Diffraction and Interference of Light

Chapter 19

19.1 When Light Waves Interfere

Grimaldi found that edges in shadows are not perfectly sharp.

Diffraction- bending of waves around a barrier

Huygens – all points of a wave front of light could be thought of as new sources of smaller waves. Fig 19-1 444

Young – dev. experiment to explain how light acts as a wave

Produced pattern of light and dark bands, interference fringes

Constructive & destructive interference Fig 19 –2 445

Monochromatic- one color –wide central band

Coherent waves – waves are in phase.

White light – central band is white, other bands depend on

Fig 19-3 445 Young also measured of light. Fig 19-4 447 = xd/L x - distance from central to next

band, d - distance between slits and L - distance from slit to screen.

Ex Prob 448

Single slit – bright central band with dimmer bands on each side

Fig 19-5 & 6 449 x = L/w x - distance from central to

next band, w – width of slit, and L - distance from slit to screen.

Fig 19-7 450 Prac Pr 451

19.2 Applications of Diffraction

Colors on butterfly wings and beetles are produced by diffraction.

Diffraction grating – transmits or reflects light & forms an interference pattern like double slit.

Holography – produces a 3-D image that is difficult to copy.

P 453

Spectroscope – used to measure Fig 19-9 454 & Fig 19-10 455 = xd/L = d sin Diffraction limits the ability of a

lens to distinguish two closely spaced objects.

Rayleigh criterion – if central band of one star falls on central band of the other can be resolved.

Can be reduced by increasing size of lens.