4.4 Waves Flashcards
How can you tell electromagnetic waves carry energy?
- things heat up
- ionisation
- loud sounds cause large oscillations in air partials which can make things vibrate
- wave power can be used to generate electricity
Define displacement.
How far a point on the wave has moved from its undisturbed - it can be positive or negative.
Define amplitude.
Amplitude is the maximum displacement - the distance from a peak to trough to the mean (rest) position.
Define wavelength.
The distance between two successive identical points that have the same pattern of oscillation. The distance a wave travels before it repeats itself.
Define period.
The time it takes for one complete cycle of oscillation go take place at any point.
Define phase difference.
Measured in radians. The amount one wave lags behind the other -the relationship between the pattern of vibration at two points.
Define frequency.
The number of oscillations per second passing a given point.
Equation for frequency.
Frequency = 1/Time
Equation for speed of wave (v).
Speed of wave (v) = frequency (f) x wavelength (y)
What is intensity?
A measure of how much energy a wave is carrying
Define progressive wave.
A type of wave that travels through a substance or space, carrying energy via the oscillation of particles
Define transverse wave.
A type of wave in which oscillations occur at right angles to the direction of wave propagation.
Define longitudinal wave.
A type of wave in which oscillations occur parallel to the direct of wave propagation.
Define coherence.
Constant phase difference.
Why is it difficult to produce coherent light waves?
It is difficult to control the frequency.
Equation for fringe spacing?
X=(wavelength x D)/ a (spacing between slits)
What is the equation for intensity?
Power/area
What’s intensity measured in?
Wm^-2
What do all electromagnetic waves have in common?
- speed
- transverse
- can be refracted, reflected, diffracted and can undergo interference
- their progressive waves carry energy
- can be polarised
Wavelength of radio waves.
10^-1 to 10^6
Wavelength of microwaves.
10^-3 to 10^-1
Wavelength of infrared waves.
7x10^-7 to 10^-3
Wavelength of visible light.
4x10^-7 to 7x10^-7
Wavelength of UV waves.
10^-8 to 4x10^-7
Wavelength of X-rays
10^-13 to 10^-8
Wavelength of gamma rays.
10^-16 to 10^-10
What happens if the second polarising filter is rotated by 45 degrees?
The intensity will be half that getting through the first filter.
When do you get the most diffraction?
When the gap is the same size as the wavelength.
What does refractive index measure?
How much a material slows down light.
What is the absolute refractive index of a material?
The ratio between the speed of light in a vacuum and the speed of light in that material.
Equation for refractive index.
N = c/ v
What does snells law do?
Uses angles to calculate the refractive index.
What is snells law?
N1sinø1 = n2sinø2
What happens to the angle of incidence when the angle of refraction is a right angle?
The angle of incidence is critical.
What happens to snells law for light hitting a material to air boundary?
Sin c = 1/n
What happens to the wavelength when the pip has a closed end?
It is a quarter wavelength.
What are progressive waves?
Waves that transfer energy away from a source, via oscillations of particles
How many radians is a full cycle of a wave?
2pi
Define intensity.
Rate of flow of energy per unit area at right angles to the direction of travel of the wave. Measured in Wm-2.
What is the refractive index of air?
1
Define interference.
When two waves superimpose at a point there is a change in overall displacement.
What is a node?
Occurs where amplitude/displacement is always 0
What is an antinode?
Occurred where the amplitude takes the maximum possible value
Big equation for intensity
(Energy/time)/area
How are intensity and amplitude related?
I=A^2
What is a plane polarised wave?
We’re all oscillations of the field and the direction of travel are confined to a single plane
What is a polarising filter?
Produces plane polarised light by selective absorption of one component of the incident oscillations - only component of light polarised perpendicular to that direction
What are differences between standing and progressive waves?
Progressive waves transfer energy,
all points on a progressive wave have different phase (in one λ),
stationary wave traps energy in pockets,
stationary wave has nodes and antinodes
What is unique about EM waves?
They travel at c (310^8) in a vacuum
When you double the frequency what happens to the speed of a wave, and the speed of oscillation of an individual point on a wave?
The wave speed will not change, the wavelength will just halve proportionally.
For an individual point, it will oscillate twice as many times in a second and so it’s speed doubles
What happens to intensity if amplitude is doubled?
Increased by a factor of 4
What happens to intensity and position if separation is halved?
Intensity unchanged
Distance doubles
What happens to intensity and position of they become 180 out of phase?
Intensity unchanged
Maxima move to minima
Define diffraction
Oaths spreading out after gal or around an obstacle
Define intensity
The incident energy per unit area per second
What are the conditions necessary to produce visible fringes?
Coherent source Constant phase relationship Slits must be close together Similar amplitude Similar intensities
Define diffraction
Wavefronts spreading after passing through a gap or around an obstacle
