Lecture 13 and 14 - Waves diffraction and interference Flashcards
What is interference
is superposition of 2 or more waves producing a resultant disturbance that is the sum of the overlapping wave contributions
What is total constructive interference ?
they are precisely in phase the resultant is the sum of the 2 wave height values
The result is that the two waves add up to give a wave having twice the amplitude of of the individual waves.
What does it mean when they waves are in phase?
they are at exactly the same point in their cycle – the peak of one wave coincides with the peak of the other wave, and hence the trough of one also coincides with the trough of the other.
-The peaks of the waves represent a positive disturbance above the zero point, and since the disturbances will add when when the waves combine, the result is an even bigger positive disturbance.
Similarly, the troughs a negative disturbance – will add up to give an even deeper (i.e. more negative) trough.
What is total destructive interference ?
When the phase difference reaches 180 degrees , waves will cancel each other.
What does it mean when waves are out of phase?
therefore the peak of one wave coincides with the trough of the other. If the waves have equal amplitudes (which would be the case if the beamsplitter in Figure 20.1 split the light by half) then the positive disturbance to the electro-magnetic field represented by a peak exactly cancels out the negative disturbance represented by the trough, the result is no disturbance, and hence no wave. There is no light on the screen since the light waves have cancelled each other out
What happens to light that goes through a geometrical point in a opaque screen?
The beam of light will go through the aperture or hole
- either light goes through or not
- the light that goes through is unaffected by the aperture- the aperture has no effect on the rays that go through.
What are parallel rays- in wave optics?
plane wavefronts that are coming from an infinitely distant object
What is diffraction?
the property of light that causes the wavefront to no longer be plane but slightly bent on the extremes of the wavefront .
- Here as light is squeezing through the aperture some of the light deviates from the straight light trajectory and there is bending of the wavefront.
- HAPPENS at the boundary or the edge of the wavefront
Why is there no refraction ?
due to the fact that light behaves in this way when it is squeezing through an aperture
-Some of the light going through this aperture is travelling at a different angle
What happens in reflection or refraction in geometrical optics?
only reflection or refraction can change the direction of the beam
What does the circular beam look like going through an aperture (FRONT VIEW) ?
- in geometrical optics this aperture will remain circular and it doesnt matter how far this beam goes- it does not change its shape- however this not happen
- -When zooming in you see a fringing effect occurring at the edges of the beam
- It is no longer a smooth beam
- the beam is changing direction in a peculiar way
- diffraction happens at the edge of the beam
- where a beam of light is being cut down by a certain aperture
- no lenses involved.
What does diffraction depend on?
size of aperture
What is diffraction significant ?
for a small aperture compared to a large aperture.
What happens for a large aperture ?
the centre of the beam is quite far from the edge of the beam , so it becomes shielded on the effect of diffraction.
- it is far enough from the edges so as it is unaffected by the perturbation introduced by the edge.
- Therefore a large part of the wavefront is still plane and only a small region close to the edges of the wavefront has been affected by diffraction.
What happens as the beam travels further away from the aperture in geometrical optics?
the effect becomes greater, further away it will effect the centre of the wavefront as well
What relationship does diffraction have with aperture size?
diffraction has an inverse relationship with aperture size
Why does diffraction have an inverse relationship with aperture size?
since the smaller the aperture the larger the amount of diffraction, and vice versa.
What happens as the beam travels further away from the aperture in geometrical optics?
however in geometrical optics it doesnt matter how far the beam has travelled it will have no effect on the beam
What happens as the beam travels further away from the aperture in wave optics?
this perturbation will affect the whole beam
What effect does the beam travelling have on forming images in a optical system? - in geometrical optics
What will give a perfect point image in a gemoertical optics?
wavefront after going through lens are spherical in shape.
What happens as light goes through an aperture?
edge of beam undergoes the effect of diffraction
the wavefront is no longer spherical
A wavefront that is not spherical would give us an image that is affected by diffraction
What gives us perfect images?
spherical wavefronts
What is an airy disk ?
the image of a point object due to effect of diffraction
- no optical system is ideal so any point object will look like an airy disk .
What will give an airy disk?
case of optical system with no aberrations
-the wavefronts after refraction by the lens are no longer spherical due to diffraction will give an image like an airy disk
What does the airy disk look like?
slightly blurred image point with a series bright and dark concentric rings around it that get fainter and fainter.