The+facts+of+Light

The Facts of Light * Light is a form of energy that can be detected by our eyes, using our sense of sight. Currently, scientists believe that nothing travels faster than light - that is approximately 300,000 kilometres per second (in a vacuum). WOW! * The light that we see is only part of the electromagnetic range, also known as the spectrum. * Light only travels in straight lines (as you'll find out in a minute). * The main sources of light found on earth come from: the sun, stars, fire, electricity (eg torches) and animals and insects, such as the glow worm. ** Reflection ** Light travels in straight lines until something prevents it from travelling further. When light hits a reflective surface such as a mirror it is reflected. Mirrors have a very smooth surface which allows them to reflect all of the light at the same angle. Reflection by a mirror reflects light in one direction only. This is what creates a clear reflection. The angle at which the light hits the mirror is called the // angle of incidence //. The angle at which the light bounces away from the mirror is called the // angle of reflection. // When light hits a plain mirror the angle of reflection is always equal to the angle of incidence.

** Scattering ** When light hits a non-reflective surface such as a piece of paper it is scattered in all directions. Paper may appear smooth but its surface is much rougher than a mirror. When light hits this rough surface it is reflected back in many directions. This is called // diffuse reflection // or // scattering //. In spite of being scattered the rays of light still travel in straight lines. Why is the sky blue? In 1867, the director of the Royal Institution John Tyndall (a very bright spark...see what I did there?) noticed that when a torch was beamed through clean, filtered air, one could not see the beam. When shone through normal air, one COULD see the beam. He concluded that the particles in the air scatter the light, with the zillions of microscopic dust particles scattering blue light more than red or yellow light. And therefore the sky is blue (if anyone ever asks).

** Refraction ** Refraction is when light appears to bend as it travels through a transparent medium such as water or glass. 90 degrees from the surface is called the // angle of normal //. When a ray of light hits water at any angle apart from 90 degrees it will refract. Water is a denser medium than air. When light travels from a less dense medium into a denser one it slows down. When it slows down it turns towards the normal. A pencil in a glass of water appears to bend because the light travelling from the water into the air speeds up again and bends away from the normal. The ability of a medium to speed up or slow down light is called the // refractive index //. Refraction also causes rainbows. The glass in a prism is denser than air. When a beam of white light passes into the prism the different coloured parts of white light are refracted at different angles so they become separated.

** Absorption ** When light hits a black object no light is reflected or scattered back because the light is absorbed. This will be covered in more detail in colour. ** Light and Seeing Colour **

How we see colour depends on:

Our understanding of the colour of an object is a constantly changing and highly dynamic process. It depends on: > front of us. > expect to see. Trying to describe exactly what we see is extremely difficult since it depends on language to describe what is really in our mind. ** Primary Colours ** Light and pigments mix differently to form colours. The visual system has three colour sensors, it is possible to pick three suitable colours and generate the other colours by mixing these. The primary colours of light are: ** Red ** ** Green **** Blue ** They produce white light when they are added together. Any of the primary colours can be combined to form a secondary colour: When a secondary colour is mixed in proper proportions with its opposite primary colour, the resulting light will be white. In pigments, a primary colour absorbs a primary colour of light while transmitting or reflecting the other two. For example, the pigment that absorbs red light and reflects blue and green would be called cyan. So, the primary pigment colours are cyan, magenta, and yellow - the secondary colours of light. The colour appearance of an object or surface depends on the light used to illuminate it. Often daylight is seen as a consistent colour. But, think about the colour of daylight. This changes with the position of the sun in the sky, how cloudy or overcast it is.
 * the wavelengths emitted by the light source.
 * the wavelengths reflected by the object.
 * the surroundings in which we see the object.
 * the characteristics of the visual system.
 * what colours surround the object.
 * how long we have been exposed to what is in
 * what we were looking at before and what we