Light, but bitesize.

Part 1Light travels in straight lines

Switch on a torch in a dusty room and you can see the beam: it goes in a perfectly straight line. Light never bends round corners on its own. That's why you can't see round a wall, and why a shadow has sharp edges.

Because light travels in straight lines, we draw it as a ray — a straight line with an arrow showing the direction the light is going. Rays are the basic tool you'll use to explain almost everything about light.

Light is also astonishingly fast. It travels at about 300 000 000 metres every second (300 million m/s) — far faster than sound. That's why you see lightning before you hear the thunder, even though they happen at the same moment: the light reaches you almost instantly, but the sound takes a few seconds to catch up.

Part 2How we see things

To see anything, light has to enter your eyes. There are two ways that happens.

Luminous objects make their own light. The Sun, a lit candle, a glowing bulb, a phone screen — light travels straight from them into your eyes. You see them directly.

Non-luminous objects don't make their own light. A book, the Moon, a tree, this page — they make no light of their own. You only see them because light from a luminous source bounces off them (reflects) and travels into your eyes. Turn off every light source and a non-luminous object disappears, because there's nothing for it to reflect.

Part 1The law of reflection

When light hits a mirror it bounces off. This bouncing is called reflection. The ray going in is the incident ray; the ray coming back off is the reflected ray.

To measure the angles properly, we draw one extra line: the normal. This is a dashed line drawn at exactly 90° to the mirror at the point where the light hits. We always measure angles from the normal, never from the mirror's surface.

Now the rule itself, the law of reflection:

angle of incidence = angle of reflection

So if light hits the mirror at 30° to the normal, it bounces off at 30° to the normal on the other side. The two angles are always equal.

Part 2Smooth vs rough — why mirrors give an image

The law of reflection is true for every surface, even a wall. So why do you get a clear reflection in a mirror but not in a brick wall?

Specular reflection happens on a smooth, shiny surface like a mirror or still water. All the rays land on a flat surface, so they all bounce off in the same neat direction. The reflected rays stay organised, and you see a clear image.

Diffuse reflection happens on a rough surface like paper or a wall. Each ray still obeys the law of reflection, but because the surface is bumpy, the normals point in all different directions, so the rays scatter everywhere. No clear image forms — which is exactly why you can read a page from any angle but can't see your face in it.

Part 1Light bends when it changes speed

Light travels at slightly different speeds in different materials. It's fastest in air, slower in water, and slower still in glass. When a ray passes from one material into another — say from air into glass — it changes speed, and this makes it change direction. This bending is called refraction.

A good way to picture it: imagine a trolley rolling at an angle off a smooth path onto thick grass. One wheel hits the grass first and slows down while the other is still on the path, so the trolley swings round. Light does the same when one edge of the beam slows before the other.

The rule for which way it bends, measured from the normal:

· Going from air into glass or water (speeding down), light bends towards the normal.
· Going from glass or water out into air (speeding up), light bends away from the normal.

Part 2Refraction in everyday life

You see refraction all the time without realising it:

· A straw in a glass of water looks bent or broken at the surface. The light from the underwater part of the straw refracts as it leaves the water, so it reaches your eyes from a slightly different direction — and your brain sees the straw as bent.

· A swimming pool looks shallower than it really is. Light from the bottom bends as it leaves the water, so the bottom appears to be raised up. Pools are always deeper than they look — which is why "it looked shallow" is a dangerous reason to dive.

· A coin at the bottom of a mug can seem to reappear when you pour water in, because the refracted light now reaches your eye over the rim.

Part 1Lenses focus light

A lens is a curved piece of glass or plastic that bends light by refraction. A converging lens is fatter in the middle. When parallel rays of light pass through it, they all bend inwards and meet at a single point called the focus. This is how a magnifying glass can concentrate sunlight to a tiny bright spot.

Focusing light onto one point is exactly what's needed to make a sharp image — and it's the job your eye does every time you look at something.

Part 2The parts of the eye

Your eye is a clever light-focusing instrument. Light goes through several parts before you "see":

· Cornea — the clear front layer that does most of the focusing as light enters.
· Pupil — the black hole that lets light into the eye.
· Iris — the coloured ring of muscle that changes the size of the pupil to control how much light gets in (small in bright light, wide in the dark).
· Lens — fine-tunes the focusing so objects near and far look sharp.
· Retina — the light-sensitive layer at the back where the image is formed.
· Optic nerve — carries the signal from the retina to the brain.

Part 3The pinhole camera

A pinhole camera is the simplest camera there is: a sealed box with a tiny hole at the front and a screen at the back. No lens at all.

Because light travels in straight lines, rays from the top of an object travel through the pinhole and land near the bottom of the screen, and rays from the bottom land near the top. The rays cross over at the hole. The result is a real image that is upside down (inverted) — and left and right are swapped too.

Interestingly, your eye does the same thing: the image on your retina is upside down. Your brain simply flips it the right way up, so the world looks normal.

Part 1White light is a mixture

White light, like sunlight, doesn't look like it has any colour in it — but it does. It's actually a mixture of all the colours mixed together. You can prove this by splitting it apart with a triangular block of glass called a prism.

As white light enters and leaves the prism it refracts (bends). Each colour bends by a slightly different amount — violet bends the most, red the least — so they spread out into a band of separate colours. This spreading-out is called dispersion, and the band of colours is the spectrum.

The colours of the spectrum, in order, are red, orange, yellow, green, blue, indigo, violet — remembered as ROYGBIV. A rainbow is the same thing made by raindrops acting like tiny prisms.

Part 2Why objects look coloured

Now the key idea: a coloured object does not make its own colour. When white light lands on it, the object reflects some colours and absorbs the rest. The colour you see is the colour it reflects into your eyes.

· A red jumper reflects red light and absorbs all the other colours.
· A green leaf reflects green light and absorbs the rest.
· A white object reflects all the colours equally.
· A black object absorbs all the colours and reflects almost none — which is why black objects warm up fastest in the Sun.

Part 3Colour filters

A colour filter is a piece of transparent coloured plastic or glass. It works in the same way as a coloured object: it lets its own colour through and absorbs the others.

A red filter lets red light pass through and absorbs all the other colours. So if you shine white light through a red filter, only red comes out the other side. A green filter only lets green through, and so on.