Chapter 4 - Waves Flashcards
What are mechanical waves?
- Waves that pass through a substance
- Cannot travel through a vacuum
- e.g. sound waves, seismic waves, waves on a string
What are mechanical waves made up of?
Vibrations passing through the substance
How are sound waves in air created?
By making a surface vibrate so it sends compression waves through the surrounding air
What are electromagnetic waves?
- Oscillating electric and magnetic fields that progress through space without the need for a substance
- Can travel through a vacuum
- A vibrating electric field generates a vibrating magnetic field, which generates a vibrating electric field further away, and so on
What are longitudinal waves?
Waves in which the direction of vibration of the particles is parallel to (along) the direction of energy transfer/direction in which the wave travels
Examples of longitudinal waves
Sound waves, primary seismic waves, compression waves along a spring
What are compressions of a longitudinal wave?
Areas of high pressure - particles closer together
What are rarefactions of a longitudinal waves?
Areas of low pressure - particles further apart
What are transverse waves?
Waves in which the direction of vibration is perpendicular to the direction of travel/energy transfer
Examples of transverse waves?
Electromagnetic waves, Secondary seismic waves, waves on a string
When are transverse waves plane-polarised?
When vibrations stay in one plane (direction) only
When are transverse waves unpolarised?
- When vibrations change from one plane (direction) to another
- Can vibrate in all directions perpendicular to the direction of propogation (travel)
Why can longitudinal waves not be polarised?
- Particles only vibrate in the same direction that the wave travels
- Can only travel in one plane, so we do not refer to them as being polarised or unpolarised
How do transverse waves become polarised?
- Unpolarised waves travel through a polariser (e.g. a polaroid filter for light), which only lets vibrations in a certain direction through, according to the alignment of its molecules
- Polariser blocks waves of other planes/polarisations so they cannot pass through
What happens when light passes through two polaroid filters at 0°/180°/360° to each other
Light intensity at a maximum (that it can be when polarised, but less than if the light was unpolarised)
What happens when a second polaroid filter is rotated from being in alignment with a first filter to being at 90° to it
- Light intensity reduces from a maximum as the second filter is rotated
- Intensity at a minimum when the second filter reaches 90°
- First filter polarises light in one plane, second filter polarises light at a right angle to this plane, so now a minimum amount can pass through
What happens when a second polaroid filter is rotated form being at 90° to a first filter to being in alignment with it
- Light intensity increases from a minimum as the second filter is rotated
- Intensity at a maximum when the second filter is aligned wioth the first
- Both filters polarising the light in the same plane, so second filter has no overall effect on intensity and it remains at a maximum
What is the plane of polarisation of an electromagnetic wave?
The direction in which the electric field oscillates
What effect do polaroid sunglasses have on light?
- Reduce the glare of light reflecting off of water or glass
- Reflected light is polarised and intensity reduced when it passes through the polaroid sunglasses
What is the displacement of a wave/vibrating particle?
Its distance and direction from its equilibrium position
What is the amplitude of a wave?
The maximum displacement of a vibrating particle:
- Height or depth of a transverse wave from its equilibrium position
- Distance between particles in areas of compression (the denser the compression, the higher the amplitude)
What is the wavelength of a wave?
The least distance between two adjacent vibrating particles with the same displacement and velocity at the same time
- Distance between adjacent crests of a transverse wave
- Distance between adjacent compressions of a longitudinal wave
What is one complete cycle of a wave?
Motion of a wave from one maximum displacement to the next maximum displacement
- From one wave peak to the next
- Wavelength is the distance of one complete cycle
What is the period of a wave?
The time for one complete wave to pass a fixed point