Across 4. the redirection of a wave as it passes from one medium to another. 6. the distance between the crests of a wave. 9. the spreading out of waves as they pass through an aperture or around objects. 14. when the source of a wave moves relative to an observer. 16. the phenomenon that occurs when two waves meet while traveling along the same medium. 17. the speed of something in a given direction. 18. a wave of compression and rarefaction, by which sound is propagated in an elastic medium such as air. 19. a wave vibrating at right angles to the direction of its propagation. 20. a form of radiation that travel though the universe.

Down 1. the horizontal line in the middle of the wave. 2. the throwing back by a body or surface of light, heat, or sound without absorbing it. 3. the lowest part of a wave. 5. a disturbance that moves energy from one place to another. 7. a central part of the theory of quantum mechanics, being half of wave–particle duality. 8. waves where the displacement of the medium is in the same direction as the direction of the travelling wave. 10. a wave that is an oscillation of matter and is responsible for the transfer of energy through a medium. 11. the number of waves that pass by each second. 12. travel more slowly through Earth material at the planet's surface and are predominantly lower frequency than body waves. 13. the distance between successive crests of a wave. 15. the highest surface part of a wave.