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 ( ) 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