Light, optics and colour

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Light, optics and colour
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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

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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.

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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).

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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

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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.

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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.

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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?

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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?

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Reflection

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

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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.

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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.

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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.

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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
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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

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Lenses

• Converging Lenses (Bi-convex) refract light
  inwards

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

• Diverging Lenses (Bi-concave) refract light
  outwards

Note the virtual focus behind the lens
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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.

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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.

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Why is the sky blue?
Source Jacaranda Core Science 3
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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
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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
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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
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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
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Light, Optics and Colour

• This PowerPoint was compiled by Robert Slider
• November 2007
• Please share this resource with others