2Is light what we see? Is there light where there is no one to see it?
3Dualism of lightIsaac Newton believe light to be a ray of corpuscles (particles)Robert Hooke believed light to have the wave-like natureNewton won… until over a century after his death…
4LIGHT INTERFERES!1801 – Thomas Young produces interference of light
5Light is a chameleon of science Particles which interfere like waves?A wave that travels in straight lines
6Three views on light Waves Light rays Photons Most common Visible light – part of EM spectrumInterference and diffraction of lightLight travels in straight linesOptics: mirrors, lenses, etc.Photoelectric effectPhotons as particles (quanta of energy)Quantum mechanics:Not so muchCovered in HS physics
7Waves or not?Waves diffract (like sound). Light doesn’t seem to
8Waves or not? Light does diffract when it goes through a tiny slit When the opening is comparable to the wavelength, shadows are cast
13ExampleRed light (664 nm) is used in Young’s experiment with slits separatedby 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.
14Formula for ‘bright spots’ Providing the screen is very-very distant,Guess: what would the formula look like for dark spots?
15Measuring 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?
20DiffractionThe wavelets going straight forward all travel the same distance to the screen. Thus they arrive in phase and interfere constructively to produce the central maximumNote the brightness (intensity)!
21Dark spotsRay 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 byDestructive interference will occur if:
22Dark spots Position of destructive interferences (dark spots) Note that p is not 0! That would be the central bright stripeHow wide would the stripe be?