Light travels in straight lines until something bends it

Word	What it means
Ray	A straight line with an arrow, showing the path light takes.
Transparent / translucent / opaque	Lets light straight through / lets light through but scatters it, like frosted glass / lets no light through.
Visible spectrum	The colours that make up white light: red, orange, yellow, green, blue and violet.
Reflection / refraction	Light bouncing off a surface / light bending as it passes at an angle from one material into another.
Normal	A line drawn at 90° to a surface, at the point where a ray meets it.
Angle of incidence / angle of reflection	The angle between the incoming ray and the normal / between the reflected ray and the normal.
Plane / concave / convex mirror	Flat / curved inward, like the inside of a spoon / curved outward, like the back of a spoon.
Convex / concave lens	Thicker in the middle; bends rays toward each other / thinner in the middle; spreads rays apart.
Focal point (F) / focal length	Where rays arriving parallel to a convex lens's axis, the line through its centre, meet / that point's distance from the lens.
Real / virtual image	Formed where rays really meet, so it can be caught on a screen / only appears to be there, like your reflection.
Retina / optic nerve	The layer at the back of the eye that detects light / the nerve carrying its signals to the brain.
Pixel	One tiny square of a digital image, with its own colour and brightness.

An old puzzle. Some ancient Greek thinkers believed the eye sends out rays that touch what it sees. About a thousand years ago, Ibn al-Haytham, working in Cairo, argued the reverse, with experiments to back it: light comes from a source, bounces off objects and enters the eye.

Straight lines. A flashlight beam in dusty air is a straight shaft, and a shadow has the shape of the object because light travels straight past its edges. So drawing light as rays predicts shadows, reflections and images.

Worked: how fast? Light crosses space at about 300 000 km every second. The Sun is about 150 million km away, so its light takes 150 000 000 ÷ 300 000 = 500 seconds, or 8 minutes 20 seconds, to reach us. You see the Sun as it was eight minutes ago.

What materials do with light. Clear glass is transparent, wax paper translucent, wood opaque. A red apple reflects red light and absorbs the other colours. A prism fans white light out into the visible spectrum because each colour bends by a slightly different amount; in the 1660s Isaac Newton showed that white light already contains the colours. Raindrops do the same to sunlight and make a rainbow.

angle of incidence = angle of reflection

Worked: measure from the normal. A ray strikes a flat mirror at 35° to the mirror's surface. The angle of incidence is not 35°. It is measured from the normal, so it is 90 − 35 = 55°. The ray reflects at 55° on the other side of the normal, and the angle between the incoming and outgoing rays is 55 + 55 = 110°.

Smooth and rough. A mirror is smooth, so rays that arrive parallel leave parallel and you see an image. Paper is rough up close, so each tiny bit reflects at its own angle and the light scatters. Both obey the same law.

A plane mirror makes a virtual image the same size as the object, as far behind the mirror as the object is in front, and reversed left to right. That is why AMBULANCE is printed backwards on an ambulance's hood: in the mirror of the car ahead, it reads the right way round.

Mirror	What it does	Used in
Plane	A same-size image, reversed left to right	Bathroom mirrors, periscopes
Concave	Brings reflected rays together; close up, a magnified image	Headlight reflectors, makeup mirrors, large telescopes
Convex	Spreads reflected rays apart; a smaller image of a wider view	Store security mirrors, a car's passenger-side mirror

Why light bends. Light travels about three-quarters as fast in water as in air, and slower still in glass. A ray entering water at an angle slows and bends toward the normal; coming out, it speeds up and bends away. That is why a straw looks broken at the surface of a drink. A ray arriving along the normal slows but does not bend.

Lenses. A convex lens bends rays together, meeting at the focal point; a concave lens spreads them apart. Hold a convex lens between a window and a sheet of paper, and at the right distance a sharp, upside-down picture of the window appears on the paper: a real image. The right half of the figure above shows why: rays from the top of the object cross through the lens and meet below the axis. Held closer to an object than its focal length, the same lens is a magnifying glass.

Worked: moving the object. A class uses a lens with a focal length of 10 cm and a candle flame 3 cm tall, and records where a sharp image forms on a screen:

Flame to lens (cm)	Lens to image (cm)	Image height (cm)
40	about 13	1
30	15	1.5
20	20	3, the same size
15	30	6
12	60	15
10 or less	no real image	—

Move the object closer and the image moves farther away and grows; at twice the focal length it is the same size. Every one is upside down. At the focal length or closer, the rays never meet, so there is no real image. A camera works like the top rows, making a small image of a distant scene; a projector works like the bottom rows.

Part of the eye	What it does	In a camera
Cornea and lens	Form a real image; the cornea does most of the bending, and the lens changes shape to focus	The lens, which focuses by moving
Iris and pupil	The iris, a ring of muscle, widens or narrows the pupil to control the light let in	The aperture
Retina and optic nerve	Turn the image into nerve signals and carry them to the brain	The image sensor

The image on your retina is upside down, like any real image from a convex lens; the brain makes sense of it.

Glasses. A nearsighted eye bends light too strongly, often because the eyeball is slightly too long, so the image of a distant thing forms in front of the retina. A concave lens spreads the rays a little first. A farsighted eye bends too weakly, and a convex lens adds the missing bending. Laser eye surgery reshapes the cornea instead.

Worked: which lens? A student reads her textbook easily but not the board. Far things are blurred, so she is nearsighted and needs concave lenses.

Seeing smaller and farther. A microscope uses two convex lenses: the objective makes a magnified real image, and the eyepiece magnifies it again. In 1610 Galileo used a telescope with two lenses to discover four moons orbiting Jupiter. Most large telescopes now gather light with a concave mirror, which can be made far larger than a lens.

Digital images. A camera's sensor is a grid of millions of light detectors, and each becomes one pixel. A 12-megapixel sensor has 12 million, for example 4000 across by 3000 down. Stored as numbers, the picture can be copied and sent without losing detail.

• Measuring the angle from the mirror. Measure from the normal.
• Saying light bends because it hits a surface. It bends because it changes speed; along the normal it slows but does not bend.
• Swapping convex and concave lenses. Convex, thicker in the middle, brings rays together.
• Expecting a real image with the object inside the focal length. The rays never meet.
• Getting glasses backwards. Nearsighted: concave. Farsighted: convex.
• Calling a mirror image real. It cannot be caught on a screen, so it is virtual.