Physics - Waves

What are Polarised Waves?

A Polarised wave is a wave in which all vibrations are in one direction or phase.

What is a Stationary Wave?

A Stationary Wave is the superposition of two progressive waves with the same frequencies (or wavelength) and amplitude, moving in opposite directions. Unlike progressive waves, no energy is transmitted from a stationary wave.

What is a Polarised Filter?

A polarised filter is a filter that only transmits vibrations of a wave in one direction or plane, called the plane of transmission.

What is a Progressive Wave?

A progressive wave is a moving wave that caries energy from one place to another without transferring any material.

How is a wave caused?

A wave is caused by something making particles or fields (e.g electric or magnetic fields) oscillate at a source.

How quickly do electromagnetic waves move in a vacuum?

All electromagnetic waves, including light, travel at the speed of 3.00x10⁸ ms⁻¹ in a vacuum.

What is Amplitude (A) of a wave?

Amplitude is measured in metres and is the maximum magnitude of the displacement i.e. the distance from the undisturbed position to the crest or trough.

How can you work out the number of nodes, antinodes, wavelengths and frequency of a harmonic?

At the x harmonic, the number of antinodes is equal to x and the number of nodes is x+1. The number of wavelengths is x/2. If you are given the first harmonic frequency, you can work out the resonant frequency f at any harmonic with f=x*resonant frequency.

What factors affect the Resonant Frequencies?

Changing the mass, length and tension of the string changes its resonant frequency.

Can amplitude or displacement be negative?

Displacement can be negative, amplitude cannot.

What is the Displacement (x) of a wave?

Displacement is a vector quantity which is measured in metres, m, and shows how far a point on the wave has moved from its undisturbed position.

What does the D stand in the double slit formula?

Distance between slits and screen in metres.

What does the S stand in the double slit formula?

Distance between slits in metres.

What are some examples of polarisation in the real world?

Examples of polarisation include: Glade reduction, improving TV and radio signals and photography.

What is the Frequency (F) of a wave?

Frequency is measure in hertz and is the number of whole wave cycles (oscillations) per second passing a given point. Also can be defined as the number of whole wave cycles (oscillations) given out from a source per second.

What does the W stand in the double slit formula?

Fringe spacing - the distance between two adjacent maxima or two adjacent minima in metres.

What is Destructive Interference?

If a wave with a positive displacement (crest) meets a wave with a negative displacement (trough), they will undergo negative interference and cancel each other out. The displacements of the combined wave is found by adding the displacements of the two waves. A crest and trough won't cancel out exactly unless they have the same amplitude.

What is Total Destructive Displacement?

If two waves with equal and opposite displacements meet (e.g. a crest and a trough with equal displacements), they cancel each other out completely. This is called total destructive displacement.

What is a Longitudinal wave?

In a longitudinal wave, the oscillation are parallel to the direction of energy transfer.

What is a transverse wave?

In a transverse wave, the oscillations are perpendicular to the direction of energy transfer.

How can you observe the diffraction of light?

Light shone through a narrow slit will diffract and sometimes produce a diffraction pattern. In order to observe a clear diffraction pattern, you should use a monochromatic, coherent light source. If a light isn't monochromatic, different wavelengths will diffract by different amounts and the pattern produced won't be very clear. Laser light can be used as it is coherent and monochromatic or you could put a colour filter in front of white light but you get clearer diffraction pattern with a laser.

What are examples of longitudinal waves?

Longitudinal wave examples include sound and P-waves.

What is Monochromatic Light?

Monochromatic light is light of a single wavelength (and frequency) and so a single colour.

What are Nodes and Antinodes?

Nodes are the points on the wave where the amplitude of vibration is zero (minimum amplitude) so they stay perfectly still. Antinodes are points of maximum amplitudes.

What waves can be polarised?

Only transverse waves can be polarised however longitudinal waves cannot as they only have 1 direction of oscillation.

What is the Period (T) of a wave?

Period is measured in seconds and is the time taken for one whole wave cycle, e.g. from one crest to another.

What is the Phase of a wave?

Phase is a measurement of the position of a certain point along the wave cycle.

How can you calculate the Wave Speed?

Speed = Distance (m) / Time (s).

What are Resonant Frequencies?

Stationary waves which are transverse are only formed at resonant frequencies which is where an exact number of half wavelengths fit on the string. Each resonant frequency has a special name.

What is Superposition?

Superposition means 'one thing on top of another thing'. It occurs when two or more waves pass through each other. At the instance that waves cross, the displacement due to each wave combine. Then each wave continues on it's way.

How is polarisation used to improve TV and Radio signal?

TV signals are polarised by the origination of the rods on the transmitting aerial. To receive a strong signal, you have to line up the rods on the receiving aerial with the rods on the transmitting aerial - if they aren't aligned, the signal strength will be lower so the rods on the TV aerials are all horizontal.

How can you tell that waves carry energy?

The 4 main ways that you can tell that waves carry energy are: ∙ Electromagnetic waves cause things to heat up. ∙ X-rays and Gamma rays knock electrons out of their orbits, causing ionisation. ∙ Loud sounds cause large oscillations of air particles which can make things vibrate. ∙ Wave power can be used to generate electricity.

How does changing the length of string affect the Resonant Frequency?

The Longer the string, the lower the resonant frequency because the half-wavelength is long.

What is the Second Harmonic?

The Second Harmonic is also known as the First Overtone. It has twice the frequency of the 1st harmonic. There are two 'loops' with a node in the middle and one at each end. Two half wavelengths fits on the string so the wavelength i the length of the sting.

What does the diffraction of monochromatic light look like?

