Waves –The Basics
Waves have amplitude, wavelength and frequency.
- Frequency is the number of waves passing past a certain point per second or the number of waves produced by a source each second. It’s measure in hertz (Hz). 1 Hz is 1 wave per second.
- Transverse waves have sideways vibrations
Most waves are transverse:
- All EM waves (light ect), Ripples on water, Waves on strings , A slinky wiggled up and down.
In transverse waves the vibrations are at 90° to the direction of the wave.
Longitudinal waves have vibrations along the same line.
- Sound and ultrasound, Shock waves (seismic waves) , A slinky spring, when you push the end.
Wave speed = Frequency x Wavelength
V = Wave speed (m/s), F = Frequency (Hz), Squiggle = Wave length (m)
· When a wave hits a material all waves can be reflected, refracted (through material but change direction) and diffracted (bends round making it spread out).
- Angle of incidence = Angle of reflection
- When a wave hits a boundary between one medium and another, some is always reflected.
Wave properties Part 2
· Refraction – Waves change speed and direction.
- Waves travel at different speeds in substances of different densities.
- EM waves normally travel slower in denser mediums, sounds waves travel faster.
- If they hit a boundary straight on, they go straight on; if they hit it at an angle they change direction.
- The reason it goes in and turns, and then comes back out in the same direction is because it bends towards the normal when entering a more dense material, and then bends back when into a less dense material, so it ends up in the same direction (if the materials are the same).
- Waves have the same change in speed and wavelength, so the frequency doesn’t change.
Wave Properties Part 3
- Total internal reflection happens above the critical angle.
- When a light wave travels from a dense substance like glass or water into a less dense subject like air, its angle of refraction is greater than the angle of incidence.
- If something going from air to glass, hits the glass at 42° or above, total internal reflection takes place, and non is refracted at all.
- This happens in wires and things.
- Diffraction – Waves spreading out
- All waves spread out (diffract) at the edges when they pass through a gap or past an object.
- The amount of diffraction depends on the size of the gap relative to the wavelength of the wave.
- The longer the wavelength and narrower the gap the more the wave spreads out.
- A narrow gap depends on the size of the wavelength.
- A narrow gap is about the same size of the wavelength in question.
· When waves meet they cause disturbance.
- When waves are in step you add the placement, when they aren’t you subtract it.
- Interference of light makes bright and dark patches.
- Constructive interference makes it bright, destructive interference makes it dark.
Light and sound must be waves because :
- They can diffracted (bend around an object)
- If light acted like particles there wouldn’t be any positive or negative disturbance, they wouldn’t interfere.
Rabbits (radio waves) Mate (microwaves) In (infra-red) Very (visible) Unusual (unltraviolet) eXpensive (X-rays) Gardens (gamma rays).
- As you go → the frequency increases (so does energy)
- And as you go ← the wavelength increases.
- All waves travel at 300,000,000 (300million) m/s in a vacuum.
- Even different colours of light have the different wavelength
- Sounds waves can’t travel through a vacuum.
- Sound waves are transmitted by vibrating particles. So no particles in a vacuum, so no sound.
- The energy of a photon depends on a frequency of EM waves.
- Energy delivered by each photo depends on the frequency of the EM waves.
- The higher the frequency, the higher the energy the photon delivers. Intensity depends on the number of photons per second.
Uses of EM waves
- Radio waves- Communication - long distances, dont get absorbed by atmosphere.
- Used for TV and FM radio ( short wavelength compared to normal radio waves)
- Microwaves Satellite communication - passes through atmosphere easily
- absorbed by sat. receivers well which sends it back to earth to be absorbed by a satellite on the ground .
- Ovens, pass through moist atmosphere but are absorbed by water.
- X-Rays - identify fractures . Bones absorb it, flesh doesnt. Where it goes through, the plate turns back, and bones stay white.
- Infrared and light - Optical fibres - The single fibre doesnt weaken much and carry a lot of information.
Analogue and Digital Signals
- Analogue signals vary. Digitals either on or off.
- The amplitude or frequency of an analogue signal varies continuously; it could be any value in a particular range.
- Digital signals are pulses.
- A digital receiver will decode the pulses to get a copy of the original signal.
- Signals have to be amplified.
- They weaken as they travel. So they are amplified on their route.
- They also pick up interference.
- Digital signals are far better quality.
- Its easier to clean up a signal that only consists of like 2 different possibilities, than one with a large range.
- So digital signals have a much higher quality sound.
- Digital signals can be processed by computers too.
- You can send more than one signal at once with digital signal.
Information and waves
- Information is converted into signals.
- 2 waves you can send information as waves, AM and FM.
- AM radio waves have varying amplitude.
- An AM radio transmitter sends out a continuous carrier wave.
- This signal is the superimposed on the carrier wave using amplitude modulation.
- FM waves have varying frequency.
- When there’s a trough in the signal, the frequency is increased, or on a peak it is increased. Or vice versa.
- Receivers recover the original signal.
- They ignore the carrier part of the wave and extract the original signal.
The topics have only been summarised. Some key points may be missing :P apologies for this inconvenience. Please use your own resources in conjunction with these revision cards.
AND GOOD LUCK BHIRUNTHA!