Light — Reflection and Refraction

Light is a form of energy that travels in straight lines and allows us to see objects around us. We see most objects not because they produce their own light, but because light from a source such as the Sun or a lamp falls on them and bounces off their surface into our eyes. This bouncing back of light from a surface is called reflection of light.

When a ray of light strikes a smooth, polished surface like a mirror, it is reflected in a very orderly way. The incoming ray is called the incident ray, and the ray that bounces back is called the reflected ray. The line drawn perpendicular (at 90°) to the mirror at the point where the ray strikes is called the normal. The angle between the incident ray and the normal is the angle of incidence (i), and the angle between the reflected ray and the normal is the angle of reflection (r).

Reflection always obeys two simple rules, called the laws of reflection: first, the angle of incidence is always equal to the angle of reflection (i = r); second, the incident ray, the reflected ray, and the normal all lie in the same plane. These laws hold true for every reflecting surface, whether flat or curved.

A plane mirror is a flat, smooth reflecting surface. The picture we see in a plane mirror is called the image. The image formed by a plane mirror has four key characteristics: it is virtual (cannot be caught on a screen), erect (upright, the same way up as the object), the same size as the object, and formed as far behind the mirror as the object is in front. In addition, the image is laterally inverted — meaning left and right appear swapped, which is why the word AMBULANCE is written reversed on the front of an ambulance so that drivers ahead can read it correctly in their rear-view mirrors.

Not all mirrors are flat. A spherical mirror is a curved mirror whose reflecting surface forms part of a hollow sphere. There are two kinds. A concave mirror curves inward like the inside of a spoon, with its reflecting surface on the inner (caved-in) side. A convex mirror bulges outward like the back of a spoon, with its reflecting surface on the outer side. You can see both effects in a shiny steel spoon: the inner side gives a concave mirror and the outer side a convex mirror.

A concave mirror is a converging mirror — it brings parallel rays of light together at a single point called the focus. Because it can gather and concentrate light, the concave mirror has many uses. It is used as a shaving or makeup mirror (it gives a large, erect, magnified image when the face is held close), in torches and car headlights (a bulb at the focus produces a strong parallel beam), in doctors' head mirrors to focus light into the ear or throat, and in solar cookers and reflecting telescopes to gather light or heat.

A convex mirror is a diverging mirror — it spreads parallel rays of light apart so that they appear to come from a point behind the mirror. A convex mirror always forms a small, erect, virtual image and gives a wide field of view, allowing the viewer to see a large area in a small mirror. This makes it ideal as a rear-view mirror in vehicles (the driver sees a wide stretch of road behind) and as a security mirror in shops and at blind corners, where a wide view is more important than the actual size of the image.

The key difference to remember: a concave mirror can form images that are either enlarged or reduced, real or virtual, depending on how far the object is, whereas a convex mirror always forms a diminished, erect, virtual image, no matter where the object is placed.

Light travels at different speeds in different transparent materials — fastest in a vacuum and air, slower in water, and slower still in glass. When a ray of light passes from one transparent medium into another, this change of speed causes the ray to bend at the boundary between the two media. This bending of light when it passes from one medium to another is called refraction of light.

The amount and direction of bending follow a clear pattern. When light goes from a rarer medium to a denser medium (for example, from air into water or glass), it slows down and bends towards the normal. When light goes from a denser medium to a rarer medium (for example, from water or glass back into air), it speeds up and bends away from the normal. If a ray strikes the surface along the normal (perpendicularly), it passes straight through without bending, because there is no sideways change.

Refraction explains many familiar everyday effects. A pencil placed in a glass of water appears bent or broken at the water surface, because light from the lower part bends as it leaves the water. A swimming pool looks shallower than it really is, because light from the bottom bends away from the normal as it leaves the water, making the bottom appear raised — this is the effect of apparent depth. A coin at the bottom of a bowl of water seems to rise up for the same reason. Even the twinkling of stars and the slightly higher position of the Sun at sunrise and sunset are caused by the refraction of light through layers of the atmosphere.

The most important practical caution from apparent depth is real: water always looks shallower than it actually is, so one should never judge the depth of a pond, river, or pool by sight alone.

A lens is a piece of transparent material, usually glass or plastic, with at least one curved surface that refracts light passing through it. Because lenses bend light by refraction, they can form images and are used in spectacles, magnifying glasses, cameras, microscopes, and telescopes. There are two main types of lens.

A convex lens (also called a converging lens) is thicker in the middle and thinner at the edges. As parallel rays of light pass through it, they are bent inward and meet at a single point called the principal focus. Because it gathers light to a point, a convex lens can form real images (which can be caught on a screen) as well as a magnified virtual image when used as a magnifying glass. Convex lenses are used in magnifying glasses, cameras, the human eye, microscopes, and projectors, and to correct the eye defect called hypermetropia (long-sightedness).

A concave lens (also called a diverging lens) is thinner in the middle and thicker at the edges. As parallel rays pass through it, they are bent outward (spread apart) so that they appear to come from a point behind the lens. A concave lens always forms a small, erect, virtual image. Its main use is to correct the eye defect called myopia (short-sightedness) and in some optical instruments and peepholes.

A simple way to tell the two apart: hold the lens close to printed text. A convex lens magnifies the print (letters look bigger), while a concave lens makes the print look smaller. You can also feel the shape — convex lenses bulge in the centre, concave lenses are hollowed in the centre.

Sunlight, or ordinary white light, looks colourless, but it is actually a mixture of seven colours. When white light passes through a glass prism, it splits into a band of these seven colours. This splitting of white light into its component colours is called dispersion of light, and the band of colours produced is called a spectrum. The seven colours, in order, are remembered by the word VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, and Red.

Dispersion happens because each colour bends by a different amount when it passes through the prism — violet light bends the most and red light bends the least. This is why the colours separate out into a fan. The most beautiful natural example of dispersion is the rainbow, which forms when sunlight is dispersed by tiny water droplets in the sky after rain; each droplet acts like a tiny prism, splitting sunlight into its seven colours.

The human eye is a remarkable natural optical instrument that lets us see. Its main parts are: the cornea, the transparent front layer where most bending of light begins; the iris, the coloured ring that controls the size of the pupil; the pupil, the central opening that lets light in and adjusts to brightness; the eye lens, a flexible convex lens that fine-focuses light; and the retina, the light-sensitive screen at the back where a real, inverted image is formed. The retina sends signals through the optic nerve to the brain, which interprets the image the right way up.

A special property of the eye is persistence of vision: an image stays on the retina for about one-sixteenth of a second after the object is removed. If still pictures are shown faster than this, the eye blends them into smooth motion — which is exactly how films, cartoons, and animations create the illusion of movement from a rapid series of separate images.