Mirrors

A mirror is a polished surface that reflects almost all the light hitting it, forming a clear image of whatever is in front of it. Mirrors have been used by humans for thousands of years. The earliest were polished pieces of metal, mostly bronze or silver, or shiny black volcanic glass (obsidian). Modern mirrors are made by coating the back of a sheet of glass with a thin layer of aluminium or silver. Curved mirrors can magnify, shrink or focus light in useful ways. From your bathroom in the morning to the giant telescopes that watch distant galaxies, mirrors are some of the most useful optical devices ever invented.

  • What it isPolished surfaceReflects light cleanly
  • First madeAround 4000 BCPolished obsidian and metal
  • Modern mirrorGlass + aluminium coatingOn the back of glass
  • Flat mirrorSame-size imageBehind the mirror
  • ConcaveMagnifies or focusesUsed in torches
  • ConvexShrinks, wider viewUsed in car wing mirrors

Flat mirrors

The most familiar mirror is a flat sheet of glass with a reflective backing. It produces an image that:

  • Is the same size as the object.
  • Appears to be the same distance behind the mirror as the object is in front.
  • Is upright (not upside down).
  • Is laterally inverted (left-right swapped, so writing appears mirrored).

Every light ray from the object follows the law of reflection (angle of incidence equals angle of reflection), so the eye traces the rays back behind the mirror to form a perfect image.

Concave mirrors

A concave mirror curves inward, like the inside of a spoon. It focuses parallel light to a single point called the focal point.

What you see in a concave mirror depends on how close you are:

  • Close up (closer than the focal length): a magnified, upright image. Used in shaving and makeup mirrors.
  • Further away (beyond the focal length): an inverted, possibly smaller image. The bigger the distance, the smaller the image. Used in some telescopes.

Concave mirrors are also used in torches and headlights, with the bulb at the focal point so the reflector sends out a focused parallel beam.

Convex mirrors

A convex mirror bulges outward. It spreads light rays apart, giving a wider field of view but a smaller, upright image.

Convex mirrors are useful where you need to see a large area in a small mirror:

  • Car wing mirrors: the passenger-side wing mirror is often convex to widen the visible area behind the car. Hence the warning "objects in mirror are closer than they appear", because the image is smaller than reality.
  • Security mirrors in shops: domed convex mirrors mounted high in the corner of a shop let staff see the whole room.
  • Blind-spot mirrors: small convex mirrors stuck to the corners of car mirrors give a wider view.
  • Side mirrors on lorries and buses: often convex for similar reasons.
Fact The first known mirrors were made of polished obsidian (volcanic glass) in Turkey around 6000 BC. Metal mirrors came later, made of polished bronze, copper or silver. Glass mirrors with a metal backing did not become common until the 1500s, when Venice perfected the technique. For centuries Venetian mirrors were a luxury for the very rich. Mass production in the 1800s finally made mirrors cheap enough for ordinary homes.

How a mirror is made

A typical bathroom mirror has 4 layers:

  • Glass: protects the reflective layer.
  • Reflective metal coating: usually aluminium for cheap mirrors, or silver for higher-quality ones. Deposited as a very thin film.
  • Copper protective layer: a backing that protects the silver from corrosion.
  • Paint: a tough back paint to protect everything.

Modern manufacturing techniques can coat huge sheets of glass with extremely thin and even layers of metal, producing big flat mirrors at low cost.

Two-way mirrors

A "two-way mirror" (also called a one-way mirror) is a normal mirror with a much thinner reflective coating, so it reflects only about half of the light and lets the other half pass through. The trick is to set up much brighter light on the "mirror side" and much dimmer light on the "viewing side". The bright side sees a normal mirror; the dim side sees through it.

Used in police interview rooms, security observation areas, scientific labs and sometimes in TV game shows.

Telescope mirrors

Most large modern telescopes use curved mirrors instead of lenses to collect light from distant stars and galaxies. Big mirrors are much easier to make than big lenses, and a mirror only needs one optical surface (rather than two for a lens).

The largest single-mirror telescopes have mirrors over 8 metres across. Even bigger telescopes use multiple mirrors working together. The James Webb Space Telescope (2021) has a 6.5-metre mirror made of 18 hexagonal segments. The future Extremely Large Telescope (under construction in Chile, finishing around 2028) will have a 39-metre primary mirror made of nearly 800 hexagonal segments.

Did you know? Some animals show interesting behaviour around mirrors. Most cannot recognise themselves; they react to their reflection as if it were another animal of the same species, trying to attack or interact with it. A small group of species (humans over about 18 months, great apes, dolphins, elephants, magpies and a few others) pass the so-called "mirror test" of self-recognition. Trying to wipe a coloured mark off their own reflection. This is taken as a clue that they have some self-awareness.

Optical illusions with mirrors

Cleverly arranged mirrors can create dramatic illusions:

  • Infinity mirrors: two parallel mirrors facing each other produce an apparently endless tunnel of images.
  • Petes ghost (and other Victorian theatre tricks): a half-silvered mirror at an angle lets ghostly images appear and disappear by switching lights.
  • Kaleidoscope: three mirrors in a tube reflect a small piece of coloured glass into beautiful symmetric patterns.
  • Penn and Teller stage tricks and many modern magic illusions rely on cleverly hidden mirrors.
  • Mirror box therapy: a mirrored box that makes an injured arm look like it is healthy. Has helped some patients with phantom limb pain.
Try this Hold a small flat mirror at different angles in front of your face. Notice how at 90 degrees (mirror flat on, looking square into it) you see a normal mirror image. Tilt the mirror downward and the image of your face appears to tilt up. Now stand two mirrors at 90 degrees in an L shape and put a small object in the corner. You see THREE images: one in each mirror, plus a third where the two mirrors bounce light to each other. With this trick, you can see the back of your head by setting up two mirrors at the right angle.
Deeper dive: the Hubble Space Telescope and the most expensive mistake in optics

The Hubble Space Telescope, launched by NASA in 1990, is the most famous telescope ever built. For decades it has produced extraordinary images of distant galaxies, nebulae and stars. But its first months in orbit were a disaster.

Hubbles main mirror is 2.4 metres across, made of glass coated with a thin layer of aluminium and a protective coating. It was carefully polished on Earth for years before launch, to a precision of less than a millionth of a millimetre across its whole surface.

When Hubble was switched on in orbit, the first images came back fuzzy. The telescope was unable to focus light to a sharp point. Within days, NASA confirmed that the main mirror had been polished to the wrong shape, by a fraction of a millimetre. A single misalignment in the polishing equipment had introduced a tiny but disastrous error called spherical aberration.

The fix was extraordinary. Engineers designed a corrective device called COSTAR: a set of small mirrors that would exactly cancel out the error in the main mirror. In December 1993, a Space Shuttle crew flew to the telescope, performed 5 spacewalks over 8 days, and installed COSTAR plus a new wide-field camera. When Hubble was reactivated, the images were crystal-clear.

The repair was one of the most successful (and dramatic) servicing missions in space history. Since then, Hubble has produced over a million observations, helped discover dark energy, measured the age of the universe, and given humanity some of the most beautiful images of the cosmos ever taken.

The lesson: even a tiny error in a mirror, less than the thickness of a human hair, can ruin a multi-billion-dollar telescope. Optics is unforgiving. But with clever engineering, even those errors can sometimes be fixed in orbit.

For more, see reflection and lenses.