from its tip parallel to the axis bends through F.

0:42A ray through the centre of the lens goes straight on.

0:46Where the rays cross, an upside-down image forms.

0:50It is a real image: light really meets there, so a screen can catch it.

0:56Cameras and projectors use convex lenses this way.

1:00Now move the object closer to the lens than F.

1:04Now the rays spread apart after the lens and never meet.

1:08Traced back, they seem to come from a bigger, upright image: a virtual image.

1:15No screen can catch it. This is how a magnifying glass works.

1:20A concave lens is thinner in the middle than at its edges.

1:25It spreads parallel rays apart, as if from F, so it is a diverging lens.

1:31Glasses use both. Concave lenses correct nearsightedness, when far things look blurry.

1:38Convex lenses correct farsightedness, when near things look blurry.

1:44A convex lens brings light rays together; a concave lens spreads them apart.