Waves

What do all waves do?

Transfer energy from one place to another

How do transverse waves move?

• They move up and down in movements called oscillations

• The oscillations are perpendicular to the direction of energy transfer

• Not all transfer require a medium

How do longitudinal waves move?

• Moves particles side to side

• Has areas of compressions (close together particles) and rarefactions (spaced out particles)

• The oscillations are parallel to the direction of energy transfer

• Require aa medium to travel through the air e.g. air, a liquid or a solid

What are oscillates?

Move backwards and forwards

Key fact on waves

In ripples on the water or sound waves in the air, it is the wave that moves and not the water or air

What is amplitude?

The maximum displacement of a point on a wave away from its undisturbed position

What is wavelength?

The distance from a point on one wave to the equivalent point on the adjacent wave

What is the symbol for wavelength?

Lambda λ

How do we measure wavelengths on longitudinal waves?

• Measure from one compression to next compression

• Measure from one rarefaction to next rarefaction

What is the frequency?

• The number of waves passing a point each second

• 1Hz = 1 wave per second

What is a period?

• The time (in seconds) for one wave to pass a point

• Period (s) = 1 / frequency Hz

What is the equation for wave speed?

V = F x λ

Required practical 8: Ripple tank

• Use a ripple tank (a shallow tray of water that has a vibrating bar that creates waves across the water surface which is connected a to a power pack) which a lamp above it and a sheet of white paper under it

• Record the waves on a mobile phone

Wave length

• To measure the wave length place a ruler on the paper and then freeze the image of the waves

• Measure the distance between one wave and ten waves further - divide the number you get buy ten to get the average length

Frequency

• Place a timer next to the paper and count the number of waves passing a point in one second

Required practical 8: Waves in a solid

• A string with one end attached to a vibration generator and the other end of the string attached to a hanging mass

• The vibration generator is attached to a signal generator which allows us to change the frequency of vibration of the string so when the power is turned on the string vibrates

• This creates a standing wave due to an effect called resonance

• To measure the wavelength of the wave we use a ruler

• Measure from the wooden bridge to the vibration generator

• Use the wave speed equation

• You can read the frequency from the signal generator

What is a electromagnetic waves?

• Transverse waves

• They transfer energy from the source of the waves to an absorber

What colour has a low/high frequency and long/short wavelength?

Red, Orange, Yellow, Green, Blue, Indigo, Violet

Low frequency High frequency

Longer wavelength shorter wavelength

What is the electromagnetic spectrum?

Radio, Microwaves, Infrared, Visible light, Ultraviolet, X-rays, Gamma Rays

Low frequency High frequency

Long wavelength Short wavelength

Abbreviation for the electromagnetic spectrum

Raw - radio

Meat - microwave

Is - infrared

Very - visible light

Unsanitary - ultraviolet

eXcept - x-ray

Giraffe - gamma rays

Features of electromagnetic spectrum

• Don’t need a medium to travel

• All travel at the same speed at a vacuum - speed is 300,000,000m/s

• Different materials absorb, transmit or reflect electromagnetic waves

What is refraction?

The change in direction of a wave (like light) as it passes from one medium to another, due to a change in speed

What are the key points of refraction?

• Waves can change direction when they change speed, moving from one medium to another

What happens when wave fronts travel through glass?

• When the wavefronts move into the glass, parts of the wavefronts slow down

• This causes those parts of the wavefronts to get closer together and gets smaller

• This causes the waves to change direction towards the normal (refracting)

• When waves speed up, they change direction away from the normal

Required practical 10: Infrared

• Use a Leslie’s cube (has a shiny metallic side, white side, shiny black side and matt black side)

• Fill the cube with hot water and point an infrared detector at each of the four surfaces and record the amount of infrared emitted (keep the same distance between the two so the experiment is repeatable)

• The matt black emits the most infrared emission and the shiny metallic emits the least infrared emission

  • Matt black , shiny black, white, shiny metallic

What can you use instead of infrared detector?

Paint light bulb black - less detectable

How is the absorbance of infrared emissions absorbed?

• Place a infrared heater and on either side place two metal plates

• One plate has been painted with shiny metallic paint and the other side painted matt black

• On the other side of the plates, use Vaseline to attach a drawing pin

• Switch on the heater and start timing

• The temp of the metal plate increases as the infrared is absorbed

• Record the time it takes for the Vaseline to melt and the drawing pins to fall off

• It falls off the matt black plate first as it absorbs more infrared and infrared tends to reflect from shiny metallic

What happens when electromagnetic waves are absorbed?’

Changes take place in atoms or the nuclei of atoms

What happens when atoms are heated?

• Electrons move from one energy level to a higher one

• When it returns to its original energy level, it generates an electromagnetic waves

What happens when there is a change on the nucleus?

• Gamma rays can be emitted by the nucleus of atoms

• Once the gamma ray has been emitted, the nucleus has less energy then it had at the start

What happens when electromagnetics are absorbed?

Cause changes to atoms e.g. electrons can change energy levels

What are the hazards of electromagnetic waves?

• Potentially very hazardous

• Ultra violet waves can cause risks of skin cancer and cause skin to age prematurely

• X-rays and gamma rays are ionising radiation - knock electrons off atoms and can cause mutation of genes

What is the dose of radiation measured in?

Sieverts or millisieverts

How are radio waves produced?

• When electrons oscillate (move backwards and forwards) in electrical circuits

• These radio waves can be absorbed

• This causes electrons in the circuit to oscillate

• This creates a alternating current with the same frequency as the radio waves

What are radio waves used for?

Radio and TV signals

What is terrestrial TV?

Received using a aerial

Why are radio waves used?

• They can travel long distances before being absorbed e.g. buildings and trees

• Can reflect off a layer of charged particles in the atmosphere

What are microwaves used for?

Heating food because most foods contain water molecules which absorb the energy of microwaves

What is infrared used for?

Used to cook food in ovens and in electrical heaters - infrared is easily absorbed

Infrared cameras - check for heat

What are visible light used for?

• Communication using fibre optics (think strands of glass which transmit pulses of light down these fibres to carry information)

• Used to carry telephone and cable TV signals

What is ultraviolet used for?

• Energy efficient light bulbs

• Tanning beds

What are gamma rays and x-rays used for?

• Medical imaging

• Gamma rays are used to detect cancers

• Used in medical treatments e.g. cancer