Wave and Wave Properties

In a transverse wave, e.g. water wave, the oscillations are perpendicular (at right-angles) to the direction of energy transfer.

• This can be demonstrated by moving a rope or slinky up and down vertically - the waves move vertically.

In a longitudinal wave, e.g. sound wave, the oscilations are parallel to the direction of energy transfer.

• This can be demonstrated by moving a slinky moving backwards and forwards horizontally - the waves also move horizonally.
• All waves transfer energy from one place to another. For example, if a stone is dropped into a pond, the ripples travel outwards carrying the energy. The water doesn't travel outwards (otherwise it would leave a hole in the middle).
• The particles makes a wave oscillate on a fixed point, in doing so, they pass on the energy on to the next particles which also oscilates, and so on.
• The waves transfer energy not matter.

All waves have:

• Frequency - the number of waves passing a fixed point per second, measured in hertz (Hz)
• Amplitude - the maximum displacement that any particle achieves from its undisturbed position in metres (m)
• Wavelength - the distance from one point on a wave to the equivalent point on the next wave in metres (m)
• Period - the time taken for one complete oscillation in seconds (s)

When observing waves (e.g. water waves):

• Amplitude is seen as the wave height.
• The period is seen as the time taken for one complete wave to pass a fixed point.
• Amplitude indicates the amount of energy a wave is carrying - the more energy, the higher the amplitude.                                                                                                       

The speed of a wave is the speed at which the energy is transferred or the wave moves. It's a measure of how far the wave moves in one second and can be found with the wave equation:

• Wave speed (m/s) = frequency (Hz) x wavelenght (m)
• V = fλ
• As waves are transmitted from one medium to another, their speed and therefore their wavelength changes, e.g. water waves travelling from deep to shallow water or sound travelling from air to water. The frequency doesn't change  because the same number of waves is still being produced by the source per second. 

As all waves obey the wave equation, the speed and wavelength is directly proportional:

• doubling the speed, doubles the wavelength
• halving the speed, halves the wavelength

• When the waves reach a boundary between one medium (material) and another, they can be reflected, refracted, absorbed or transmitted.
• When waves are reflected at a surface, the angle of incidence is equal to the angle of reflection. When a wave passes from one medium to another it can be refracted and change direction. The direction of the refraction depends on:

• the angle at which the wave hits the boundary
• the materials involved

For light rays, the way in which a material affects refraction is called its refractive index. When light travels:

• from a material with a low refractive index to one with higher refractive index, it bends towards the normal.
• from a material with a high refractive index to one with a lower refractive index, it bends away from the normal.

Refraction is due to the difference in the wave speed in the different medium.

When a light wave enters at an angle, a medium in which it travels slower:

• the first part of the light wave to enter the medium slows down
• the rest of the wave contiunes at the higher speed
• this causes the wave to change direction, towards the normal.

• Sound waves have a frequency, an ampliltude and wavelength, the amplitude of a sound wave relates to the loudness, the frequency and wavelength of a sound wave relates to the pitch - the higher the frequency, the higher the pitch.
• The normal range of human hearing is from 20Hz to 20kHz (20000Hz).
• Sound in any medium, is due to vibrations of the particles that make up the medium. 
• In a solid these oscillations can cause the entire object to vibrate with the same frequency as the sound wave.
• The conversion of sound waves to vibrations only occurs over a limited range of frequencies. The range of frequencies converted is dependent on the structure of the object.
• Within the ear, sound waves cause the ear drum to vibrate and other internal structures to vibrate and it's this vibration that is heard by sound. 
• The limited range of conversion is what limits human hearing.
• You need to be able to gibe examples of sound waves being converted  to vibrations, e.g. in the ear drum or by a mircophone.

• Ultrasound waves have a frequency greater than 20kHz, so they cannot be heard by humans.
• When an ultrasound wave meets a boundary between two different mediums it is partially reflected.
• It is possible to determine how far away the boundary is by measuring the time taken for reflected ultrasonic waves to return to a detector.
• Uses in the industry include detecting defects in materials without cutting them. These defects could be manufacturing faults (e.g. cracks and air bubbles) or damage (e.g. corrosion).
• Uses in machine include pre-natal scanning, detecting of kidney stones and tumours, and producing images of damaged ligaments and muscles.

