Light forms the part of the radiation given out by the Sun that we can detect with our eyes. One familiar property of light is that it travels in straight lines. This explains why objects cast sharp shadows in the Sun, and why we can't see round corners.
When light falls on an object, a variety of things may happen. One, the light may bounce off the object, that is, be reflected. Two, the light may pass through the object. And three, the light may be absorbed by the object. In practice a combination of these things usually happens.
All objects reflect light to a lesser or greater extent. Indeed, we see objects usually because they reflect light into our eyes.
When light passes through an object, we say it is translucent. And if it doesn't, we say the object is opaque. If light passes through an object without being distorted, we say it is transparent. Thus a sheet of frosted glass is translucent but not transparent, while ordinary glass and water are both translucent and transparent.
The way in which an object absorbs light determines what colour it appears. This happens because ordinary white light is actually made up of many different colours, which correspond to different wavelengths of light. We see these colours in a rainbow.
Different materials absorb different wavelengths, or colours, and reflect the rest. And the colour we see is the colour that is reflected. For example, the leaves of plants absorb all colours except green. They reflect this colour into our eyes, and we see leaves as green. The paint on fire engines absorbs all colours except red, which it reflects. And we see fire engines as red.
Some materials absorb practically all the light that falls on them and reflect hardly any. These materials appear black to our eyes. On the other hand some materials absorb hardly any light but reflect practically all of it. These materials appear white.
Materials that are flat, very smooth and shiny, such as polished metals, reflect light in a special way. They not only reflect most of the light that falls on them, but also reflect it back in straight lines. They turn into mirrors, which reflect back a clear image of whatever object is in front of them. Most ordinary mirrors are sheets of glass with a silvery coating on the back.
When an incoming light ray strikes the mirror at a certain angle, it is reflected off the mirror at the same angle. The perpendicular line from the point at which the ray strikes the mirror is called the normal; the angle between the incoming ray and the normal is called the angle of incidence; and the angle between the reflected ray and the normal is called the angle of reflection. The basic law of mirrors states that the angle of incidence is equal to the angle of reflection.
When you look in a mirror, you see what appears to be an accurate picture of yourself looking back. But the picture you see is not completely accurate. If you raise your right hand, your mirror image raises its left hand. If you turn your head to the left, your mirror image turns its head to the right. In a mirror image, right and left are reversed.
The image of yourself you see in a mirror appears as far behind the mirror as you are in front. It looks real enough, but in fact it doesn't really exist! It is what we call a virtual image. You can't capture the image or project it onto a screen.
Reflection from a mirror surface is one of the basic properties of light. Another basic property is refraction. Refraction is the bending of a light ray that occurs when it travels from one transparent medium into another. Refraction takes place, for example, when a light ray travels from air into water, water into air, air into glass, and glass into air.
Refraction explains why a straw in a glass of lemonade looks bent when you look down on it. And if you look at it from the side, it looks broken. Both these effects occur because of the way light bends at the surface, the boundary between the air and the lemonade. Refraction also explains why water in a bath and swimming pool looks shallower than it really is.
A basic law of refraction is that when a light ray travels from a less dense to a more dense medium (such as air into water), the refracted ray is bent towards the normal in the denser medium.
Conversely, when a light ray travels from a more dense to a less dense medium (such as water to air), the refracted ray is bent away from the normal in the less dense medium.
Different media, such as water, glass and clear plastic, bend light entering them by different amounts. It depends on an optical property called their refractive index.
So far we have dealt with the reflection and refraction of light at flat surfaces. But reflection and refraction at curved surfaces produce some interesting and very useful effects.
The image in a plane, or flat, mirror is exactly the same size as the object that produces it. But if you curve the mirror, you can produce smaller or bigger images.
The wing mirrors used on cars are curved. Their surface curves backwards from the centre like an upside-down saucer. We call this a convex shape. When you look into a wing mirror, you see a smaller image of yourself and your surroundings. It is erect, or the right-way up, and is virtual. The reason convex mirrors are used for this purpose it that they have a wider field of view than ordinary mirrors. This means that drivers have better vision of the traffic situation around them.
