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© Boardworks Ltd 2005 1 of 31 KS4 Physics Diffraction, Interference and Resonance

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© Boardworks Ltd 2005 2 of 31 Contents Diffraction, Interference and Resonance Diffraction Interference Standing waves Summary activities Natural frequency

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© Boardworks Ltd 2005 3 of 31 The ripple tank A ripple tank is a device used to study the behaviour of waves, because all waves behave in a similar manner. A ripple tank produces water waves that can be reflected, refracted and diffracted. paddle vibrates to produce waves

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© Boardworks Ltd 2005 4 of 31 Reflection and refraction paddle What do you think will happen if a barrier is placed in front of the water waves? If it is a plane barrier then the waves are reflected. barrier What do you think will happen if a block is submerged in the ripple tank? The change in depth of the water causes a change in speed of the waves – they are refracted.

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© Boardworks Ltd 2005 5 of 31 Barrier with small gap What do you think will happen if a barrier with a gap in it is placed in front of the water waves? It depends upon the size of the gap. If the gap is smaller than the wavelength of the waves what do you think will happen? The waves are reflected by the barrier.

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© Boardworks Ltd 2005 6 of 31 Barrier with wavelength-sized gap What will happen if the width of the gap in the barrier is similar in width to the wavelength of the waves? Circular waves are produced. This effect is known as diffraction.

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© Boardworks Ltd 2005 7 of 31 Barrier with large gap What will happen if the width of the gap in the barrier is larger than the wavelength of the waves? The waves pass through the gap unchanged apart from slight diffraction of the waves near their ends.

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© Boardworks Ltd 2005 8 of 31 Effect of diffraction… How can the teacher in the corridor hear the school band even though he isn’t in the hall?

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© Boardworks Ltd 2005 9 of 31 Effects of diffraction When you are in a room, why can you hear people in the corridor even though you can’t see them? Light and sound are both waves. Waves travel in straight lines. Sound waves have a wavelength similar in magnitude to the width of the doorway and so diffraction occurs. Light waves have a much shorter wavelength than sound waves and are not diffracted by the doorway.

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© Boardworks Ltd 2005 10 of 31 Contents Diffraction Interference Standing waves Summary activities Natural frequency Diffraction, Interference and Resonance

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© Boardworks Ltd 2005 11 of 31 Constructive interference When two waves meet, they interfere with each other. If they meet each other exactly in phase, the amplitudes ‘add up’ to produce large crests and troughs. += This is called constructive interference.

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© Boardworks Ltd 2005 12 of 31 Destructive interference This is called destructive interference. If two waves meet each other exactly out of phase, the amplitudes ‘subtract’ to produce no peaks or crests. +=

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© Boardworks Ltd 2005 13 of 31 Young’s slits To get two waves of light to interfere, the waves must be very similar. A single source of monochromatic light is used, and split into two waves by using a diffraction grating like this: In 1801, the English physicist Thomas Young first performed this classic investigation, which showed the interference of light waves.

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© Boardworks Ltd 2005 14 of 31 Diffraction and interference The light source emits rays of light, which diffract towards the double slit. S1S1 S2S2 S 1 and S 2 act as two light sources. The waves interfere:constructively (bright fringes) destructively (dark fringes) fringes

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© Boardworks Ltd 2005 15 of 31 Diffraction patterns What would the fringes look like if white light was used as the source instead? This is the front view of fringes produced by Young’s slits.

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© Boardworks Ltd 2005 16 of 31 Diffraction effects The coloured fringes on these CDs are the result of interference. Light reflecting from the aluminium coating diffracts and interferes. Some colours are diffracted more than others.

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© Boardworks Ltd 2005 17 of 31 Resonance All objects have a natural frequency of vibration. If an object if forced to vibrate at its natural frequency it will vibrate at its maximum amplitude. This effect is called resonance. The larger the mass of an object, the lower its natural frequency. What will happen to a glass made to vibrate at its natural frequency? Resonance causes it to shatter!

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© Boardworks Ltd 2005 18 of 31 Waves, diffraction and interference

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© Boardworks Ltd 2005 19 of 31 Contents Diffraction Interference Standing waves Summary activities Natural frequency Diffraction, Interference and Resonance

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© Boardworks Ltd 2005 20 of 31 Natural frequency Fill a test tube rack with test tubes containing varying amounts of water. Blow across the top of the test tubes. What do you notice? The test tubes resonate at their natural frequency. How does the length of air column affect its natural frequency? The shorter the air column, the higher the frequency. The shorter the air column, the higher the natural frequency.

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© Boardworks Ltd 2005 21 of 31 Mass and natural frequency If these strings are identical apart from their mass, which one will have the highest natural frequency? The smaller the mass, the higher the natural frequency. The string with the smallest mass will have the highest natural frequency.

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© Boardworks Ltd 2005 22 of 31 Length and natural frequency If these strings are identical apart from their length, which one will have the highest natural frequency? The shorter the string, the higher the natural frequency. The string with the shortest length will have the highest natural frequency.

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© Boardworks Ltd 2005 23 of 31 Tension and natural frequency If these strings are identical apart from the tension they are under, which one will have the highest natural frequency? The greater the tension on the string, the higher the natural frequency. The string with the greatest tension will have the highest natural frequency. 10 N 15 N 20 N 25 N

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© Boardworks Ltd 2005 24 of 31 Changing natural frequency

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© Boardworks Ltd 2005 25 of 31 Contents Diffraction Interference Standing waves Summary activities Natural frequency Diffraction, Interference and Resonance

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© Boardworks Ltd 2005 26 of 31 Standing waves Some waves, such as light, move. Other waves, such as those on a guitar string, do not move. These waves are called standing waves. Some parts of the standing waves actually move. These are called antinodes. Some parts of the standing waves do not move. These are called nodes. The lower the number of nodes, the lower the frequency. node antinode

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© Boardworks Ltd 2005 27 of 31 Standing waves and frequency Wave 1 has the highest frequency, because it has the most nodes. N NN N A A A NN A 1. 2. Which standing wave has the highest frequency?

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© Boardworks Ltd 2005 28 of 31 Contents Diffraction Interference Standing waves Summary activities Natural frequency Diffraction, Interference and Resonance

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© Boardworks Ltd 2005 29 of 31 Glossary antinode – The part of a standing wave that moves. constructive interference – When two in-phase waves interfere to produce a larger wave. destructive interference – When two out-of-phase waves interfere to cancel each other out. diffraction – The spreading out of waves as they pass through a gap or round a barrier. natural frequency – The frequency at which an object resonates. node – The part of a standing wave that does not move. resonance – The vibration of an object at its maximum amplitude when it is subject to its natural frequency. standing waves – Waves that do not move.

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© Boardworks Ltd 2005 30 of 31 Anagrams

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© Boardworks Ltd 2005 31 of 31 Multiple-choice quiz

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