• Echo sounding, or sonar, is the use of ultrasonic waves for the detecting objects in deep water and measuring the depth of water.
• It involves sending an ultrasound pulse into the water, which is then reflected back when it hits a surface.

The time between the pulse being sent and reflection being detected is used to calculated the distance travelled by the sound waves:

• distance = speed x time
• The speed of sound in water is 1500 m/s

This will then the total distance travelled by the pulse, which is then divided by two to find the depth of the water, for example:

• distance = speed x time
• = 1500 x 0.5 = 750m
• water depth = 750/2 = 375m

Two types of seismic waves are produced during an earthquake:

P-waves (Primary waves):

• are longitudinal waves.
• travel at the speed of sound and are twice as fast as S-waves.
• travel at different speeds through solids and liquids.

S-waves (Secondary waves):

• are transverse waves.
• are not be able to travel through liquids.

When seismic waves are produced, the difference in time between the arrival of P-waves and S-waves at different detectors can provide evidence about the location of the earthquake and the material they have travelled through.

• Seismic waves travels outwards from the earthquake and are capable of travelling all the way through the Earth.

• The seimic waves travel in a curved path through the Earth, due to the Earth  increasing in density with depth.

• Detectors placed around the Earth measure when and where the different waves arrive.

• There two key pieces of evidence, which come from the S-wave shadow zone and the P-wave shadow zone.

• Seismic waves have provided evidence about the structure of the Earth.

S-wave shadow zone:

• S-waves aren't able to travel through the liquid outer core of the Earth.
• This results in a large shadow zone on the opposite side of the Earth to where the earthquake originated.
• This shadow zone provides evidence of the size of the Earth's core.

P-wave shadow zone:

• P-waves are able to travel through liquid outer core. 
• However, they are refracted at the boundary between semi-solid mantle and liquid outer core.
• These refractions result in P-wave shadow zones.
• The study of these shadow zones is used to determine the size and composition of the inner and outer core.

• Electromagnetic waves are transverse waves.
• All types of electromagnetic waves travel at the same velocity (the speed of light) in air or in a vaccum.
• The electromagnetic spectrum extends from low frequency, low energy waves to high frequency, high energy waves.
• Human eyes are only capable of detecting visible light, i.e. a very limited range of electromagnetic waves.
• The wavelength of an electromagnetic wave affects how it is absorbed, transmitted, reflected or refracted by different substance. This also affects its uses.

Uses:

• Television
• Radio
• Bluetooth

Explanations:

• Radio waves are low energy waves therefore they are not harmful, making it ideal for radio transmissions.

Uses:

• Satellite
• Communications
• Cooking food

Explanations:

• Microwaves travel in straight lines through the atmosphere 
• This makes them ideal for transmitting signals to sateillites in orbit and transmitting them back down to receivers.

Uses:

• Electrical heaters
• Cooking food
• Infrared cameras

Explanations:

• Electrical heaters, grills, toasters, etc. glow red hot as the electricity flows through them.
• This transmits infrared energy that is absorbed by the food and converted back into heat.

Uses:

• Fibre optic communications
• Energy for plants through photosynthesis

Explanations:

• Visible light travels down optical fibres from one end to the other without being lost through the sides

Uses:

• Energy efficient light bulbs
• Security marking
• Sunbeds

Explanations:

• In energy efficient light bulbs, UV waves are produced by the gas in the bulb when it is given an electric current.
• These UV waves are absorbed by the coating of the bulb, which fluoresces giving off visible light.

Uses:

• Medical imaging and treatments

Explanations:

• X-rays are able to penetrate soft tissue but not the bone
• A photographic plate behind a person will show the shadows of where the bones are

Uses:

• Sterilising food
• Treatment of tumours

Explanations:

• Gamma rays have the most energy compared to all the electromagnetic waves and it can be used to destroy bacteria and tumours

• Radio waves can be caused by oscillations in electrical circuits (by an alternating current)
• The frequency of the radio wave produced matches the frequency of the electrical oscillation, this is how a radio signal is produced.
• When radio waves are absorbed by a conductor this creates an alternating current with the same frequency as the radio wave, this is how a signal is recieved.
• When this oscillation is induced in an electrical circuit it creates an electrical signal that matches the wave.

Changes in atoms and the nuclei of atoms can result in EM waves being generated or absorbed over a wide frequency range:

• Electrons moving between energy levels as a result of heat or electrical excitation can generate waves, e.g. infrared waves, visible light, ultraviolet waves and X-rays.
• Changes in the nucleus of an atom can generate waves, i.e. an unstable nucleus can give out excess energy as gamma rays.

