Light, optics and colour

Sources of Light The sun – the main source of light on Earth The moon – this is reflected light from the sun Fire – creates light energy and heat energy Electricity – from the burning of fossil fuels or from a variety of renewable sources

Beams and Rays Light travels in straight lines at a speed of 300,000,000 m/s. A line that is used to represent the path that light travels is known as a ray. Our eyes cannot see a single ray. However, our eyes can detect a stream of rays known as a beam.

When light strikes a surface When light strikes a surface, it undergoes a variety of behaviours depending upon the material. Reflection –the light may bounce off the surface such as a mirror. Scattering – the light may bounce off the particles in the substance in many different directions such as light in fog. Absorption – the light may be absorbed by a substance such as a solid wall. (note that most solid objects also reflect some light).

Types of substances The type of substance that light strikes determines whether it is transmitted, reflected or absorbed. There are 3 basic types of substances: Transparent Translucent Opaque

Reflection When light rays strike a reflective surface at an angle (angle of incidence), it is reflected at the same angle (angle of reflection). The normal is the line perpendicular to the surface that the light strikes. This is known as the Law of Reflection.

Reflection When the eyes are looking at a reflected image the rays appear to come from behind the mirror. This type of image is known as a virtual image.

Reflection Concave mirror This is a curved mirror that causes the light rays to converge to a focal point. The distance from the focal point to the centre of the mirror is known as the focal length. This type of mirror is used in car headlights. Can you think of any other applications?

Reflection Convex mirror This is a curved mirror that causes the light rays to diverge outwards. The focal point of this mirror is virtual. This type of mirror is used to see around blind corners. Can you think of any other applications?

Reflection The outside and inside of a spoon make very different reflection images. Can you explain why? Hint: draw light ray diagrams.

Refraction Refraction occurs when light passes into a new material and starts to move at a different speed. If the ray hits the surface of the material at an angle, the ray will be bent.

Refraction It is important to note that light will refract differently depending on the different densities of the two materials. When moving from a less dense to more dense material: the light will bend towards the normal. When moving from a more dense to less dense material: the light will bend away from the normal.

Refraction Refraction can be understood through this analogy. Imagine that the fronts are rows of soldiers and the rays are lines of soldiers. The soldiers move more slowly through mud. If they approach it at an angle, then the soldiers on the left will reach the mud first, and slow down first. This will cause the lines of soldiers (the rays) to bend.

Total Internal Reflection When light travels from a substance such as glass into air, it bends away from the normal. If the incident ray is sufficiently small enough, the ray does not pass through, but is reflected back into the glass. This process is known as total internal reflection. An example of this is seen in optical fibres as seen in the diagram to the right. Source: www.spectris.com

Lenses A lens makes use of refraction. If a curve or curves is formed by a transparent piece of glass or plastic, it can bend the light rays to a focus as they pass through. There are 2 types: Converging lenses These lenses focus the rays to a focal point after they pass through the lens. Diverging lenses These lenses cause the rays to spread outwards after they pass through the lens

Converging Lenses (Bi-convex) – refract light inwards Light rays focus in front of the lens. The wider the lens , the shorter the focal length.

Diverging Lenses (Bi-concave) – refract light outwards Note the virtual focus behind the lens

Colour In 1666, Sir Isaac Newton discovered that white light is made up of different colours. These colours are known as the visible spectrum. The colours are ROYGBIV. Can you tell which colours this anagram represents? Source: www.crystalinks.com This spectrum can be seen when white light is passed through a prism. Each colour (or wavelength) is refracted at a slightly different angle resulting in a ‘rainbow’. This process is known as dispersion.

Why is the sky blue? Because the wavelength of blue light is closest to the size of the particles in the atmosphere, blue light is scattered much more than the other colours, making the sky appear blue. When the sun is low in the sky, there is more atmosphere for the light to pass through. Because of this, the blue end of the spectrum is scattered more than usual. This means that the red and orange colours pass through.

Why is the sky blue? Source: Jacaranda Core Science 3

Seeing Colours White light shines on a surface and reflects only the colour of the object. All other colours are absorbed by the object. A red object absorbs all of the colours except red, which is reflected. A green object reflects only green light. What do you think happens with a black object? Source: whyfiles.larc.nasa.gov/.../light/sim1.html

Adding Colours of Light In light, the three primary colours are red, green and blue (RGB). These are known as primary colours because when we add these colours together, they produce white light. Mixing two primary colours produces one of the three secondary colours, magenta, yellow or cyan. Colours on TV screens are made by mixing RGB beams together. Source: retina.umh.es A TV mixes colours to produce the image we see. Source: www.danalee.ca

Subtracting Colours of Light Light filters of a specific colour absorb all of the colours of the spectrum except the colour of that filter. The colour of paint or text pigments that we see is the result of reflected colour. Pigment colours work by absorbing or subtracting colours. Source: Jacaranda Core Science 3

Subtracting Colours When all of the secondary colours filters of light are used together, all of the colours are subtracted. This produces no light which is black. Source: www.steve.gb.com

Light, Optics and Colour This PowerPoint was compiled by Robert Slider November 2007 Please share this resource with others