Physical Optics Ch 37 and 38

Physical Optics Light is an electromagnetic wave. Wave properties: Diffraction – wave bends around corners, spreads out from narrow slits Interference – waves from two or more sources interfere

Electromagnetic Waves Usually we keep track of the electric field E : amplitude E B EoEo v

The Electromagnetic Spectrum The Electromagnetic Spectrum (m) f (Hz) x x x x x x x Radio TV Microwave Infrared Visible Ultraviolet X rays  rays Infrared 780 nm Red Yellow600 nm Green550 nm Blue450 nm Violet 380 nm Ultraviolet

Infrared 780 nm Red Yellow600 nm Green550 nm Blue450 nm Violet 380 nm Ultraviolet Visible-Light Spectrum Spectrum

The EM Spectrum

Coherent sources are sources that have a constant phase difference between them (lasers). In phase sources are sources that have zero phase difference between them. Monochromatic sources have the same wavelength. Plane waves mean the wavefronts are planes Wavefronts are surfaces on which we have only crests, or troughs for example.

Double Slit Double Slit (Thomas Young, 1801) Result: Many bright and dark fringes or lines on the screen: screen Two slits, separation d θ m=2 m=1 m=0 m=-1 m=-2 incident light,plane waves

The slits act as sources of light in phase. Due to diffraction, the light spreads out after it passes through each slit. When the two waves arrive together at some point P on the screen, they are out of phase, due to the difference in the length of the paths. This path difference varies from place to place on the screen. P x1x1 x2x2 d  screen

Constructive Interference: (bright fringe) Δx = mλ, or d sin θ = mλ, m = 0, 1, 2, … Destructive Interference: (dark fringe) Δx = (m + ½)λ, or d sin θ = (m + ½) λ, m = 0, 1, 2, …

Example 1 Where area) the m=0,1 bright fringes? b) m=0,1 dark fringes? (give values of y) 3 m y 0 2 slits, 0.20 mm apart; red light ( = 667 nm) screen

Solution

Example 2 If the distance between the first and the third minima in the diffraction pattern is 3.0mm, what is the width of the slit? 50 cm y 0 2 slits, red light ( = 690 nm) screen

Solution

Which of the following would cause the separation between the fringes on the screen to decrease? (There may be more than one answer.) A)Increasing the wavelength B)Decreasing the wavelength C)Moving the slits closer together D)Moving the slits farther apart E)Moving the screen further from the slits F)Moving the screen closer to the slits Quiz

Test Discussion