Light

= Light =

**Some Key Facts...**
Light is a form of energy which our senses can detect and is therefore responsible for our sight. Light travels in straight lines and will continue to do so unless it is blocked, reflected or refracted. This video demonstrates light travelling in straight lines: []

Light travels as transverse waves and the speed in a vacuum (an empty area of matter) is around 186,000 miles per second or 300,000 kilometres per second. Physicists refer to light as electromagnetic radiation however the light we refer to in everyday life is the VISIBLE SPECTRUM (the part of the electromagnetic spectrum that the human eye can see). Interesting fact: other animals can see parts of the spectrum that humans cannot e.g. a large number of insects can see ultraviolet (UV) light.

Our main source of light on Earth comes from the Sun. These are both sources of natural light. Other examples of natural light sources are the stars and fire. As well as natural light sources, there are also artificial sources of light. These are man-made sources and are often powered by electricity, for example an electric light bulb is an artificial light source.

A common misconception in this area is that the Moon is a source of light. Light from the Moon does reach the Earth however this light originates from the Sun and is only being reflected by the Moon.

**Reflection**
Light waves can reflect off smooth surfaces such as mirrors. The light waves are reflected uniformly which thereby forms an image. The angle at which the light hits the mirror (the angle of incidence) is equal to the angle at which it is reflected (angle of reflection.)

Rule of thumb: The angle of INCIDENCE is equal to the angle of RELECTION.

Image :Taken from www.desktopclass.com

Rough surfaces however scatter light in all directions.

**Refraction**
Refraction is the bending of light. When light travels from one transparent material to another, such as when light passes through glass or water, it slows down and bends or refracts.

Light which is travelling through transparent materials of different densities will usually behave as follows:
 * Light falling at right angles onto the surface of a denser (transparent) material will pass straight through the material

(Image taken from: Parsons & Gannon (1998) Key Stage 3 Science: The Revison Guide L3-6)

(Image taken from: Parsons & Gannon (1998) Key Stage 3 Science: The Revison Guide L3-6)
 * Light falling at an angle, and travelling into a denser material, will be refracted (i.e. bends) towards the normal.
 * Light leaving a dense material (and entering a less dense medium such as air) will be refracted away from the normal.

If you look at the photograph below, the straw appears to be broken. This is due to the refraction of light as it passes from the drink into the air (i.e. from a more dense to a less dense medium).



Refraction also explains why water always looks shallower than what it is and why it is hard to tell where an underwater object really is.

**Colours**
White light consists of a mixture of colours. We commonly know these colours as the colours of the rainbow – Red, Orange, Yellow, Green, Blue, Indigo and Violet. These colours have different wavelengths with red having the longest wavelength (lowest frequency) and violet having the smallest wavelength (highest frequency). The light waves are refracted as they enter and leave a transparent material. The shorter the wavelength of the light, the more it is refracted. As a result, red light is refracted the least and violet light is refracted the most, causing the coloured light to spread out to form a spectrum.

We can see the dispersal of white light into its constituent colours when white light enters a glass prism or a rain drop. This rainbow effect is known as a **spectrum**.

(Image taken from: @http://www.school-for-champions.com/science/light_dispersion.htm)


 * TIP:** The colours of the spectrum can be remembered as: **R**ichard **O**f **Y**ork **G**ave **B**attle **I**n **V**ain
 * Did you know?...** Sir Isaac Newton was the first person to look at white light through a prism and see it split into the colours of the spectrum. When we see a rainbow, white light from the sun is being split into colours because raindrops in the air act as prisms.

Watch this video on how to make a rainbow: []

We see objects around us because they reflect light from their surfaces, and some of this reflected light enters our eyes. We see them in colour because objects and surfaces absorb some coloured light (i.e. some wavelengths) and reflect others. The colour that they reflect will be the colour that we see. For example, a pair of blue jeans will reflect blue light and absorb all other colours.
 * Why do we see objects in colour?**

Objects or surfaces that reflect all wavelengths / colours will be seen as white. Objects or surfaces that absorb all light that falls on them, and therefore do not reflect light of any colour, will be seen as black.



** **Coloured Filters** **
Transparent materials that are colourless allow light of all colours (i.e. wavelengths) to pass through. If a coloured filter is added, it will only allow light of the same colour to pass through – all other colour will be absorbed by the filter. Therefore coloured filters can be used to change the colour of light. An example of this is a red traffic light. This is a bulb giving off white light behind a red filter that only lets red light through.

**Key words:**

 * Medium** - Any material or substance that light passes through
 * Normal** - a line at a right angle to the surface of the material
 * Prism -** a block of glass with a triangular cross-section.
 * Reflection** - when light hits the surface of a material and bounces off the surface again.
 * Refraction** – is the change of direction as a ray of light passes from one medium to another. Caused by its change in speed.
 * Spectrum –** the rainbow effect seen when white light is dispersed.
 * Vacuum -** an area devoid of matter.