L1 - Light Rays and Waves

objectives

For you to see something, light must enter your eyes. The Sun, lamps, lasers, and glowing TV screens all emit (send out) their own light. They are luminous. However, most objects are non-luminous.
You see them only because daylight, or other light, bounces off them. They reflect light, and some of it goes into your eyes.

If you can see a beam of light, this is because tiny particles of dust smoke, or mist in the air are reflecting some of The light into your eyes.

You can see this page because it reflects light. The white parts reflect most light and look bright. However, the black letters absorb nearly all the light striking them. They reflect very little and look dark.

Most surfaces are uneven, or contain particles that scatter light. As a result, they reflect light in all directions. The reflection is diffuse.
However, mirrors are smooth and shiny. When they reflect light, the reflection is regular.
Transparent materials like glass and water let light pass right through them. They transmit light.

Light is a form of radiation This means that light radiates (spreads out) from its source. In diagrams, lines called rays are used to show which way the light is going.
Light travels in straight lines You can see this if you look at the path of a sunbeam or a laser beam.
Light transfers energy Energy is needed to produce light. Materials gain energy when they absorb light. For example, solar cells use the energy in sunlight to produce electricity.
Light can travel through empty space Electric and magnetic ripples do not need a material to travel through- That is why light can reach us from the Sun and stars.
Light is the fastest thing there is In a vacuum (in space, for example), the speed of light is 300 000 kilometres per second. Nothing can travel faster than this. The speed of light seems to be a universal speed limit.
Light travels as waves Light radiates from its source rather as ripples spread across the surface of a pond. However, in the case of light, the 'ripples' are tiny, vibrating, electric and magnetic forces.
Light waves have wavelengths of less than a thousandth of a millimetre (see below). Like other waves, they can be diffracted, but the effect is too small to notice unless the gaps are very narrow, for example, as in a fine mesh.
Some effects of light are best explained by thinking of light as a stream of tiny 'energy particles'. Scientists call these particles photons.

When light enters the eye, the brain senses different wavelengths as different colours. The wavelengths range from 0.000 4 mm (violet light) to 0.000 7 mm (red light), and white light is made up of all the wavelengths in this range.
Most sources emit a mixture of wavelengths. However, lasers emit light of a single wavelength and colour. Light like this is called monochromatic light.

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