What is light?

Light as electromagnetic waves

Light consists of electromagnetic waves. Electromagnetic waves are a combination of vibrating electric and magnetic fields which travel through space at a very high speed. If the rate of vibration of these fields is millions of times per second, we can detect them as radio waves. Radio waves carry the signals for radio, television and mobile phones. If the rate of vibration is very much faster, around five hundred million million times per second (ie 5 × 1014 vibrations per second), these electromagnetic waves will become visible to us as light, and they will be seen as light of various colours.

Different colours of light have electromagnetic waves that vibrate at different frequencies. For example, red light vibrates with a frequency of about 4.3 × 1014 vibrations per second. Blue light has a frequency of about 7.0 × 1014 vibrations per second.


Polarised light

Some people like to wear sunglasses that are labelled polarising. Why is this?

If you look at the surface of a calm fish pond, you will see a reflection of the sky and clouds in the surface of the pond water. This makes it difficult to see the fish swimming around in the pond. However, if you put on a pair of polarising sunglasses, the reflection of the clouds and the sky above is greatly reduced. Now you can easily see the fish below the surface.

If you tried this with an ordinary pair of tinted sunglasses, you would find that they not only make the reflection of the sky and clouds dimmer, but also reduce the brightness of the fish and everything under the water.

It is the nature of light waves that enables polarising glasses to have this effect.

Electromagnetic waves vibrate as they travel. They are like waves sent along a skipping rope. Electromagnetic waves can vibrate from side to side, up and down and in all directions in between.

Electromagnetic waves that vibrate in all direction are said to be unpolarised.

Light waves that vibrate only in one direction are said to be polarised. We call this light polarised.

When the light from the clouds and sky is reflected off the surface of water (or any other shiny transparent surface) to form the reflection you see, the reflected electromagnetic waves only vibrate from side to side (or horizontally). The up and down (or vertical) vibrations do not get reflected. So the light is described as polarised horizontally.

The lenses in polarising sunglasses are not just coloured pieces of plastic. They contain minute crystals that allow light to pass through only if it is polarised vertically.

Thus, light from the surface of the pond cannot pass through the lens of the sunglasses because it is polarised horizontally, not vertically.

However, light reflected off the fish and the bottom of the pond is unpolarised. It contains waves that vibrate in all directions. The vertical vibrations will pass through the lenses of the sunglasses and so the fish can be seen even although the clouds in the sky cannot be.

Polarised light


Lasers

Laser beams are often used as laser pointers, and more powerful ones are seen at concerts and other displays. They are seen as very thin, straight light rays. Often smoke is added to the air to make the laser beam easy to see.


Laser light

Laser beams are light beams but they are different to the light beams that come from torches or the headlights of cars.

The most obvious difference is that laser beams do not spread out very much as they travel. They continue in a beam that hardly gets bigger as it travels, whereas the beam from a torch spreads out as it travels from the torch.

Also, laser beams consist of one pure colour only, whereas the white light from a torch globe is a mixture of most of the colours of the spectrum. For example, the helium/neon gas laser produces a red laser beam.

The other major difference is that the electromagnetic waves in the laser beam all vibrate in step, whereas the waves in the light from a torch light are random and out of step. The laser beam is said to be coherent light. What this means may be illustrated by comparing it to sound waves. White light is like the noise made by hitting all the keys of a piano at once. Laser light, however, is like a pure note made by a flute, which could be called coherent sound.

Uses of lasers

Because light travels in straight lines at a fixed speed, lasers are used by architects and builders to measure distances and to get buildings level and in the correct positions.

Looking directly into a laser beam can be very dangerous becase it can burn the retina. Eye surgeons can use a laser beam instead of a scalpel when performing operations.

Very high-powered lasers are used in industry to cut sheets of metal or sheets of plastic to exact shapes and sizes or, in tailoring, to cut many layers of cloth at once.

Lasers are also used in CD players to scan the surface of the CD to produce music, and in shopping centres to scan the barcodes of things being purchased.


Fibre optics

Cable television is a method of receiving television broadcasts through an underground cable, rather then receiving the broadcast using an antenna on the roof of the house.

Cables with copper wires can provide a telephone service, but copper wires cannot cope with the huge amount of information that is required for a television picture.

The underground cables transmitting a television signal use optical fibres. Optical fibres are extremely thin fibres of pure, very clear, silica glass.

If a ray of light is shone into one end of an optical fibre, it will travel down the fibre for a long distance. The light cannot escape from the fibre because it is reflected off the wall of the fibre.

Optical fibre


This process is called total internal reflection. It occurs when light is travelling through a denser medium and meets the boundary to a less dense medium. If the angle of refraction at the boundary is great enough, none of the light will leave the fibre as total internal reflection has occurred.

Internal reflection


If the light entering the fibre is made to flicker, these flickers will travels down the optical fibre. If they are made to flicker in the right way, they can be made to carry a digital message such as a telephone conversation, computer data or the information needed to make a television picture.

Fibre optics is becoming increasingly important in technology because in certain applications light in optical fibres can carry more information than electrons can carry in a cable.


© Commonwealth of Australia, 2003