# Light Wave nature of light.

## Presentation on theme: "Light Wave nature of light."— Presentation transcript:

Light Wave nature of 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
Particles which interfere like waves? A wave that travels in straight lines

Three views on light Waves Light rays Photons Most common
Visible light – part of EM spectrum Interference and diffraction of light Light travels in straight lines Optics: mirrors, lenses, etc. Photoelectric effect Photons as particles (quanta of energy) Quantum mechanics: Not so much Covered in HS physics

Waves or not? Waves diffract (like sound). Light doesn’t seem to

Waves or not? Light does diffract when it goes through a tiny slit
When the opening is comparable to the wavelength, shadows are cast

Young’s Experiment

Constructive and destructive interference

Constructive and destructive interference
The waves coming from the slits interfere constructively or destructively, depending on the difference in distances between the slits and the screen.

Example Red light (664 nm) is used in Young’s experiment with slits separated by m. The screen is located a distance 2.75 m from the slits. Find the distance on the screen between the central bright fringe and the third-order bright fringe.

Formula for ‘bright spots’
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??

Grating Diffraction

Sometimes More IS More