Ultraviolet waves, X-rays and gamma rays carry enough energy to have hazardous effects on the human body:

• Ultraviolet waves can cause the skin to age prematurely and increase the risk of skin cancer.
• X-rays and gamma rays are ionising radiation - they can damage the cells by ionising atoms and, if absorbed by the nucleus of the cell, can cause gene mutations and cancer.

Visible light describes an electromagnetic wave that can be detected by the human eye.

• When light is incident on (arrives at and hits) an object, it can be absorbed, reflected or transmitted.
• Reflection by a smooth surface in a single direction (e.g. by a mirror) is called specular reflection.
• Reflection from a rough surface, where the light is scattered, is called diffuse reflection.

All objects are either:

• Transparent - they transmit light coherently (the light rays do not get jumbled up) so objects on the other side can be seen clearly.
• Translucent - they transmit light, but the rays are scattered so objects cannot be seen clearly through them, e.g. frosted glass.
• Opaque - they either reflect or absorb all light incident on them, so no light passes through.

Each colour within the visible spectrum has its own narrow band of wavelength and frequency.

When an opaque object appears coloured:

• it's reflecting  light of that particular wavelength
• it's absorbing other wavelengths

If all wavelengths are reflected equally, the object appears white whereas if all wavelengths are absorbed, the object appears black.

Coloured filters work by absorbing some wavelengths and not others, it filters out some of the colours while letting others through.

If the filter is the same colour as the object, the object will appear its true colour. 

If the object is a different colour to the filter then a few things will happen:

• A red and blue striped object seen through a red filter will appear red and black. This is because the filter will allow the red light through, but not the blue light.
• The same object seen through a green filter it will appear completely black. This is because the filter will not allow red or blue to pass through it.

All bodies (objects) emit and absorb infrared radiation.

The rate at which an object emits radiation depends on the nature of the surface and on its temperature - the hotter the body, the faster it emits infrared radiation.

A perfect black body:

• absorbs all the infrared radiation incident on it
• doesn't reflect or transmit any infrared radiation

Since a good absorber is also a good emitter, a perfect black body is also the best possible emitter.

The temperature of an object (or body) determines:

• the rate at which it emits radiation
• the wavelength of the radiation it emits

As the temperature increases the amount of radiation an object emits at all wavelengths increases, but the intensity of shorter wavelengths increase faster.

As an object is heated, it first glows red hot. As it gets hotter, it emits even shorter wavelengths and it becomes whiter.

• The temperature of an object is related to the balance between radiation absorbed and radiation emitted.
• The ground on a sunny day will increase in temperature when the sun is out, this is because it absorbs radiation from the sun faster than it emits radiation. As the ground gets warmer, the rate at which it emits radiation will increase. Eventually, the rate of emission is equal to the rate of absorption, and the ground will then be at constant temperature. This effect applies to many other situations, e.g. a radiator cooling down, a house, an object in front of radiant heater and the planet Earth itself.

The temperature of the Earth depends on many things:

• how much energy it recieves from the sun
• how much energy is reflected back into space
• how much energy it emits into space

The Earth's atmosphere also affects how much of the radiation emitted from the surface escapes into space.

• A lens forms an image by refracting light. 
• There are two main types of lens: convex and concave.

A convex lens is wider in the middle than at the edges:

• Parallel rays of light entering a conve lens are brought to a focus at the principal focus or focal point. As parallel rays of light entering a convex lens converge (come together), thye are sometimes called converging lenses. 
• The distance from the lens to the principal focus is the focal length.

A concave lens is wider at the edges than it is at the middle:

• Parallel rays of light entering a concave lens spread out.This makes the rays appear to have come from the principal focus on the same side of the lens that they originated.
• Because the parallel rays of light entering a concave lens diverge, they are sometimes called diverging lenses.

Convex lenses can produce real or virtual images.

Concave lenses only produce produce virtual images.

A real image is on the opposite side of the lens to the object and can be projected onto a screen.

A virtual image is on the same side of the object and can only be seen by looking through the lens.

Magnification is the ratio of image height to object height, i.e. a magnification of 2 means the image is twice the size of the object. As magnification is a ratio, it has no units.

Magnification = image height/object height