Waves Flashcards
(39 cards)
What is Superposition
When two or more waves overlap, the resultant displacement at a point is equal to the sum of the individual displacements at that point. This is the principle of superposition.
What are the rules of refraction
The angle of incidence = angle of reflection, i = r
The reflected ray is in the same plane as the incident ray.
All angles are measured from the normal, a line running at 90o to the surface.
What is refraction
Refraction is the change of direction a light ray undergoes when it enters a medium with a different optical density.
Light travels at different speeds in materials with different optical densities resulting in a change in the speed of the light which causes change in direction.
What is the critical angle
This angle is called the critical angle, C - it is the angle of incidence at which the corresponding angle of refraction is 90o (along the boundary).
What are the necessary conditions for total internal reflection to occur?
The wave must be travelling from a more dense medium to a less dense medium
Angle of incidence must be greater than the critical angle
What are optic fibres
Optical fibres are basically long thin bits of glass that carry electromagnetic waves over long distances (usually visible light).
come up with a short experimental method of identifying the refractive index of a solid.
Trace around the solid on a blank sheet of paper.
Use a ray box to draw a range of values for i and measure their corresponding values of r.
Plot sini vs sinr and calculate the gradient.
The gradient will be the refractive index of the solid.
How do optic fibres work?
Light is directed into the narrow fibre.
The light is travelling from air into a glass medium.
This means light is refracted and bends towards the normal.
In a normal glass fibre the light would refract out of the glass at this point.
With the cladding in place light is now travelling from a more dense medium to a less dense medium.
Provided the i > C, TIR will occur
One of the inherent problems with fibre optics is that light can take more than one path within the fibre.
What are the problems with optic fibres and how do you overcome them?
Using very high purity glass.
Using an infrared light source.
The glass is surrounded by another layer of glass that has a much lower refractive index. This CLADDING is optically less dense than the CORE glass.
The cladding is surrounded by a buffer coating that protects the fibre from damage and moisture.
What is multi path dispersion
Different rays of light take DIFFERENT PATHS
So they will arrive at DIFFERENT TIMES
Which will SMEAR/DISTORT the signal
How can multi path dispersion be corrected?
So how do we reduce multipath dispersion?
Use a monochromatic light source (more about these later in the course).
Use a very thin piece of optic fibre – this forces the light to travel almost entirely along the axis (the difference in the path of any reflected ray is negligible).
Choosing a cladding with a refractive index as close to the core as possible.
Optical Bench
Very simply, move the lens back and forth until the image is in focus
Measure u and v
Move the position of the object closer to the screen and repeat steps 1 and 2. do this for a range of object distances.
Plot 1/u verses 1/v
Find the intercepts of both axis. When 1/u = 0, 1/v = 1/f!
What is polarisation
Polarisation occurs when oscillations of electric and magnetic fields are in one plane only in the direction of the travel of light
Why is the EM wave polarised
The EM wave is oscillating in all directions.
Only the oscillations parallel to the first polaroid filter are transmitted.
the wave is now plane polarised.
When these waves arrive at the second the waves are perpendicular to the direction of propagation.
So no light will be transmitted
How do sunglasses polarise
Point 1 – light from the Sun is unpolarised.
Point 2 – light entering the glasses directly are only partially polarised so glare is reduced a little.
Point 3 – light that reflects off a surface (like water) is partially polarised in the horizontal plane.
Point 4 – partially polarised light is polarised further vertically
What is phase difference
The phase difference isthe difference in the phase angle of the two waves. It tells us how much a particle (or wave) is in front or behind another particle (or wave).
What is path difference
Path difference isthe difference in the path travelled by two waves from their source.
What is constructive interference
Constructive interference occurs whenever the path difference between coherent sources is nλ, where n is a whole number.
What is coherent
COHERENT: Light that is composed of waves that are in phase or have a constant phase difference. They have the same amplitude and frequency.
What is destructive interference
Destructive interference occurs whenever the path difference between coherent sources is nλ/2, where n is a whole number.
Difference between standing and progressive waves.
Standing Wave:
Energy is stored
Amplitude Varies
Oscillations are in phase
Progressive Wave
Energy is transferred
Amplitude stays constant
Phase varies continuously
What factors affect the frequency at which the fundamental frequency will occur?
The length of the string
The tension of the string
The thickness of the string – the mass per unit length
What types of waves can be diffracted? When will the greatest amount of diffraction occur?
All types of waves can be diffracted, but the amount depends on the ratio of the wavelength to the size of the opening or obstacle.
Diffraction is greatest when the wavelength is approximately the same as the width of the gap.
Why is the diffraction greatest when wavelength is approximately same as the width of the gap
Why this happens can be explained by Huygen’s principle: every point on a wave front may be considered as a source of secondary waves.
