The sun The sun – the main source – the main source of light on Earthof light on Earth

The moon The moon – this is – this is reflected light from the reflected light from the sunsun

FireFire – creates light energy – creates light energy and heat energyand heat energy

ElectricityElectricity – from the – from the burning of fossil fuels or burning of fossil fuels or from a variety of from a variety of renewable sourcesrenewable sources

Light travels in straight lines Light travels in straight lines at a speed of 300,000,000 at a speed of 300,000,000 m/s.m/s.

A line that is used to A line that is used to represent the path that light represent the path that light travels is known as a travels is known as a rayray..

Our eyes cannot see a Our eyes cannot see a single ray. However, our single ray. However, our eyes can detect a stream of eyes can detect a stream of rays known as a rays known as a beambeam..

When light strikes a surface, it undergoes a When light strikes a surface, it undergoes a variety of behaviours depending upon the variety of behaviours depending upon the material.material.

ReflectionReflection –the light may bounce off the –the light may bounce off the surface such as a mirror.surface such as a mirror.

ScatteringScattering – the light may bounce off the – the light may bounce off the particles in the substance in many particles in the substance in many different directions such as light in fog.different directions such as light in fog.

AbsorptionAbsorption – the light may be absorbed by – the light may be absorbed by a substance such as a solid wall. (note that a substance such as a solid wall. (note that most solid objects also reflect some light).most solid objects also reflect some light).

The type of substance that The type of substance that light strikes determines light strikes determines whether it is transmitted, whether it is transmitted, reflected or absorbed. reflected or absorbed.

There are 3 basic types of There are 3 basic types of substances:substances:

TransparentTransparent TranslucentTranslucent OpaqueOpaque

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

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

Concave mirrorConcave mirror

This is a curved mirror that This is a curved mirror that causes the light rays to causes the light rays to convergeconverge to a focal point. to a focal point.

The distance from the The distance from the focal focal pointpoint to the centre of the to the centre of the mirror is known as the mirror is known as the focal length.focal length.

This type of mirror is used This type of mirror is used in car headlights.in car headlights.

Can you think of any other Can you think of any other applications?applications?

Convex mirrorConvex mirror

This is a curved mirror This is a curved mirror that causes the light that causes the light rays to rays to divergediverge outwards. The focal outwards. The focal point of this mirror is point of this mirror is virtualvirtual..

This type of mirror is This type of mirror is used to see around used to see around blind corners. blind corners.

Can you think of any Can you think of any other applications?other applications?

The outside and The outside and inside of a spoon inside of a spoon make very different make very different reflection images. reflection images.

Can you explain why?Can you explain why?

Hint: draw light ray diagrams.Hint: draw light ray diagrams.

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

It is important to note that light will refract It is important to note that light will refract differently depending on the different differently depending on the different densities of the two materials.densities of the two materials.

When moving from a When moving from a less dense to more less dense to more dense dense material: the light will bend material: the light will bend towardstowards the normal. the normal.

When moving from a When moving from a more dense to less more dense to less dense dense material: the light will bend material: the light will bend awayaway from the normal.from the normal.

RefractionRefraction can be understood can be understood through this analogy. through this analogy. Imagine that the Imagine that the frontsfronts are rows of soldiers and the are rows of soldiers and the raysrays are lines of soldiers. are lines of soldiers.

The soldiers move more slowly The soldiers move more slowly through mud. If they through mud. If they approach it at an angle, approach it at an angle, then the soldiers on the left then the soldiers on the left will reach the mud first, will reach the mud first, and slow down first. and slow down first.

This will cause the lines of This will cause the lines of soldiers (the soldiers (the raysrays) to bend.) to bend.

When light travels from a When light travels from a substance such as glass substance such as glass into air, it bends away from into air, it bends away from the normal.the normal.

If the incident ray is If the incident ray is sufficiently small enough, sufficiently small enough, the ray does not pass the ray does not pass through, but is reflected through, but is reflected back into the glass.back into the glass.

