OP: Diffraction, Wave Optics, Coherence - Week 4 Flashcards
(35 cards)
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)
