2.4b - Interference and diffraction

Conditions for observable interference

sources coherent, amplitudes of sources nearly same, fringes sufficiently far apart

Path difference for constructive interference

n x wavelength

Path difference for destructive interference

(n + 1/2) x wavelength

Theory of Youngs double slit experiment

monochromatic light through narrow vertical slit, light diffracts as it passes, light falls on two narrow vertical slits, diffracted light overlaps and interference occurs, alternate bright/dark fringes

Youngs equation

fringe seperation = (wavelength x distance to screen) / slit seperation

Define diffraction

waves spread as pass through opening/around object

As gap decreases what happens to diffraction?

increases

When does greatest diffraction through slit occur?

gap size same as wavelength

What does decreasing aperature size do to diffraction?

width of pattern increases, maxima heights decrease

order of fringe equation

distance slit to screen x sinO = order of bright fringe x wavelength

equation for distance slit to screen

d = 1/lines per metre on grating

Describe experiment using diffraction grating and laser to find wavelength

shine laser through diffraction grating onto screen, measure distance to screen/distance to 1st to 5th bright fringe, plot graph using dsinO = n x wavelength