Light

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  • Created by: AIV17
  • Created on: 30-03-21 08:39

Reflection

  • Reflection is the change of direction of light by a shiny surface (mirror).
  • When an object is placed in front of a mirror, an image can be seen.
  • The image is the same size as the object, is directly in line with it, and is the same distance behind the mirror as the object is in front of it
  • Angle of incidence i = angle of reflection r, measured to the normal
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Refraction investigation

Experimental demonstration:

  • Place a rectangular glass block in the middle of a sheet of white paper and draw around it to record its position.
  • Direct, with a ray box, a ray of light at the midpoint of one of the long sides of the block and observe the refracted ray passing through the block.
  • Mark with a pencil some points on where it hits the block and where it leaves the block.
  • Remove the block and draw straight lines connecting the points; the line will show the path of the ray.
  • Mark the angles, measure them and record their values.
  • Repeat the experiment for 3 or 4 more values of the angle of i, and record the results in a table.
  • Calculate the value of sin sin i/sin r for each value of i.
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Refraction (theory)

  • When light enters a denser material it slows down and bends towards the normal
  • refractive index (n): the quantity that describes how much light is slowed down. = speed of light in a vacuum / speed of light in the material. It has no units.
  • Snell's law: n = sin i / sin r
  • When light is moving from a more dense material to a less dense material, most of it is refracted; but a little is internally reflected. (TIR) happens when the angle of i is greater than the critical angle, so light is entirely reflected inside
  • Critical angle: The angle of i where the angle of r is 90°
  • (TIR) is used to reflect light along the optical fibres, allowing high-speed transmission of data on the internet. They can also be used in medicine to see the inside of the body
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Thin converging lens

  • When parallel rays of light pass through a converging lens, they are brought to the principal focus. The focal length is the distance between the lens and the principal focus. Lens can be used to form images of objects.
  • The location and nature of the image is found by drawing a ray diagram:
  • 1. Draw the lens with a horizontal axis through the middle
  • 2. Mark the 2 principal focuses on either side at equal distances from the lens, and mark the object as an arrow standing on the axis
  • 3. Draw ray 1 from the top of the arrow passing undeflected through the middle of the lens
  • 4. Draw ray 2 from the top of the arrow parallel to the axis, passing through the lens, then deflected through the principal focus.
  • 5. The position of the top of the image is at the point where the 2 rays cross
  • The image formed is enlarged, same size or diminished, and upright or inverted
  • A real image is formed by the convergence of rays of light, and can be projected onto a screen. A virtual image is formed by the divergence of rays from the image, and cannot be projected onto a piece of paper; it only appears to have converged
  • If an object is placed closer to the lens than the focal length, the rays diverge; it is no longer a real image. By this way, lens can be used as a magnifying glass
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