Reflection

Reflection happens when light hits a surface and bounces back. Reflection is what lets you see yourself in a mirror, watch the moon shimmer on a pond and notice your reflection in a shop window. Almost everything you see is visible because light has reflected off it into your eyes. Even objects that look "dull" reflect light, just not as cleanly as a polished mirror. The behaviour of reflected light follows a simple, exact rule called the law of reflection.

• What it isLight bouncing off a surfaceReturns into the same medium
• Law of reflectionAngle in = angle outMeasured from the surface normal
• Specular reflectionSmooth surfaceMirror-like, clean reflection
• Diffuse reflectionRough surfaceLight scattered every direction
• Best mirrorsHighly polished silverReflect 95%+ of light
• ExamplesMirrors, water, shiny metalPlus you (slightly)

The law of reflection

The law of reflection is one of the simplest laws in physics: the angle of incidence equals the angle of reflection.

• Angle of incidence: the angle between the incoming light ray and an imaginary line at right angles to the surface (called the normal).
• Angle of reflection: the angle between the outgoing reflected light ray and the same normal line.

Both angles are measured from the normal, not from the surface itself. Light hitting a mirror straight on (zero angle of incidence) bounces straight back (zero angle of reflection). Light hitting at 30 degrees from the normal bounces off at 30 degrees on the other side.

Two kinds of reflection

• Specular reflection: from a smooth, polished surface. All the light rays bounce off in parallel, producing a clear mirror image. Examples: a flat mirror, a still pond, polished metal, glass.
• Diffuse reflection: from a rough surface. Each tiny bump on the surface points in a slightly different direction, scattering the reflected light in many directions. You see the surface clearly but no mirror image. Examples: a wall, a piece of paper, the page of a book, almost everything in your room.

Without diffuse reflection, you could not see anything except light sources and their direct reflections. Your bedroom would be pitch black except where a lamp pointed. Diffuse reflection from every wall, every object and every surface is what lets you see your whole room from any angle.

Fact A mirror does NOT really flip your image left-right, even though it looks that way. It flips it front to back. Your right hand still looks like your right hand when you wave it. But because you have rotated mentally to face your reflection (instead of standing back-to-back with it), the result is that words appear reversed and the figure feels mirrored sideways. It is a very neat optical and psychological trick.

Why mirrors work

A typical bathroom mirror is a sheet of glass with a thin coating of metal (usually aluminium or silver) on the back, protected by a layer of paint. Light passes through the glass, hits the metal coating, and bounces back almost perfectly. The reason metals reflect so well is that their free electrons can quickly rearrange to absorb and re-emit the light without much loss.

The flat surface gives a perfect specular reflection: each light ray bounces at the exact angle predicted by the law of reflection. The result is a clear, sharp image of whatever the mirror is facing.

Curved mirrors

Bend a mirror into a curve and the image changes:

• Concave mirror (curved inward, like a spoons inside): focuses light to a point. Used in torches, headlights, telescopes and shaving mirrors. Up close, gives a magnified image.
• Convex mirror (curved outward, like a spoons outside): spreads light outward. Gives a smaller, wider-angle view. Used in side mirrors on cars, security mirrors in shops and the bulges at blind road junctions.

What you can do with reflection

• See yourself: bathroom mirrors.
• See behind you: car rear-view mirrors and side mirrors.
• Make small spaces look bigger: mirrored walls in restaurants and gyms.
• Direct or focus light: torches use a curved mirror behind the bulb to send light forward in a beam.
• See in tight spaces: dentists use small angled mirrors to look at the backs of teeth.
• Take photos: digital SLR cameras have an internal mirror that reflects the image into the viewfinder.
• Study the stars: reflecting telescopes use giant concave mirrors to gather and focus light from distant galaxies.
• Lasers: built around carefully positioned mirrors that bounce light back and forth thousands of times.

Did you know? The mirror in NASAs James Webb Space Telescope (launched in 2021) is made of 18 hexagonal segments arranged into a giant 6.5-metre-diameter mirror. The mirror is gold-coated to maximise its reflection of infrared light. It is so smoothly polished that if the mirror were scaled up to the size of the United Kingdom, the biggest bumps would be just a few centimetres tall. The mirror gathers faint light from galaxies billions of light-years away.

The angle trick

The law of reflection is precise enough to play tricks with light. Here are a few classic examples:

• Periscopes: two mirrors at 45 degrees in a tube let you see over a wall (or out of a submarine).
• Whispering galleries: domes shaped so that sound (or light) emitted at one point reflects to focus at another point.
• Kaleidoscopes: three mirrors at 60 degrees create endlessly repeating symmetric patterns.
• Retroreflectors: a special arrangement of three mirrors at right angles bounces any incoming light back exactly the way it came. Used in road signs (catseyes), bike reflectors and on the Moon (left by Apollo astronauts so we can bounce lasers off them and measure the Earth-Moon distance precisely).

Try this Place a small mirror flat on a table and put a coin on top. Look down at the coin from different angles. Notice how the coin and its reflection always appear to be the same distance below and above the mirrors surface. Now stand a second mirror at 90 degrees to the first (in an L shape) and place a small object in the corner. You will see TWO reflections, one from each mirror, plus a third reflection where the two mirrors bounce light to each other.

Deeper dive: how reflection lets us bounce lasers off the Moon

One of the most remarkable scientific instruments ever set up by humans is sitting on the surface of the Moon: an array of retroreflectors left by the Apollo astronauts in the 1960s and 70s.

A retroreflector is a clever arrangement of three flat mirrors set at right angles to each other (a "corner cube"). Any beam of light entering the corner bounces off all three mirrors and comes out exactly parallel to the way it went in, just shifted slightly to the side. This means the reflected beam goes back to the source no matter what angle the original beam came in at.

The Apollo missions left several retroreflector arrays on the lunar surface, the largest containing 300 individual corner cubes. The Soviet Luna 17 and Luna 21 rovers also left smaller arrays.

Today, ground-based observatories on Earth fire short pulses of laser light at these arrays. The reflectors bounce a tiny fraction of the light back to Earth. By measuring how long the round trip takes (about 2.5 seconds), scientists can calculate the distance to the Moon to within a few millimetres.

This precision lunar laser ranging has revealed many things:

• The Moon is slowly moving away from Earth at about 3.8 cm per year (because of tidal friction).
• The Moons orbit is mildly distorted by the gravity of Earths bulging equator.
• Einsteins theory of gravity is correct to within tiny tolerances.
• The Moon may have a small liquid core, based on subtle wobbles in its motion.

Five decades after the last astronauts left the Moon, we can still bounce light off the mirrors they left behind. All thanks to a simple law: angle of incidence equals angle of reflection, applied three times in a corner-cube prism.

For more, see mirrors and refraction.