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Across 5. time to complete one cycle 6. material that light rays can get through without creating a shadow 7. material that blocks light rays to produce a shadow 9. displacement at right angles to direction of wave 10. a wave spreading after travelling through a gap 14. measure of change over a single period 16. object emitting light after being radiated 17. a dark area created when light wave is blocked by an opaque object 20. displacement parallel to direction of wave 21. the highest point of a wave

Down 1. studies of light 2. failure of a lens to focus all wavelengths of light at the same point 3. they use refraction to change the direction of rays of light 4. number of waves occurring in time 8. a type of reflection, when light bounces off a non-smooth surface 11. wavelengths of light that can be detected by human eye 12. light bouncing off a surface 13. phenomenon when a driving frequency is equal to a natural frequency 15. happens when light wave slows down, travelling through a material, e.g. glass 18. electromagnetic radiation that can be visible or not 19. perception of different wavelengths of light