Shaving or make-up mirrors curve in the opposite direction from wing mirrors. They have a surface that curves forwards from the middle, like a saucer. We call this a concave shape. When you look into the concave mirror from a close-up position, it produces a magnified image of your face. As with a plane mirror, the image is erect and virtual.
But if you move back from the mirror, you notice that your image suddenly turns upside-down. We say it becomes inverted. And if you hold a card in the right place, you can capture the image. In other words, it is a real image. Astronomers make use of this property of curved mirrors to form real images in reflecting telescopes, as we shall see later.
Just as curved reflecting surfaces can produce a variety of different images - real or virtual, erect or inverted, same size, smaller or magnified, and so on - so can curved refracting surfaces. The most useful of these are the curved pieces of glass we call lenses. In general a lens has two curved surfaces. Light is refracted, or bent, twice as it passes through each surface - from air to glass, and from glass back into the air.
As with curved mirrors, there are basically two types of lenses, concave and convex. Concave lenses have saucer-shaped surfaces and are thinnest in the middle. Convex lenses have the opposite shape, being thickest in the middle.
These two types of opposite-shaped lenses are also optical opposites. Because of the different curvature of the lens surfaces, light is bent in different directions as it passes through the two types of lenses.
When a beam of light rays enters one side of a concave lens, the rays are bent away from the axis of the lens - an imaginary line passing through the middle of the lens. In other words, they spread out, or diverge, when they leave the other side of the lens. That is why we call the concave lens a diverging lens.
On the other hand, when a beam of light enters one side of a convex lens, the rays are bent towards the axis. In other words, they come together, or converge, when they leave the other side of the lens. The convex lens is a converging lens.
Of the two kinds of lenses, the convex lens is the most useful. When you look through a convex lens held close to an object, you see a magnified image of the object. This is the principle behind the simple magnifying glass. The magnified image is erect, but virtual, and is on the same side of the lens as the object.
A convex lens will also produce a real image of a distant object which can be captured on a screen. The real image is inverted and on the opposite side of the lens to the object. Again, astronomers make use of this property to design telescopes to view the heavens more clearly. These telescopes are called refractors because they rely on the refraction of light in their lenses. And again they produce an inverted image.
Historically, refractors were the first telescopes used. The Italian scientist Galileo made the first good refractor in 1609 and turned it on the heavens in the winter of that year. Among his startling discoveries were the four large moons of Jupiter, still called the Galilean moons, and the phases of Venus. These discoveries, among others, convinced him that Copernicus's revolutionary theory (1543) that the Sun, and not the Earth, was the centre of the universe, was correct.
One major drawback with the early refractors was that they were affected by colour blurring of the image. This chromatic aberration happens because the different colours, or wavelengths, that make up white light bend by differing amounts when they are refracted in the lens. Consequently, they are not all brought into focus at the same point. This results in blurring.
The English scientist Isaac Newton realised the problem and in about 1668 designed a different kind of telescope to get around it. He used a curved mirror rather than a lens to gather the light. The common type of reflecting telescope, or reflector, used by amateur astronomers today still uses Newton's design.
The Newtonian reflector has a large concave primary mirror to gather the starlight, set at the base of an open tube. The mirror then reflects the light back up the telescope tube to a plane mirror, angled at 45 degrees. This plane mirror in turn reflects the light through a right-angle into an eyepiece set in the side of the tube. The eyepiece produces a magnified image of the star field viewed. The image is inverted, or upside-down. But for astronomical work, this doesn't matter.
Larger reflecting telescopes have different mirror arrangements, and are usually able to form images at different points. The prime-focus position is the place above the primary mirror where the mirror forms a sharp image, that is, brings the light rays to a focus. Most photography is done in this prime-focus position.
The Cassegrain-focus position is a point beneath the primary mirror. Light rays gathered by the concave primary mirror are reflected up the tube to a small convex mirror. This mirror then reflects the light back down the tube and through a hole in the primary mirror. There it is viewed through an eyepiece.
All the major telescopes in the world are reflectors. This is because they can be built with large mirrors. The famous Hale telescope of Palomar Observatory in the USA has a mirror over 5 metres across. The largest refractor, however, has a lens only a little over 1 metre across. The reason you can build big reflectors is that you can support a mirror from behind. You can only support a lens at the sides, which sets up stresses and causes distortion as the size and weight of the lens increases.
