# Diffraction, Interference and Resonance

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Diffraction, Interference and Resonance
KS4 Physics Diffraction, Interference and Resonance

Diffraction, Interference and Resonance
Contents Diffraction, Interference and Resonance Diffraction Interference Natural frequency Standing waves Summary activities

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

Reflection and refraction
What do you think will happen if a barrier is placed in front of the water waves? paddle If it is a plane barrier then the waves are reflected. 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. barrier

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.

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.

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.

Effect of diffraction…
How can the teacher in the corridor hear the school band even though he isn’t in the hall?

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

Diffraction, Interference and Resonance
Contents Diffraction, Interference and Resonance Diffraction Interference Natural frequency Standing waves Summary activities

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.

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

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.

Diffraction and interference
The light source emits rays of light, which diffract towards the double slit. S1 fringes S2 Teacher Notes Click ‘fringes’ to jump to the next slide, which contains a front-on photograph of the fringes S1 and S2 act as two light sources. The waves interfere: constructively (bright fringes) destructively (dark fringes)

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

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.

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!

Waves, diffraction and interference

Diffraction, Interference and Resonance
Contents Diffraction, Interference and Resonance Diffraction Interference Natural frequency Standing waves Summary activities

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 shorter the air column, the higher the frequency. 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 natural frequency.

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.

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.

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

Changing natural frequency

Diffraction, Interference and Resonance
Contents Diffraction, Interference and Resonance Diffraction Interference Natural frequency Standing waves Summary activities

The lower the number of nodes, the lower the frequency.
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. antinode node node The lower the number of nodes, the lower the frequency.

Standing waves and frequency
Which standing wave has the highest frequency? 1. A A N N N N A Wave 1 has the highest frequency, because it has the most nodes. A 2. N N

Diffraction, Interference and Resonance
Contents Diffraction, Interference and Resonance Diffraction Interference Natural frequency Standing waves Summary activities

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.

Anagrams

Multiple-choice quiz