Chapter 19-2

Applications of Diffraction

Diffraction by Reflection

• Tiny ridges of beetle or CD have spacings only a few hundred nanometers apart

• Each space acts as a slit and diffracts light

• Two-slit Interference pattern results

Diffraction Grating

• Diffraction by transmission or reflection• Transmits or reflects light• Forms an interference pattern (double-slit)• Made by scratching very fine lines on

glass or impressing scratches on plastic• Spacings can be as small as 1,000 nm

Holograms1. coherent light beam

passes through semi-transparent mirror

2. Light split: object beam/reference beam

3. Object beam: passes thru lens, reflected off mirror; illuminates object, light falls on photographic plate

4. Reference beam: reflected off mirror, spread by lens, interferes with object beam and falls on photographic plate

5. Interference pattern on photographic plate which is the hologram

Grating Spectroscope

• Comparison with double slit:• Interference pattern has

bands in same locations• Grating:

– color bands narrower, dark bands wider

– Individual colors more easily distinguished

– Wavelengths measured more precisely

Red light White light

DiffractionGratingPatterns

Ans:4 x 10^-4 cm

Ans:Red: 49 degreesBlue: 30 degrees

Ans:660 nm

Two radio transmitters are 25.0 m apart and each one sends out a radio wave with a wavelength of 9.80 m. The two radio towers act exactly like a double-slit source for light. How far from the central band is the first order image if you are 19.0 km away? (Yes, this really happens. Radio stations can and do fade in and out as you cross the nodals and anitnodals.) [Ans: x = 7.45 km]

End 19-2Formulas for Chap 18 & 19

Single slit formula

Double slit formula