1. Diffraction

2. You will be able to Describe qualitatively double slit interference
Solve problems involving diffraction gratings.

3. Interference
Waves interfere (superposition) in a predictable way.

4. antinode
Two crests meet: maximum displacement of the medium

5. node
Where a crest meets a trough: complete destructive interference

6. Young discovered this in 1801

7. What Young proved!

8. The observed pattern has regular intervals that can be measured.
Just like water waves

9. Diffraction of Red Light
102
103
Double slit
Red filter
Light diffracts
Screen Po – central max P1
1st order image
P2’ P1’ P P P2
Constructive interference Destructive Interference
Antinode Node
Bright band (fringe) Dark band

10. Monochromatic light’s wavelength can be calculated from the observed interference patterns using:

11. Example:
Light is incident on a pair of slits 1.28 x m apart. The maxima are 4.11 x m apart and the screen is 1.00 m from the slits. What is the wavelength and colour of the light? See p. 676 for color

12. Solution

13. Solution

14. Example:
Yellow monochromatic light, ʎ = 580 nm, shines on a pair of slits. The second dark spot is 21.8 cm from the central anti-node on a wall that is 5.00 m away. What is the slit separation?

15. Solution:

16. Solution:

17. Diffraction Gratings (interference gratings)
Rather than a double slit to perform Young's experiment, multiple slits, called diffraction gratings, are more commonly used.

18. Diffraction Grating
Transparent glass (or plastic) with opaque scratches placed very close together so it acts like multiple slits.

19. The bright maxima are much sharper and narrower for a grating than for a double slit, and, therefore, better for measuring wavelength.

20. The angle of diffraction depends on
wavelength.
If light of multiple wavelengths is used, the grating sorts out the wavelengths and produces a spectrum.

21. The formula for a diffraction grating is:

22. Review of Second Equation for DiffractionP2 P3
n=2
n=3 P0
P1’
Screen
wavelength x d P1
Diffraction grating
For angles less than 10⁰, the sine and tangent values are nearly equal

23. Eg) Light falls onto a diffraction grating ruled with 500 line /cm and casts a second order image at 2.00⁰. Determine the wavelength of light falling onto the grating in nanometers.

24. Eg 2) Light falls on a pair of slits 1. 35 um apart
Eg 2) Light falls on a pair of slits 1.35 um apart. The maxima are measured to be cm apart and the screen is a distance of 1.00 m from the slits. Determine the wavelength of light using each diffraction equation: