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Teacher's Notes
INTERFERENCE
OF
LIGHT
Duration: 18 minutes
Years: 10 - 12
PROGRAM SUMMARY
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* Show the ray paths in the air gap and beyond:
* Examples of thin film interference and diffraction
* Interference of waves
* Twin slits with water waves
*Twin slits with coherent light
* Young's experiment - twin slits with incoherent light
* Diffraction gratings
* Thin film interference
* Colours of a soap bubble
* Newton's rings
* Coatings on lenses
* How a compact disc works
QUESTIONS
* Draw the ray paths showing interference producing
annulment and reinforcement:
* Draw an explanation of how the compact disc
is read:
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* Show what happens to the light hitting a lens
coating 50% thicker that this:
FURTHER QUESTIONS
1. What causes the colour of:
- a pearl shell
- a coloured beetle
- feathers
- opals
- oil film on water
- a compact disc
- two sheets of clear cellophane
- two sheets of glass
- a crack in clear crystal?
2. Has a rainbow anything to do with diffraction?
3. Try different oils floating on water and explain the
different patterns.
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4. If a soap film is tapered and each side is flat, what
will the pattern look like under green light?
5. How would this be different to an air gap that was
tapered between two glass plates?
6. Draw the path of monochromatic light interfering
in a soap film, and an air gap.
7. Draw a diagram to explain why a film can
become invisible. How would you make it
invisible to two wave lengths? Three wave
lengths? More?
8. If the film was less than a quarter of a wave length
thick, would it still be invisible? Draw a diagram.
9. When would an air gap ever be invisible?
Visible?
10. How would you design a joke lens that would
have very little light arriving at the film surface?
What would the image look like?
11. Draw a diagram showing how two slits can cause
maxima and minima.
12. On your diagram find out how many maxima
there will be and where.
13. Derive a formula for the position of the maxima.
14. If the number of slits changed, what difference
would this make to the image formed?
15. If light did not show maxima and minima in a
diffraction grating, what would it mean?
16. What is a monochrometer?
17. Why do we need to use a diffraction grating?
18. What happens to the energy when two waves
are annulled?
19. If two waves were heading off together and they
interfered and annulled each other, where would
the energy go?
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20. Explain how a compact disc is read.
21. How are Newton's rings used?
22. Think up other uses for Newrton's rings
23. How would you use the colours of thin film
interference to measure the evaporation
rate of mineral turpentine?
24. Design an instrument to measure the thermal
expansion of solids.
25. Design a weighting device with very little
movement.
26. How would you measure the wave length of
light?
27. If you coated something with many layers of
clear lacquer, when would you have
colours?
Credits
Written and produced by
John Davis
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