4Wave-9

Outcomes

• Pupils can use a raybox.
• Pupils can construct ray diagrams
• Pupils can use a raybox.
• Pupils can define the term 'reflection'.
• Pupils understand the law of reflection.

Specification References

• 3.14 recall that light waves are transverse waves which can be reflected, refracted and diffracted
• 3.15 recall that the angle of incidence equals the angle of reflection
• 3.16 construct ray diagrams to illustrate the formation of a virtual image in a plane mirror

Starter

• Give pupils one minute to list words they associate with light.

Main Body of Lesson

• Demonstrate to pupils how a raybox works.
  • It is now standard to tape down a piece of A3 paper to the bench before using a raybox.
• Have pupils (in pairs) set up a raybox. Walk around the room with semicircular prisms and issue prisms to those pupils who have correctly set up their raybox.
• Ask pupils to "experiment" with the prisms and the rayboxes, to see if they can create any interesting effects. Have them demonstrate these effects to you.
• Collect in prisms but not rayboxes.
• Demonstrate the key features of ray diagrams and reference the similarities/parallels with the rayboxes.
  • Rays of light are always straight lines.
  • Rays of light begin at something.
  • Rays of light have arrowheads to show their direction (you might want to point out here that diagrams wouldn't change if direction reversed therefore the reflection interaction is symmetric with respect to time.)
  • The normal is an imaginary line drawn at right angles to a surface.
• Demonstrate how to measure angle using a protractor. Yes, I'm serious. A lot of them really don't know how to use a protractor effectively. They have a vague idea, but aren't sure. Use WRK181 to check ability to measure angles.
• Emphasise the necessity of measuring between ray of light and normal. Angles are never measured between surface and ray. Use WRK182 to practice measuring angle of incidence/reflection.
• Explain to pupils that when light strikes a surface, three things (or a combination thereof) can happen:
  • Absorption (e.g. blue light on a red surface)
  • Transmission (e.g. light through glass)
  • Reflection (e.g. image formation by a mirror)
• Be careful to note that there is usually a combination of processes: light hitting glass is not 100% transmitted - a small portion is reflected.
• There is a good opportunity here to talk about "one-way mirrors" and to demonstrate the effect of light levels on reflection with Pepper's Ghost. Another good example is train carriages: during the day you see out, during the night you see a reflection of the carriage's interior.
• Question pupils on what the "rules" governing reflection are likely to be. One will doubtlessly get the idea of angle of incidence = angle of reflection, but not in those words.
• Give pupils the "official" form of the law of reflection angle of incidence equals the angle of reflection
• Revise how to use a raybox. Remind pupils to tape down an A3 sheet of paper before starting.
• Revise how to draw the normal to a surface.
• Pupils use raybox and small mirror to create three lines of incidence and three lines of reflection (in different colours).
• Demonstrate how virtual images are formed by reflected rays of light.

Additional Information

Resources Required

• Demonstration raybox
• Class set of rayboxes
• Perspex blocks - semicircular prisms only.
• A3 paper
• Protractors

Safety/Hazards

Rayboxes get hot avoid touching. When operating in limited light walkways need to be kept clear and pupils should take care when moving around.