When light is reflected from any mirror the same rules of reflection apply. For a plane mirror the angle at which the light ray strikes the surface (the angle of incidence) is equal to the angle at which the same light ray leaves the surface (the angle of reflection).

For mirrors with curved surfaces this law of reflection still applies, but the curved surface of the mirror creates different images to that of a plane mirror.

Convex and concave mirrors Curved mirrors have a variety of forms, two most common types are convex and concave. A convex mirror has a surface that bows outwards and a concave mirror has a surface that caves inwards. Each has distinctive characteristics in terms of size of image and whether the image is real or virtual.

Real and virtual images

A real image can be projected onto a screen, while a virtual image appears to be behind the mirror, and cannot be projected onto a screen.

Concave mirrors can produce both real and virtual images depending on the distance from the mirror to the object and the curvature of the mirror, while convex mirrors produce only virtual images.

An image produced by light reflected from a convex mirror of a far off object, is diminished in size (smaller than the object), and the right way up. A strange thing happens with regard to the image size and orientation of an image produced by a concave mirror. An image of an object very close to the surface of the mirror, that is if the object is closer than the focal point, is the right way up and magnified. When the object is further away, its image is upside down, diminished and real.

Try this with a highly polished spoon, or a pair of reflective Sun glasses. Start with the spoon at arms length and observe your image, then move the spoon closer and closer, watch what happens to the image.

Caustic curves So far we have considered parallel rays of light. What happens if there is a wide beam of light? If you look at an image produced by the outside of a highly polished ring, or a white mug, on a table, with light shining on it, it’s similar to that formed by a convex mirror. Now look inside the ring. You will observe an image on the table called a caustic curve. A caustic curve can also be seen on the surface of a cup of tea or coffee.

This curved shape image is produced because the rays of light reflecting from the surface do not meet at a single point of focus but are spread over a focal plane. To focus light from a wide beam source an adaptation of the concave mirror is needed.

Parabolic mirrors A parabolic mirror is a concave mirror adapted to focus any beam of light originating at a single point, no matter how close or far away the source of light is from the mirror. The parabolic shape focuses each ray of light accurately into a single focal point. Parabolic mirrors are used in solar cookers to focus the Sun’s rays and in car headlights to create beams of light from a single light globe.

Funny mirrors At an amusement park you might find mirrors made like a combination of concave and convex mirrors. These can produce very funny results as the image of the body is distorted!

Using curved mirrorsRound convex mirrors are useful as security aids in shops where a broad (though distorted) image of an otherwise hidden aisle is clearly shown. They are also valuable to show traffic at a blind junction. Wing mirrors on late model cars sometimes are convex, with an appropriate warning that the image is deceiving.