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