Reflection

Mirror Mirror on the wall...

• What type of wave is visible light? [2]

  • Visible light is an EM wave
  • Visible light is a transverse wave
    • How does visible light allow us to see 'glow in the dark' objects? [1]
      • Glow in the dark objects give off visible light which comes straight into our eyes
        • So could we still see them if there were no other light sources? [1]
          • Yes - because they give off their own visible light, they are a source of light (a light source)
    • How does visible light allow us to see normal objects? [1]
      • Reflection - Since normal objects do not give off their own light, what we see must be light bouncing off the object into our eyes
      • • So could we still see them if there were no light sources? [1]
          • No - they do not give off their own visible light
        • So could we see objects that didn't reflect light? [1]
          • No! If it absorbed light comlpetely it would look very black.
          • If it let light pass through it (transparent) we would also not see it
      • What happens when light reflects from an uneven surface, such as a piece of paper? [1]
        • The light reflects off at all different angles (a diffuse reflection)
      • So, what happens when light reflects from an even surface (smooth and shiny, like a mirror)? [1]
        • Then it's all reflected at the same angle (a clear reflection)

• What is the 'law of reflection'? [1]

  • Angle of INCIDENCE = Angle of REFLECTION
    • What would the angle of reflection be if the incident angle was 60º? [1]
      • 60º
    • What would the angle of incidence have been if the reflected angle was 36º? [1]
      • 36º
    • Are the angles drawn between the ray and the surface? [1]
      • No! Careful - the angle is ALWAYS between the ray and the NORMAL!
      • What is the 'normal'? [1]
        • An imaginary line that's at right angles to the surface (at the point where the light hits the surface)

• What is an image? [1]

  • This is where the light from an object comes together to form a real image on a 'screen', like the image formed on an eye's retina (the 'screen' at the back of an eye)
    • What is a 'virtual image'? [1]
      • This is when the rays are diverging, so the light from the object apears to be coming from a completely different place.
      • e.g. When looking at an object through a magnifying lens you can get a virtual image that looks BIGGER and FURTHER AWAY than the object actually is
      • e.g. When you look at a reflection you see a virtual image because the object appears to be BEHIND the mirror (even though the object is really in front of it)
        • What 3 properties does an image in a plane mirror always have? [3]
          • The image is the same size as the object
          • It is as far behind the mirror as the object is in front
          • It's formed from diverging rays, which means it's virtual
        • How do you draw a ray diagram for an image in a plane mirror? [4]
          • 1 - Don't try to draw the rays first.  Start by drawing the virtual image (same size, as far behind the mirror as the object is in front)
            • 2 - Draw a reflected ray going from the top of the virtual image to the top of the eye (draw a dotted line for the part behind the mirror)
              • 3 - Draw the incident ray going from the top of the object to the mirror at the point where the reflected ray meets the mirror (follow the law of reflection).
                • 4 - Now you have the incidence and reflected rays for the top of the image (Do steps 2 & 3 again for the bottom of the eye)

• Where is the 'centre of curvature'? [1]

  • This is the centre of the sphere (since curved mirrors are like a round portion of a sphere)
    • What is the vertex? [1]
      • The centre of the mirror's surface
        • Where is the focal point? [1]
          • Halfway between the centre of curvature and the vertex.
          • This is where rays parallel to the axis of a concave mirror reflect and meet at the focal point
          • This lies in front of a concave mirror & behind a convex mirror
          • • What is the axis? [1]
              • A line down the middle of a mirror (the centre of curvature, vertex & focal point all lie on this)

• What are the two types of curved mirror? [2]

