1Refraction
Light slows down when it goes from air into glass. If it hits at an angle, it bends towards the normal: this is refraction. Leaving, it speeds up and bends away.
refraction
Lesson 24.3 notes
Refraction, lenses and the eye
Light bending as it changes speed, lenses that focus, the eye and the energy light carries.
2Why it bends
Think of light as waves. The side of each wave that reaches the glass first slows down first, so the waves swing round and change direction.
3Pools look shallow
Light from the bottom of a pool bends away from the normal as it leaves the water. Your brain traces the rays back in straight lines, so the bottom looks nearer.
4Convex lenses
A convex lens is fatter in the middle. It refracts parallel rays so they meet at the focal point. A fatter lens bends light more, so its focal point is closer.
convex lensfocal point
5Inside the eye
Light enters through the pupil, a hole the iris can widen or narrow. The cornea and lens focus it onto the retina, which sends signals along the optic nerve to the brain.
corneairispupillensretinaoptic nerve
6Eye and camera
A camera works like an eye. Its lens focuses an upside-down image onto a light-sensitive sensor (or old-style film), just as your eye focuses one onto the retina.
7Light carries energy
Light transfers energy from a source to whatever absorbs it. In a solar cell it makes electricity. In photographic film it causes a chemical change. In your retina, a chemical change makes electrical signals.
Explore: Ray lab
Aim a ray into glass or water and see how much it bends, then focus light with a convex lens. Drag the torch or use the sliders.
Common mistake: Light only bends at a boundary
A ray bends where it crosses into a new material, then travels in a straight line again. A ray that hits the boundary along the normal slows down but doesn't bend.
Remember
- Light refracts (bends) when it changes speed at a boundary: towards the normal as it slows down entering glass or water.
- A convex lens brings parallel rays together at its focal point. A fatter lens has a shorter focal length.
- In the eye, the cornea and lens focus light onto the retina, whose cells turn it into electrical signals for the brain.
Going further at GCSE