OP: Diffraction, Wave Optics, Coherence - Week 4 Flashcards
What is the corpuscular theory of light?
A.K.A particle theory of light.
states that light is made up of small discrete particles called ‘corpuscles” (little particles) which travel in a straight line with a finite velocity and possess impetus/energy
How did Newton describe the travelling of particles of light?
As travelling through a medium of an invisible aether
How did Newton’s particle model explain reflection?
the corpuscles bounce of the boundary, with their vertical velocity reversed and horizontal velocity unchanged
How did Newton’s particle model explain refraction?
he argues that the corpuscles traveled faster in the glass so that their vertical velocity was accelerated by some attractive force exerted on the corpuscles by the medium
He also said that particle size differed with colour, and therefore refract differently, to explain the light spectrum
Why did people accept Newton’s particle theory?
Because nobody at the time could measure the speed of light in a medium
- and he was a famous genius
Who came up with the wave model of light? But in what way did he describe the waves that differs to today’s understanding?
Cristiaan Huygens. But he said the waves were longitudinal, rather than our current understanding that they are transverse
How did Huygens describe the speed of waves when travelling from air to a denser material
He said that the waves slow down! (contradicting Newton’s idea that particles sped up)
In Huygens’ model, what is significant about each point on a wavefront?
Each point along a wavefront is the source for new ‘wavelets’, i.e. each point is a source of circular (hemi-spherical) waves
Which theory better explains interference patterns (newton’s rings)? Newton or Huygen?
Huygen’s wave theory
How does diffraction work?
It is the phenomenon where light energy spreads out as it travels through an aperture. (or bends around a corner as it travels through)
When diffracted light impinges on a screen, what is the result?
(- i.e. when diffracted wave comes through a single slit/hole/aperture)
A pattern of maxima and minima is formed. (i.e. a pattern of light and dark bands).
The pattern is called “Airy’s disk”, when in 2-dimensions
When light passes through an aperture and diffracts, in what region is the light brightest?
At the centre of the aperture/airy disk
How does the size of the aperture affect the level of diffraction?
Effect of diffraction is much larger with smaller aperture size
Explain the basis behind Thomas Young’s 2-slit experiment
He theorised, based on sound waves interference, that light waves could also interfere with each other. So he passes light through 2 nearby apertures
What determines the way in which waves interfere?
Amplitude and Phases (assuming frequencies and wavelengths are equal)
How do you obtain Constructive vs Destructive interference?
Constructive: waves in-phase
Destructive: waves out-of-phase
What is (spatial) Coherence?
The degree to which the local phase of a wave can be predicted from watching other waves nearby at the same time
What is (temporal) Coherence?
The degree to which the local phase of a wave can be predicted by observing what has happened locally in the immediate past
How does coherence influence the ability of light passing through 2 slits to form an interference pattern?
Coherent = interference pattern is formed (with alternating bright and dark spots) Non-coherent = NO interference pattern
Are you able to get interference from 2 independent emitters of light?
NO
When light is passing through 2 slits, when will we see constructive interference?
When the distance between each slit and the image location point is the same or differ by exactly ‘m’ wavelength (m = 1, 2, 3 etc.)
Where will the dark fringes of light appear in an interference pattern?
Appear where you have ‘destructive’ interference.
- i.e. when one wave is 180deg out of pahse with the other
Do you still get an interference pattern when you turn down intensity and only one photon reaches the 2 slits at one time? If so, why?
YES, because the particles have an inherent wave-like nature
Explained by Heisenberg’s Uncertainty Principle - you can’t simultaneously know both a particle’s position and velocity at the same time
What happens when you pass a laser beam through a very small aperture?
You get an interference pattern (Airy’s Disk)
Is it possible to achieve point-to-point imagery?
NO. No matter how small the object. This is because it is prevented by diffraction (so the wave-like nature of light).
- best you can achieve is blob-to-blob imagery
Describe the brightness pattern of an Airy Disk
The bands of light and dark start of brighter and get progressively dimmer as you go outwards towards the periphery of the disk
Describe the formula that relates aperture diameter, to the angle of the aperture and to the wavelength of the light?
Dsin(theta) = 1.22lambda
Theta = the angle of the aperture subtended by the middle of the airy disk and it’s first minima
D = aperture diameter
theta = 1.22lambda/D
Explain Rayleigh’s criteria
The smallest angular separation of things that we can image in a system is limited by diffraction when the peak of one point lands on the minima of the airy disk produced by a neighbouring point
- i.e. two point sources separated by less than the radius of Airy’s disk cannot be resolved/distinguished from each other
For a natural pupil size (5mm) and light in the middle of the visible spectrum (500nm), is visual acuity limited by Diffraction? Explain.
No.
For 6/3 V.A: limbs separated by 0.146mrad can be resolved
Rayleigh’s criterion: 0.122mrad in this case (theta = 1.22x5x10^-5cm/0.5cm)
0.146 is greater than 0.122. So 6/3 can be resolved (ideally).
For a small pupil size (2.5mm) and light in the middle of the visible spectrum (500nm), is visual acuity limited by Diffraction? Explain.
Yes.
For 6/3 V.A: limbs separated by 0.146mrad can be resolved
Rayleigh’s criterion: 0.244mrad in this case (theta = 1.22x5x10^-5cm/0.25cm)
0.146 is less than 0.244. So 6/3 cannot be resolved.
What is the goal of laser interferometry? How does it work?
Goal: to make an interference pattern happen on the retina itself
since laser light has a long coherence “length”, the 2 beams can interfere with each other, giving rise to a grating pattern on the retina
How can laser inferometry be used clincally?
Can be used as a “Potential Acuity Meter” to assess the potential visiual acuity that a person with optical problems such as cataract or keratoconus might achieve after surgery
How does rayleigh’s criterion relate to reading DVDs, CDs etc?
The formula theta = 1.22lmabda/d is important for detecting the microscopic features such as the little pits that encode data on CDs, DVDs etc.
Is microscopic resolution limited by rayleigh’s criterion?
Yes
Describe our current understanding on how light operates
Photons are particles that behave as waves
