wave nature of light light. is light what we see? is there light where there is no one to see it?
TRANSCRIPT
WAVE NATURE OF LIGHT
Light
Is light what we see? Is there light where there is no one to see it?
Dualism of light
Isaac Newton believe light to be a ray of corpuscles (particles)
Robert Hooke believed light to have the wave-like nature
Newton won… until over a century after his death…
LIGHT INTERFERES!
1801 – Thomas Young produces interference of light
Light is a chameleon of science
A wave that travels in straight lines
Particles which interfere like waves?
Three views on light
Waves Light rays Photons
Most commonVisible light – part of
EM spectrumInterference and
diffraction of light
Light travels in straight lines
Optics: mirrors, lenses, etc.
Photoelectric effectPhotons as particles (quanta of energy)
Quantum mechanics:
Covered in HS physics
Not so much
Waves or not?
Waves diffract (like sound). Light doesn’t seem to
Waves or not?
When the opening is comparable to the wavelength, shadows are cast
Light does diffract when it goes through a tiny slit
Young’s Experiment
Constructive and destructive interference
Constructive and destructive interference
The waves coming from the slits interfere constructively ordestructively, depending on the difference in distances betweenthe slits and the screen.
sind
,3,2,1,0 sin md
m
,3,2,1,0 sin 21 md
m
Red light (664 nm) is used in Young’s experiment with slits separatedby 0.000120 m. The screen is located a distance 2.75 m from the slits.Find the distance on the screen between the central bright fringe andthe third-order bright fringe.
Example
Formula for ‘bright spots’
,3,2,1,0 sin md
m
Providing the screen is very-very distant,
Guess: what would the formula look like for dark spots?
Measuring wavelength of light
A double-slit interference pattern is observed on a screen 1.0 m behind two slits spaced 0.30 mm apart. The distance from the center of one particular fringe to the center on the ninth bright fringe from this one is 1.6 cm. What is the wavelength of the light?
Double-slit interference pictures
Single-slit diffraction
Huygens’ Principle
Diffraction
The wavelets going straight forward all travel the same distance to the screen. Thus they arrive in phase and interfere constructively to produce the central maximum
Note the brightness (intensity)!
Dark spots
Ray 2 is a/2 away from ray 1. If the two rays arrive counter-phase (trough / crest), destructive interference occurs. Ray 2 travels a longer distance by
Destructive interference will occur if:
Dark spots
Position of destructive interferences (dark spots)
Note that p is not 0! That would be the central bright stripe
How wide would the stripe be?
How does the picture change??
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Grating Diffraction
Sometimes More IS More