This process is known as This process is known as total total internal reflectioninternal reflection..

An example of this is seen in An example of this is seen in optical fibres as seen in the optical fibres as seen in the diagram to the right.diagram to the right.

Source: www.spectris.com

A lens makes use of refraction. A lens makes use of refraction.

If a curve or curves is formed by a transparent If a curve or curves is formed by a transparent piece of glass or plastic, it can bend the light piece of glass or plastic, it can bend the light rays to a focus as they pass through. There are rays to a focus as they pass through. There are 2 types:2 types:

Converging lensesConverging lenses

These lenses focus the rays to a focal point after These lenses focus the rays to a focal point after they pass through the lens. they pass through the lens.

Diverging lensesDiverging lenses

These lenses cause the rays to spread outwards These lenses cause the rays to spread outwards after they pass through the lensafter they pass through the lens

Converging Lenses (Bi-convex) – refract light Converging Lenses (Bi-convex) – refract light inwardsinwards

Light rays focus in Light rays focus in front of the lens. front of the lens. The wider the The wider the lens , the shorter lens , the shorter the focal length.the focal length.

Diverging Lenses (Bi-concave) – refract light Diverging Lenses (Bi-concave) – refract light outwardsoutwards

Note the virtual Note the virtual focus behind the focus behind the lenslens

This spectrum can be seen when This spectrum can be seen when white light is passed through white light is passed through a prism. Each colour (or a prism. Each colour (or wavelength) is refracted at a wavelength) is refracted at a slightly different angle slightly different angle resulting in a ‘rainbow’. resulting in a ‘rainbow’.

This process is known as This process is known as dispersiondispersion..

In 1666, Sir Isaac Newton In 1666, Sir Isaac Newton discovered that white light is discovered that white light is made up of different colours. made up of different colours. These colours are known as These colours are known as the the vivisisiblble se spepectrctrumum..

The colours are ROYGBIV. The colours are ROYGBIV.

Can you tell which colours this Can you tell which colours this anagram represents?anagram represents? Source:

www.crystalinks.com

Because the wavelength of Because the wavelength of blue light is closest to the blue light is closest to the size of the particles in the size of the particles in the atmosphere, blue light is atmosphere, blue light is scattered much more than scattered much more than the other colours, making the other colours, making the sky appear blue.the sky appear blue.

When the sun is low in the sky, When the sun is low in the sky, there is more atmosphere for there is more atmosphere for the light to pass through. the light to pass through. Because of this, the blue end of Because of this, the blue end of the spectrum is scattered more the spectrum is scattered more than usual. This means that the than usual. This means that the red and orange colours pass red and orange colours pass through.through.

Source: Jacaranda Core Science 3

White light shines on a White light shines on a surface and reflects only surface and reflects only the colour of the object. the colour of the object. All other colours are All other colours are absorbed by the object.absorbed by the object.

A A redred object absorbs all of object absorbs all of the colours except red, the colours except red, which is reflected. which is reflected.

A A greengreen object reflects object reflects only green light.only green light.

What do you think What do you think happens with a happens with a blackblack object?object?

Source: whyfiles.larc.nasa.gov/.../light/sim1.html

In light, the three primary colours In light, the three primary colours are are redred, , greengreen and and blue blue ((RRGGBB).).

These are known as These are known as primary primary colours colours because when we add because when we add these colours together, they these colours together, they produce white light.produce white light.

Mixing two primary colours Mixing two primary colours produces one of the three produces one of the three secondary colourssecondary colours, , magentamagenta, , yellowyellow or or cyancyan..

Colours on TV screens are made Colours on TV screens are made by mixing RGB beams together.by mixing RGB beams together.

Source: retina.umh.es Source: retina.umh.es

A TV mixes colours to A TV mixes colours to produce the image we produce the image we see.see.Source: www.danalee.caSource: www.danalee.ca

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

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

This PowerPoint was compiled by Robert Slider

November 2007

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