  • Concave Mirrors (shiny on the inside of the curve)
    • What happens to light reflected off a concave mirror? [1]
      • It converges
        • What will the image look like when the object is placed at the centre of curvature? [4]
          • Real
          • upside down
          • same size
          • in the same place
        • What will the image look like when the object is placed inbetween the centre of curvature and the focal point? [4]
          • Real
          • upside down
          • bigger
          • behind the mirror.
        • What will the image look like when the object is placed in front of the focal point? [4]
          • Virtual
          • right way up
          • bigger
          • behind the mirror
        • How do you draw a ray diagram for an image in a concave mirror? [6]
          • 1 - Pick a point on the top of the object. Draw a ray going from the object to the mirror parallel to the axis of the mirror
            • 2 - Draw another line going from the top of the object to the mirrr, passing through the focal point on the way
              • 3 - The incident ray that's parallel to the axis is reflected through the focal point. Draw a reflected ray passing through the focal point.
                • 4 - The incident ray that passes through the focal point is reflected parallel to the axis. Draw a reflected ray passing parallel to the axis
                  • 5 - Mark where the two reflected rays meet. The top of the image.
                    • 6 - repeat the process for a point on the bottom of the object. When the bottom of the object is on the axis, the bottom of the image is also on the axis.
  • Convex Mirrors (shiny on the outside of the curve)
    • What happens to light reflected off a convex mirror? [1]
      • It diverges
        • What are the advantages of using a convex mirror over a concave mirror? [1]
          • you can see a wide area in a convex mirror
            • What aplication can you use a convex mirror in to exploit this advantage? [1]
              • Doggy road corners
        • What does an image of an object always look like in a convex mirror? [5]
          • virtual
          • upright
          • smaller
          • behind the mirror
          • closer than the focal point
        • How will it affect the size of the image if you move the object further away? [1]
          • The further away the object is from the mirror, the smaller the image
        • How do you draw a ray diagram for an image in a convex mirror? [6]
          • 1 - Pick a point on the top of the object. Draw a ray going from the object to the mirror parallel to the axis of the mirror. Make it a bold line when it's in front of the mirror, and a dotted line when it's behind
            • 2 - Draw another line going from the top of the object to the mirror, passing through the focal point on the other side. Make it dotted when its behind the mirror
              • 3 - The incident ray that's parallel to the axis is reflected as if it starts at the focal point. Make sure the reflected ray meets the incident ray at the mirror surface
                • 4 - The incident ray that passes through the focal point is reflected parallel to the axis. Make sure the reflected ray meets the incident ray at the mirror surface.
                  • 5 - Mark where these two reflected rays meet behind the mirror. That's the top of the image
                    • 6 - Repeat the process for a point on the bottom of the object

• Classwork

  • 1) What is the difference between a real and a virtual image?
  • 2) What 3 pieces of information do you need to give to describe the nature of an image?
  • 3) A ray of light hits the surface of a mirror at an incidenct angle of 10° to the normal. What is the angle of reflection for the ray of light?
  • 4) Copy and complete this diagram, showing the reflected rays and the position of the virtual image
  • 5) An object is put in front of a concave mirror, between its centre of curvature and focal point. What type of image will b formd? Where will this image be?
  • 6) The diagram shows a curved mirror. On the diagram, label the: axis, focal point, vertex, centre of curvature.
  • 7) A car's rear view mirror often has a convex shape
    • State one other practical use for a convex mirror
    • What property of convex mirrors makes them useful as rear-view mirrors?
  • 8) Harold writes "I know that no reflection occurs when I look at a wall because if the wall reflected any light I'd see my reflection in it." Explain why he is wrong.

• Homework

  • 1) What is the relationship between the angle o incidence and the angle of reflection?
  • 2) State whether the following images are real or virtual:
    • an image you see in a plane mirror
    • an image formed on the retina of your eye
    • an image you see when looking through a magnifying glass
  • 3) The diagram shows a pinhole camera. Light reflected from a goat passes through a pinhole to form an image on a tissue paper screen
  • 4) The diagram shows a candle bein reflected in a plane mirror. Some of the rays have already been drawn in.
    • On the diagram, draw in the rays showing how light is reflected to form an image of the top of the pencil
    • Is this image in a plane mirror real or virtual?
  • 5) Below are four sentences about curved mirrors. Say whether each refers to concave or convex mirrors.
    • Reflective on outside of curve
    • Reflected light converges
    • Reflected light diverges
    • Reflective on inside of curve
  • 6) Julie is holding a small concave mirror in front of her in order to do her lipstick. The focal length of the mirror is 3m. State whether each statement is true or false
    • She sees a real image in the mirror
    • The distance from the vertex to the centre of curvature is 6m
    • The miror gives a magnified picture of her face
    • The mirror gives a wider view than a plane mirror
  • 7) In the sentence below, circle the correct words from the choices given in brackets:
    • An .............. ( extended / incident ) ray that can be .................... ( extended / refracted ) to pass through the focal point of a convex mirror will be reflected ..................... ( along the normal / parallel to the normal )