Diffraction & Interference of Light

1. Diffraction & Interference of Light Chapter 19

2. 19.1 Interference • Diffraction- • Interference- • Incoherent light-unsynchronized wave fronts • Coherent light-synchronized wave fronts, waves are in phase

3. When Light Waves Interfere • Thomas Young (1772-1829) studies the human eye and the voice • This lead him to study waves • He applied this to study wave interference in oceans and lakes • Later proved that light has wave properties by producing interference patterns

4. The Two-Slit Interference Pattern • Italian, Francesco Maria Grimaldi first noted that the edges of shadows are not sharp • He named this spreading of the wave diffraction

5. Young used monochromatic light (a source of only one color and one wavelength) • He also created coherent light from incoherent sources.

6. He placed a barrier with a very narrow single slit in front of incoherent light. • Only coherent light will pass through the slit because it is so narrow. • The light is then diffracted through the narrow slit • The wave spreads out in a cylindrical shape so that when this wave reaches the next barrier that has two slits the waves will be coherent • These two waves will then undergo either constructive or destructive interference and create interference fringes

7. The dark and bright bands are called interference fringes • The bright bands were from constructive interference • The dark bands were from destructive interference

8. Measuring the Wavelength of a Light Wave • Young used his experiment to determine the wavelength of light • similar triangles

9. 1. If L is much large than d, then the line segments S1P1 and S2P1are nearly parallel to each other and to line segment QP1, and triangle RS1S2 is very nearly a right triangle. Thus, sinθ≈λ/d. • 2. If the angle θ is small, then sinθ is very nearly equal to the tanθ

10. Wavelength Equation • Where: x is distance from central bright band to first bright band • l is wavelength • L is distance from slits to screen • d is distance between slits

11. Be careful of the units • wavelength is nm, nanometers, 10-9

12. 19.2 Single Slit Diffraction • Door of a room • difference between double and single slit is a wider central bright band • Similar equation • m is the position of the of the dark band

13. Insect’s bodies and wings are diffraction gratings • Hundreds of tiny ridges • The light hits them and produces interference patterns

14. Diffraction Gratings • plastic or glass that has thousands of lines per centimeter scratched into it • 10,000 lines per cm • 10-6 m is the spacing between lines

15. Gratings form interference patterns similar to the double slits • The bright bands are in the same location, but they are narrower • The dark regions are wider

16. Individual colors are seen easier because they are separated • Better measurement of wavelengths • The same equation is used where d is the distance between the lines

17. Grating Spectrometer • An instrument used to measure wavelength with a diffraction grating • Quality control in labs • Uses the angle instead of L and x • l = d sinθ