KS4 Waves: Diffraction, Interference and Resonance

Using the ripple tank A ripple tank is a device used to study the behaviour of waves, because all waves behave in a similar manner. Paddle vibrates to produce waves A ripple tank produces water waves that can be r_______, r_______ and d________. eflected efracted iffracted

Reflection and refraction What do you think will happen if a barrier is placed in front of the water waves? Paddle vibrates to produce waves If it is a plane barrier than 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.

Barrier with a small gap What do you think will happen if a barrier with a gap in it is placed in front of the water waves? Paddle vibrates to produce waves It depends upon the size of the gap. Barrier 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 a medium gap What do you think will happen if a barrier with a gap similar in width to the wavelength of the waves is used? Paddle vibrates to produce waves Barrier Circular waves are produced, this effect is known as DIFFRACTION.

Barrier with a large gap What do you think will happen if a barrier with a gap larger than the wavelength of the waves is used? Paddle vibrates to produce waves Barrier The waves pass through the gap unchanged apart from slight diffraction of the waves near their ends.

Why??? Diffraction effects Light and sound are both waves. Waves travel in straight lines. When you are in a room, why can you hear people outside of the room in the corridor even though you can not see them. Why??? Light waves have a much shorter wavelength then 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.

Interference + =  When 2 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.

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

Diffraction & Interference  To get 2 waves of light to interfere, the waves must be very similar.  We use a single source of monochromatic light and split it into 2 waves using a diffraction grating like this:  In 1801, a physicist called Young first performed this classic investigation which showed the interference of light waves.

Diffraction & Interference S1 and S2 act as 2 coherent light sources The light source emits rays of light which diffract towards the double slit S1 Fringes S2  The waves interfere - constructively [bright fringes]. destructively [dark fringes].

Diffraction & Interference  What would the fringes look like if white light was used as the source instead?

Diffraction & Interference  The coloured fringes on these CDs are the result of interference. Light reflecting from the Aluminium diffracts and interferes. Some colours are diffracted more than others.

Resonance causes it to shatter!!! Natural frequency 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. Resonance causes it to shatter!!! We call this effect 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?

Columns of air 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? Shorter the air column, the higher the natural frequency.

Vibrating strings - mass 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. This string, with the smallest mass, will have the highest natural frequency.

Vibrating strings - length 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. This string, with the shortest length, will have the highest natural frequency.

Vibrating strings - tension 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 This string, with the greatest tension, will have the highest natural frequency.

Waves that do not move Some waves such as light move, other waves, such as those on a guitar string do not, do you know what these waves are called? Standing waves Some parts of the standing waves actually move what are they called? Antinodes (A) Some parts of the standing waves do not move what are they called? Nodes (N) Fundamental mode of vibration A N N The lower the number of nodes, the lower the frequency.

Which standing wave has the highest frequency? 1. Because it has the most nodes. 1. A A N N N N A A 2. N N

What effect causes a glass to shatter if it is vibrated at its natural frequency? Reflection Refraction Total internal reflection Resonance 

What happens to the natural frequency of an object as its mass decreases? No change Increases Decreases Goes to zero 

What effect causes your car radio signal to go up and down as you drive along? Resonance Interference Reflection Refraction 

What size must the gap in a barrier be to get the greatest diffraction effect for an approaching wave? Twice the wavelength of the wave A lot smaller than the wavelength of the wave A lot larger than the wavelength of the wave The same size as the wavelength of the wave 

Why is light not diffracted by a doorway but sound is? Light has a longer wavelength than sound Light does not travel in waves Light has a much shorter wavelength than sound Sound travels in waves 