The diffraction of light is shown by shining a laser beam through a very narrow slit onto a screen. If the wavelength of a light wave is roughly similar to the size of the apparatus (slit), you get a diffraction pattern of light and dark fringes. The pattern has a bring central fringe (central maximum) with alternating dark and bright fringes on either side of it.

Why is there alternating bright and dark fringes in the diffraction of monochromatic light?

The fringe pattern is due to interference: ∙ The bright fringes are due to constructive interference, where waves from across the width of the silt arrive at the screen in phase. ∙ The dark fringes are due to total destructive interference, where waves from across the width of the silt arrive at the screen completely out of phase.

How does changing the mass of string affect the Resonant Frequency?

The heavier (the more mass per unit length) the string, the lower the resonant frequency - because waves travel more slowly down the sting.

How does changing the tension of string affect the Resonant Frequency?

The lower the tension of the string, the lower the resonant frequency because waves travel more slowly down a loose string.

How is energy transferred in a wave?

The oscillations pass through the medium (or field) as the wave travels, carrying energy with it. A wave transfers this energy away from its source - so the source of the wave loses energy.

What is the Phase Different of a wave?

The phase difference is the amount by which one wave lags behind another. It is the difference between two identical points (such as maximum displacement) on two waves.

What is Diffraction?

The way that waves spread out as they come through a narrow gap or round obstacles is called diffraction.

What is the First Harmonic?

This stationary wave is vibrating at its lowest possible frequency called the first harmonic. It has one 'loop' with a node at each end and an antinode in the middle. One half wavelength fits onto the string and so the wavelength is double the length of string.

How can you convert between radians and degrees?

To go from radians to degrees you multiply by (180/π). To go from degrees to radians you multiply by (π/180). π radians is equal to 180 degrees.

What are examples of Transverse waves?

Transverse wave examples include electromagnetic waves, S-waves, ripples on water and waves on string.

When are two waves in phase?

Two waves are in phase when they have a phase difference of 0 degrees or 360 degrees (2π radians).

What is Wavelength (λ) of a wave?

Wavelength is measured in metres and is the length of one whole wave oscillation or wave cycle, e.g. the distance between two crests (or troughs) of a wave.

How can waves change direction?

Waves can change direction in 3 main ways: ∙ Reflection - The wave is bounced back when it hits a boundary (Such as seeing your reflection in a mirror). ∙ Refraction - The wave changes direction as it enters a different medium. The change of direction is a result of the wave slowing down or speeding up. ∙ Diffraction - The wave spreads out as it passes through a gap or round an obstical

How is Diffraction affected by obstacles?

When a wave meets an obstacle, you get diffraction around the edges. Behind the obstacle is a 'shadow', where the wave is blocked. The wider the obstacle compared with the wavelength of the wave, the less diffraction you get, and so the longer the shadow.

How is polarisation used to reduce glare?

When light reflects off some surfaces such as water it becomes partially polarised (the light starts to oscillate parallel to the surface) this causes glare. The fact that reflected light is partially polarised allows us to filter some of light out with polarised filters. If you view partially-polarised reflected light through a polarising filter at the right angle, you can block out some of the reflected light, while still letting through light which vibrates at the angle of the filter. This reduces the intensity of light entering you eye.

Why does more diffraction occur with sound than light?

When sound passes through a doorway, the size of the gap and the wavelength are roughly equal so lots of diffraction occurs so you can hear someone well. The reason that you can't see them is that when light passes through the doorway, it is passing through a gap around a million times bigger than its wavelength - the amount of diffraction is tiny. So to get noticeable diffraction with light you must shine it through a very narrow silt.

What is Constructive Interference?

When two waves meet, if there displacements are in the same direction, the displacements combine to give a bigger displacement. A crest plus a crest gives a bigger crest. A trough plus a trough gives a bigger trough. This is known a constructive interference.

What happens when white light is diffracted?

White light is actually a mixture of lots of different colours each with different wavelengths. When white light is shone through a single narrow silt, all of the different wavelengths are diffracted by different amounts. This means that instead of getting clear fringes (as you would with monochromatic light source) you get a spectrum of colours.

How can you form a Stationary Wave?

You can demonstrate stationary waves by setting up a driving oscillator/vibrator generator at one end of a stretched sting with the other end fixed. The wave generated by the oscillator is reflected back (off the boundary) and forth. This creates a stationary wave as the two progressive waves are superposed.

How many Harmonics are there?

You can have as many harmonics as you like - an extra loop and an extra node are just added with one, the number of wavelengths that fit goes up by 1/2, and the frequency increases by the value of the frequency of the first harmonic.

What does the amount of diffraction depend on?

∙ All waves diffraction but the amount of diffraction depends on the wavelength of the wave compared with the size of the gap: ∙ When the gap is bigger than wavelength - diffraction is unnoticeable. ∙ You get noticeable diffraction though a gap several wavelengths wide. ∙ The most diffraction occurs when the gap is the same size as the wavelength. ∙ If the gap is less that the wavelength, the waves are mostly just reflected back.

How can you demonstrate stationary waves?

∙ You can set up a stationary wave by reflecting a microwave beam at a metal plate. The superposition of the wave and its reflection produces a stationary wave. You can find the nodes and antinodes by moving the probe between the transmitter and reflecting plate. The meter or loudspeaker receives no signal at the nodes and maximum signal at the antinodes. ∙ Powder in a tube of air can also show stationary sound waves. A loudspeaker produces stationary waves in the glass tube. The powder laid along the bottom of the tube is shaken away from the antinodes but left undisturbed at the nodes.