LIGHT

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LIGHT
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Refraction

• Refraction Fact 1 As light goes from one medium
  to another, the velocity CHANGES!
• Refraction Fact 2 As light goes from one medium
  to another, the path CHANGES!

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Refraction Going from Air to Water

• If a light ray goes from a LOW refractive Index
  (FAST MEDIUM) to a HIGH refractive index (SLOW
  MEDIUM), its speed DECREASES and the angle BENDS
  TOWARDS the normal

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Refraction Going from Water into Air

• If you are going from a HIGH refractive index to
  a LOW refractive index, your speed INCREASES and
  the angle BENDS AWAY the normal

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There is one exception

• That is when light hits head-on, perpendicular
  to the boundary. The light does not bend but its
  speed still changes.

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Total internal reflection

• happens when the angle between the light ray and
  the normal is greater than the critical angle.
• The critical angle is formed when the light ray
  travels along the boundary between the two
  substances.
• Optical fibres use multiple total
• internal reflections to transmit light.

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• Copy the diagrams into your workbook.
• Draw a normal wherever the light rays enter a new
  substance.
• Demonstrate what will happen to the rays as they
  enter and exit from the substances by continuing
  the ray through the shape and out the other side.

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Apparent Depth Prac

• Coin and Fish

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Why does it look bent away from normal and not
towards??

• Light from the lower part of the ruler is
  travelling into a region of lower optical density
  (air), and so has been bent away from the normal.
  
• The image in the water is actually an illusionan
  image our brain constructs based on where light
  from the lower part of the ruler appears to come
  from.
• It assumes that the light travelled in a
  perfectly straight line, even though it didnt.

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• Part B Refraction in water - ANSWERS
• 1 a Refract towards.
• b Emerge away from.
• c Incident a glass surface no.
• 2 When light rays at an angle to the surface
  travel from a medium of low density to one of
  higher density, the light rays refract
• towards the normal.
• 3 a From the tip, to the eye.
• 4 a From the coin to the eye.

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Lenses

• There are two main types of lenses
• convex lensesthese curve outwards and are
  fatter in the middle
• concave lensesthese curve inwards (a little
  like a cave) and are thinner in the middle.

Convex lenses Concave Lenses
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Lenses An application of refraction

• There are 2 basic types of lenses

A diverging lens (concave) takes light rays and
spreads them outward.
A converging lens (Convex) takes light rays and
bring them to a point.
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Movement of Light through Lenses

• CONVEX LENS
• In a convex lens, an incoming ray parallel to the
  principal axis is refracted through the principal
  focus (F).

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Concave Lens

• In a concave lens, an incoming ray parallel to
  the principal axis is refracted so that it
  appears to come from the principal focus (F).

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Movement of Light through a Lens

• The distance from the centre line (plane) of the
  lens to the principal focus is called the focal
  length of the lens.
• A ray passing through the centre of either type
  of lens is unaffected.
• As with all images, rays of light that come from
  a part of the object come together again at that
  same part of the image.

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Focal Length

• The greater the curvature of a lens, the more it
  bends light and hence the shorter the focal
  length.

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Image type and Location

• Convex lenses produce two different types of
  images, depending on where the object is located.
• If the object is at a distance greater than the
  focal length of the lens, a real image is formed.
• A real image can be projected onto a screen

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Convex Lenses

• If the object is at a distance less than the
  focal length of the lens, a virtual image is
  formed.
• This image cant be projected onto a screen.

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Concave Lenses

• Concave lenses produce only virtual images.

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Finding the focal length

• Rays coming into a lens from a distant object are
  almost parallel and form an image very close to
  the focus.
• We can then measure the distance from lens to
  image to determine the focal length of the lens.

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Images in a Convex and Concave Lens Prac

• AIM To investigate the image formed by different
  convex lens and concave lens
• Complete Convex prac using 2 different convex
  lenses

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Questions

• CONVEX
• 1) What happened as the object was brought
  closer to the lens?
• 2) When does
• a a real image (on a screen) is obtained
• b a virtual image (one that cannot be caught on
  a
• screen) is obtained
• c no image is obtained
• CONCAVE -
• 1) Assess whether it is possible to form a real
  image (one that may be caught on a screen)
  using a concave lens.
• 2) Explain how the image changes as the
  object-to-lens distance is varied.

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Answers
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Optical instruments

• Telescopes
• Telescopes make small, far objects appear larger.
• Two lenses are used.
• The objective lens produces a real, inverted
  image just inside the focus of a second lens,
  called the eyepiece lens. The image produced by
  the first lens now acts as the object for the
  second lens. Because the first image is inside
  the focus of the second lens, the second image
  (the one seen by the telescope user) is virtual
  and enlarged compared to the first one
• The thinner the first lens (objective lens), the
  larger the first image. But thin lenses have
  longer focal lengthsthis is why telescopes are
  long instruments.

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Eye ball prac
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