Waves

What do waves do?

• Transfer energy from one place to another (but don’t transfer any matter)

• Waves vibrate and oscillate to travel from one place to another

What do the distance and displacement represent on the graph?

• Distance (horizontal) how far the wave has travelled

• Displacement (vertical) how far from equilibrium the wave has oscillated (up or down)

What is the amplitude?

The maximum displacement (from the start line)

What is the wavelength?

One entire oscillation

It could be crest to crest or trough to trough

What is one complete oscillation on a displacement time graph?

The time period (the time it takes for one complete oscillation)

We can then work out frequency from this

What unit should wavelength be in?

meters

What are transverse waves?

• The oscillations are perpencicular to the direction of energy transfer (why the vibrations are going up and down whilst the wave is moving from left to right)

• All EM waves, ripples, waves on strings

What are longitudinal waves?

• Oscillations are parallel to the direction of energy transfer

• Waves vibrating back and forth- spread out and compressed

• Compression moves from left to right

• Sound waves, seismic p waves

What happens when a wave arrives at a boundary of a material?

• The wave could be absorbed- energy transferred

• Wave could be transmitted- enters material but passes out other side- leads to refraction

• It could be reflected- never enters

• Depends on length of wavelength and properties

How to draw ray diagram?

• Angle of incidence = angle of reflection

• Start at bottom- with boundary (surface)

• Draw incoming raw- arrow to show down

• Draw the normal- perpendicular to the surface

• Measure angle between incoming ray and normal (angle of incidence)

• Draw the angle of reflection (same) on the other side of the normal

What is the point of incidence?

Where the incoming ray touches the boundary/surface

Different types of surfaces

• Flat boundary- normal is same direction so reflections are the same

  • Specular reflection- clear image e.g. mirror

• Rough surface- bumpy boundary- normals will be in different directions- 90o to different surfaces

  • Light will be reflected in different directions

  • Diffuse/scattered reflection

What is the refraction of light waves?

When waves change direction as they pass from one medium to another

Wave speeds

• Waves travel at different speeds in different materials (mediums)

• Because different mediums have different densities

• Higher density- slower wave will travel through

• If wave travelling perpendicular (straight forwards) it will travel straight through the next material

• However if it hits the next medium at an angle, it will be refracted (direction will change)

What happens if a wave is refracted into a more dense medium?

It will bend towards the normal

How to draw ray diagrams?

1. First, draw normal at the point of incidence (where the ray hits the medium)

2. More dense- slow down and bend towards normal

3. Draw a new line across but closer to normal (the refracted ray)all through medium

4. After exits medium, from more dense to less dense, bend away from normal (the emergent ray)

5. Add the angle of incidence (between incident ray and normal) and refraction (between refracted ray and normal)

What happens as the wave speed changes?

• the wave length also changes because the frequency always stays the same

How are different wavelengths refracted by different amounts?

White light (all wavelengths) through triangular prism, different colours will bend to different degrees and spread out

Mnemonic for EM spectrum

• Rude Mum Is Very Useful eXcept Grumpy

• All transverse waves

• In a vacuum- all travel at same speed

• Different mediums- travel at different speeds

Trends as you go along the line, left to right

Frequency increases

Wavelength increases

Inversely proportional

Which part of the spectrum can we detect?

Visible light

Different wavelengths give us different colours

Which waves on the EM spectrum are ionising?

Ultra violet, X-rays and gamma

They damage our cells

What are some sources of waves?

• Radioactive decay for gamma

• Visible, UV and X-rays emitted when electrons drop down energy levels

• Infrared- when bonds holding molecules vibrate

• Can all either be reflected, absorbed or transmitted

Radio waves

• Longest wavelength, lowest frequency

• Use transmitter connected to oscilloscope (see frequency of AC)

• Receiver absorbs energy from transmitter- another oscilloscope

• Frequency of each AC the same

• Allows us to transfer information trough generating and interpreting

What does EM mean?

• EM means made up if oscillating electric and magnetic fields- can be generated by electric current made up of oscillating charges

Use of radio waves for communication

• Long, short and very short

• Long- transmitted huge distances because they diffract around the curved surface of the area

• Short- long distances but are reflected from the ionosphere- long distances

• Short distances- Bluetooth

• Very short- TV, directly from transmitter to receiver

Microwaves

• Aren’t absorbed by water molecules- used for satellite- have to pass through earths atmosphere- received by satellites and transmitted back down to earth where they can be detected with satellites dish

• Absorbed by water molecules- microwaves- energy absorbed causing to vibrate, transfer energy to spread through food

Infrared radiation

• Emitted from all objects that have thermal energy

• Hotter object- more IR radiation

• Infrared cameras to spot living organisms- warm so emitting IR so appear bright

• Cooking and heaters- make metal emit IR radiation which can heat food by transferring heat energy

Are microwaves and IR waves harmful?

In large quantities- background radiation

Visible light

• Light we use to see- different colours depending on wavelength

• Red- longest, violet- shortest

• Used for communication with optical fibres- transmit pulses of light over long distances - transmit data

  • Specular reflection of light

• Copper wires to transmit electricity- much less and signals more likely to be distorted

UV radiation

• Sun radiation- from sun beds artificially

• Fluorescence- UV light absorbed and energy re-emitted as visible light (fluorescent lights) which we can see, energy efficient

• Security- make codes and mark properties, bank notes

• Sterilise water- destroys microorganisms

X-rays

• Help us view internal structure of objects

• X-rays absorbed by dense materials (bones) but can pass through parts that are mostly air (lungs)

• Dense areas appear white because receive radiation

• Detect broken bones

• Small amount of radiation

Gamma Rays

• Sterilise medical equipment and food

• Kill microorganisms without causing other damage

• Treat disease