Some properties of waves

1. Some properties of waves The Professional Development Service for Teachers is funded by the Department of Education and Skills under the National Development Plan

2. Waves on water • The picture shows straight waves (“plane waves”) approaching a beach. • What will the waves do when they come to the rock in the water? PDST Resources for Leaving Certificate Physics2

3. Possibilities • The parts of the wave that are not blocked will continue straight ahead. The rest will be stopped. • The parts of the wave that are not blocked will continue straight ahead. The rest will be reflected. • The parts of the wave that are not blocked will largely continue straight but the parts near the rock will go around the rock into the ‘shadow’ and form circular waves. The rest will be reflected. What do you think? PDST Resources for Leaving Certificate Physics3

4. Possibility 1 • Wave blocked PDST Resources for Leaving Certificate Physics4

5. Possibility 2 • Wave blocked and some reflection from the front PDST Resources for Leaving Certificate Physics5

6. Possibility 3 • Wave reflection from the front but also going around the back (into the ‘shadow’) • What do you think? PDST Resources for Leaving Certificate Physics6

7. Watch the videos • Video 1: waves at a small rock(There is a reflected wave but it is not so obvious.) • Video 2: waves at large rocks PDST Resources for Leaving Certificate Physics7

8. Terms • Part of the wave is REFLECTED from the front of the rock. • Part of the wave is DIFFRACTED around the edges and forms circular waves in the ‘shadow’ of the rock. • Where the diffracted waves meet they can INTERFERE CONSTRUCTIVELY forming a large wave. PDST Resources for Leaving Certificate Physics8

9. Another question What will happen when the reflected waves meet the incoming waves? • Will they be stopped by the larger incoming waves? • Will they reflect off one another? • Will they go through one another? Have a look at the next video and see for yourself. (Video 3) PDST Resources for Leaving Certificate Physics9

10. Superposition of waves • Here reflected waves are seen to have ‘gone through’ the incoming waves. This is referred to as ‘SUPERPOSITION’ of waves. (Video 3) PDST Resources for Leaving Certificate Physics10

11. Superposition • At any point at which waves meet their amplitudes add together. Two crests will then form a bigger crest; this is constructive interference. • If a crest of one wave meets a trough of another wave they interfere destructively. • They can cancel out completely (at a point) if their amplitudes are equal and opposite (i.e. if they are exactly out of step or ‘180° out of phase’) PDST Resources for Leaving Certificate Physics11

12. Implications • It has long been known that sound can go around corners. In this way sound behaves as a wave. • The fact that light can ‘go around corners’ is not as obvious. However it was first noted by Francesco Grimaldi (1618-1663). He coined the term ‘diffraction’. • (web link) PDST Resources for Leaving Certificate Physics12

13. The shadow of a pin • Note again what happens to the two sets of waves diffracting around a rock. (Video 1) • In the middle of the ‘shadow’ of the rock a large crest forms. PDST Resources for Leaving Certificate Physics13

14. Diffraction of light • If light diffracts around objects then what would the shadow of a pin look like? (Note: a large light source would not form sharp images, especially of small objects. A ‘point source’ will give the best results.) PDST Resources for Leaving Certificate Physics14

15. Possibilities (magnified) • A sharp shadow? • A shadow with fuzzy edges? • A fuzzy shadow with a bright line in the centre? What do you think? PDST Resources for Leaving Certificate Physics15

16. The shadow of a pin • Using a small ‘point source’ such as a laser, the shadow of a pin or thin wire looks like this. • There is bright line in the middle of the shadow. Note also the faint interference pattern on each side. PDST Resources for Leaving Certificate Physics16

17. Waves from two sources • Waves of the same wavelength from two sources that are relatively close together produce a characteristic interference pattern. • In the next slide there are two identical sets of concentric circles whose centres differ by exactly three wavelengths. (The dark bands are the lines of destructive interference.) PDST Resources for Leaving Certificate Physics17

18. Two sets of concentric circles PDST Resources for Leaving Certificate Physics18 Rory Geoghegan. Science for the Primary School Teacher: Light 18

19. Interference of water waves • If two stones are thrown into still water two sets of concentric waves are generated. They produce a characteristic interference pattern. • The next two slides show the beginning of the interference pattern produced in this way. PDST Resources for Leaving Certificate Physics19

20. Interference of water waves PDST Resources for Leaving Certificate Physics20 Rory Geoghegan. Science for the Primary School Teacher: Light 20

21. Interference of water waves PDST Resources for Leaving Certificate Physics21 Rory Geoghegan. Science for the Primary School Teacher: Light 21

22. Another example • The interference pattern is visible in the distance and in the foreground. PDST Resources for Leaving Certificate Physics22

23. Waves at gaps • When waves go through a small gap circular waves appear in the ‘shadow’ just as they do at edges. • When waves go through a pair of relatively small gaps then two sets of concentric waves are produced. The waves can then generate the same kind of interference pattern that is formed by two separate sources of circular waves. PDST Resources for Leaving Certificate Physics23

24. Thomas Young • In the early 1800s Thomas Young used a ripple tank to study how water waves behave. He noted the interference pattern produced by two sources or by waves coming through a pair of gaps. • He then observed a similar pattern produced by light shining through two close slits. This showed that light behaves as some kind of wave. (Link) PDST Resources for Leaving Certificate Physics24

25. Laser diffraction • Interference pattern produced by a red laser shining through a pair of narrow slits that are close together. PDST Resources for Leaving Certificate Physics25

26. 1803 • In 1803 Thomas Young published his findings in Experiments and Calculations Relative to Physical Optics. • He had confirmed the views held by Hooke and Huygens over a hundred years earlier that light had wave-like properties. PDST Resources for Leaving Certificate Physics26

27. Summary • Waves can go around corners or edges; this is called diffraction. • Waves can interfere constructively or destructively producing a characteristic pattern. • Sound and light can generate such interference patterns and so behave as waves. PDST Resources for Leaving Certificate Physics27

28. Complications • Interference patterns can be produced at single edges or single slits but a mathematical treatment of this phenomenon is more complicated than that of double slits or double sources. PDST Resources for Leaving Certificate Physics28

29. Refraction (just a mention) • Water waves can change direction when the water depth changes. This is called refraction. • Refraction is the result of a change in the velocity of the waves. • Sound and light can also change speed when going from one medium into another. The change in speed causes a change in direction (refraction). PDST Resources for Leaving Certificate Physics29

30. End PDST Resources for Leaving Certificate Physics30