Wave behaviour - light Flashcards
How are colours on an oil film created?
Thin film interference
Briefly describe how thin film interference works
Light falls on the film and some is reflected from the top boundary while some travels into the film to be reflected from the bottom boundary. The light which has gone through the film traveled slower in this medium so is delayed, causing a phase difference. When both rays emerge this can cause constructive or destructive interference for different wavelengths, creating colours.
What can happen at the boundaries in thin film interference?
Phase change (ie the waves are inverted)
What are Newton’s rings?
When two pieces of glass are put together there is a very thin layer of air between them which acts like a thin film. An interference pattern can be seen and on curved surfaces they look like contour lines - Newton’s rings
What can Newton’s rings be used for?
Glass-making
If the interference pattern is stable then the surface is smooth
What happens in thin film interference with monochromatic light?
Dark and light fringes (no colour)
Why are pearls iridescent?
They have lots of thin layers of different thicknesses, causing thin film interference
What are anti-reflection coatings?
They are used in cameras to let light enter by stopping it all from being reflected. They are made with a certain thickness to cause destructive interference in a specific colour/wavelength of light, preventing reflection of that colour
What were Ole Romer’s observations about light?
He noticed that the moons of Jupiter would fall behind for six months and then catch up in the next six. Earth and Jupiter’s orbits take each other further and further away for half the year. If light took a finite amount of time to reach the earth then that would explain this. He did not know the diameter of Earth’c orbit but he knew it had to be about 22 minutes of light time
What did Isaac Newton think about light?
Wave and particle ideas of light were mixed together. He talked about wavelengths and phases but thought light could work by particles setting up vibrations in matter.
he also thought light traveled faster in glass to try and explain refraction.
What were Huygens thoughts on light?
- Light does no hinder other light so it is a motion which can pass through other motions (like wave superposition)
- An longitudinal wave passing through small hard particles (since proven incorrect)
What is the theory of Huygen’s wavelets?
- Light spreads out spherically from a source, like ripples
- Each point on a wavefront acts as a new source of wavelets
- Wavefront are formed where wavelets are in phase and so interfere constructively
What is a wavefront?
Formed where wavelets superimpose constructuvely ( a plane can be created). Everywhere else wavelets are out of phase and interfere destructively, making zero
What are wavelets?
Small, sperical waves (like ripples) which spread out from different points on a light source
What rules did Huygen’s light theories explain?
- angle of incidence = angle of reflection
- sini/sinr = refractive index
- light travels more slowly in glass and water than in air
What is path difference?
The extra distance a second wave has to travel to reach the same point as the first wave
Why can rays emerging from one slit in one direction be considered parallel?
The angle between them is so small compared to the distance of several metres they travel to reach the screen.
What is d in a double slit experiment?
The distance between the slits
What is L in a double slit experiment?
The distance from slits to screen
What is x in a double slit experiment?
The distance from fringe to centre fringe
What is the formula for path difference in a double slit experiment?
dsinӨ OR d(x/L)
What is the formula for distance between fringes in a double slit experiment?
Δx=(λL)/d
What is sinӨ equivalent to in a double slit experiment?
x/L
What is the formula for distance from the centre fringe to the nth fringe in a double slit experiment?
x = (nλL)/d
What are the condition for constructive interference in a double slit experiment?
The path difference is a multiple of one wavelength
dsinӨ = nλ
What are the condition for destructive interference in a double slit experiment?
The path difference is a multiple of half a wavelength (must be an odd number)
dsinӨ = (n +1/2)λ
What is diffraction grating?
A grid of multiple slits which light is diffracted through
follows the same set of rules as double slit
What affect does adding more slits have on the diffraction pattern?
The fringes become further apart
How are diffraction patterns on a screen created with a single slit?
Every part of the aperture is imagined to send out wavelets. Where these are received (the screen) the wavelets are added up using their phasors. Where they add up constructively there is a bright fringe and where they add up destructively is a dark fringe
What happens directly in front of the aperture in single slit diffraction?
All the wavelets travel in parallel and so have travelled equally far. They are all in phase so their phasors add up to make a very large amplitude and an intense bright fringe in the centre
What happens at an angle to the aperture in single slit diffraction?
As you go further and further to the side, wavelets travel increasingly different distances. This causes lag and so phasors point in different directions. When they are added they could make zero or a smaller positive amplitude than the central bright fringe.
How is a dark fringe created in single slit diffraction?
Phasors of the wavelengths add up to a complete circle when added tip to tail, making zero resultant. This is a path difference of one wavelength.
What are the conditions for constructive and destructive interference in single slit diffraction experiments?
destructive = path difference a multiple of one wavelength dsinθ = nλ constructive = half way between destructive
What is the central fringe called?
Zero order fringe
What is d in a single slit diffraction experiment?
The width of the slit
What is L in a single slit diffraction experiment?
The distance from slit to screen
What is W in a single slit diffraction experiment?
The beam width (how wide the diffraction pattern is from horizontal) in radians
What is the formula for beam angle in radians?
λ/d
What happens in diffraction through a circular aperture?
Light is diffracted in two dimensions
What is the equation for diffraction through a circular aperture?
θ = 1.22 λ/d (θ = angle of the first MINIMA in radians d = diameter of aperture)
What is the minimum angular distance apart at which instruments can resolve objects?
How can this problem be solved?
θ=1.22 λ/d
Two telescopes can be placed as far away as possible on Earth and then used as if they were a single instrument
What is the Rayleigh criterion?
The criterion for minimum resolvable detail
Diffraction is limited when the first minimum of the image of one source coincides with the maximum of another image source
