Basics, relationships and diffraction Flashcards
Why are waves important?
Because they can carry energy from one place to another
What is a wave
a method of transferring energy through a medium without the medium moving from its rest position
Difference between mechanical and electromagnetic wave
Mechanical waves need a medium to travel in, while electromagnetic waves do not. This is because the creation of the wave is due to the physical movement of the medium.
Once the mechanical wave has passed through, the medium is left in the same position that it was in before the wave arrived.
Can electromagnetic waves travel in a medium
Yes but it is not needed.
We know this because all the different types of electromagnetic waves reach Earth from the Sun, and space is a vacuum.
What are the electromagnetic waves
Radiowaves Microwaves Infrared Visible light UV light X-ray Gamma
How is a wave generated
The medium must be disturbed in some way.
Eg when you speak, air is passed over your vocal cords, making them vibrate and the vibrating vocal cords give repeated pushes to the air molecules surrounding them. This is the disturbance that creates the sound wave.
What is the amplitude
The maximum distance the medium goes out, in any direction, from rest position. Half the distance between crest and trough.
What is the wavelength
Distance from one point of the wave to the next equivalent point.
Relationship between energy, amplitude and wavelength
The more energy used to generate wave, the greater the amplitude. The greater the rate (shakes per second), the smaller the wavelength.
How is amplitude experienced
Sound: louder the sound, the more energy, so bigger amplitude.
Light: more energy, brighter the light so greater the amplitude.
What is the local effect of a mechanical wave
make each point in the medium complete one swing (out and back followed immediately by out and back in the opposite direction) ie from rest position.
Transverse vs longitudinal
Transverse: when local movement of the medium is at right angles to the direction in which the wave is travelling in
Longitudinal: if local movement is parallel to the direction in which the wave is travelling
What is frequency
The frequency of a transverse wave is the number of crests of the medium that pass a point every second. Number of waves produced per second.
The frequency of a longitudinal wave is the number of compressions of the medium that pass a point every second.
What is frequency measured in?
Hertz
Hz
Symbol: f
What is the period
The time taken for one complete wave. Time for two adjacent crests or compressions to pass a point.
What is period measured in
second
s
What is the relationship between frequency and time
Inverse. As time period increases, the frequency decreases. Vice versa
Equation for frequency in terms of time period
f = 1/T
or T = 1/f
How is frequency experienced
Sound: pitch. Higher frequency = higher pitch
Light: colour
Does frequency change when it goes from one medium to another?
NO IT DOESN’T.
eg if red light goes from air to glass, it stays red.
eg if a sound goes from cold air to hot air it stays at that pitch.
What speed do electromagnetic waves travel at
3 x 10^8 ms^-1
Wavelengths of the electromagnetic waves
Radio waves = longest
Gamma = shortest
Relationship between wavelength and frequency
Inverse. As wavelength increases, frequency decreases.
Frequencies and wavelengths of colours
Red: longest wavelength, lowest frequency
Violet: shortest wavelength, highest frequency
Does speed change in one medium
No. It only changes when the wave changes mediums
Eg of not obvious medium changes
Hot air to cold air
Deep water to shallow water
Do electromagnetic waves change speed when changing mediums
YES. They travel fastest through a vacuum.
Equation for velocity, frequency and wavelength
v = f λ
How does a reflected wave differ from the incident wave
If a wave strikes a surface, it will be reflected. The reflected wave is still travelling in the same medium as the incident wave, so it will have the SAME frequency and speed (and therefore wavelength). Only the DIRECTION has changed.
What happens if the wave strikes the boundary between the two mediums?
The wave is able to travel through the boundary from one medium into the other, and so a wave will both be reflected back into the first medium and be TRANSMITTED into the second medium. Because FREQUENCY DOES NOT CHANGE, the second wave will have the same frequency as the incident wave. But its speed, and therefore wavelength will change (due to a proportional relationship between v = fλ)
The direction of the incident wave often changes when it is transmitted into a different medium, but not always.
The AMPLITUDES of the reflected and transmitted wave will be less than the amplitude of the incident wave, because the energy that the incident wave was carrying has been shared between the reflected and transmitted waves.
What is diffraction
The bending of a wave around a barrier
What does the amount of bending depend on
The wavelength of the wave - the longer the wavelength, the more it bends around the edge of a barrier
Why do shadows have relatively sharp edges
Because light waves have very short wavelengths, so they hardly bend at all when they pass the edge of a barrier.
What happens when waves pass through a gap in the barrier?
Diffraction occurs at both edges of the gap, so the wave passing through the gap tends to be circular in shape.
Relationship between gap of barrier and wavelength
Bending increases when width of the gap is about the same as the wavelength of the wave
The longer the wave compared to the obstruction, the greater the diffraction is.
An image produced by a plane mirror is:
- upright
- same size
- virtual
- same distance behind as the real object is in front of the mirror
