1. MSTC Physics C
Chapter 24
Section 3

2. Diffraction Spreading of waves into a region behind an obstruction
Occurs when waves pass through small openings, around obstacles, or by sharp edges

3. Diffraction
Longer the wavelength, compared to the width of the opening, the greater the diffraction

4. Diffraction
Resemble interference patterns because they result from constructive and destructive interference

5. Diffraction
According to Huygen’s principle, each portion of a slit acts as a source of waves
Hence, one portion of a slit can interfere with light from another portion
Suppose slit is composed of 5 particles
Point 1 travels the shortest distance
Point 5 travels the farthest
The first dark fringe occurs when the extra distance traveled by the wave from source 3 is 1/2λ longer
The waves from 1 and 3 are opposite and destructive interfere
At this angle each wave from the upper half cancels a wave from the bottom half that travels 1/2λ farther
For other angles some of the light remains uncanceled and the screen appears bright

6. Diffraction
According to Huygen’s principle, each portion of a slit acts as a source of waves
Hence, one portion of a slit can interfere with light from another portion
Suppose slit is composed of 5 particles
Point 1 travels the shortest distance
Point 5 travels the farthest
The first dark fringe occurs when the extra distance traveled by the wave from source 3 is 1/2λ longer
The waves from 1 and 3 are opposite and destructive interfere
At this angle each wave from the upper half cancels a wave from the bottom half that travels 1/2λ farther
For other angles some of the light remains uncanceled and the screen appears bright

7. Diffraction For destructive interference, w sinΘ = m λ m=1,2,3,…..
where w=width of slit and Θ=angle of particular minimum

8. Sample problem
Light of wavelength 580 nm is incident on a slit of width 0.3 mm. The observing screen is placed 2 m from the slit. Find the positions of the first dark fringes and the width of the central bright fringe. What is the width of the first order bright fringe?

9. Single Slit diffraction
Top photo is with monochromatic light.
Bottom photo is with white light.
Note: brightest and widest in the middle

10. Single slit diffraction
As slit width decreases, note that the width of the central max also increases

11. Double slit interference
Top picture is monochromatic light
Bottom picture is white light
Note: intensity stays pretty consistent
Note that all bands are equal in width and the brightness stays fairly constant

12. Diffraction grating
Consists of a large number of equally spaced parallel slits
Can be made by engraving parallel lines on a glass plate with a precision machining technique
Typical grating consists of several thousand lines per centimeter
ex – 5000 lines/cm
d ( distance between slits) = 1/5000 cm
= 2 x 10-4 cm

13. Diffraction grating
Each slit acts as a source of waves where all waves start at the slits in phase

14. Diffraction grating
Each slit produces diffraction

15. Diffraction grating
The diffracted beams interfere with each other to produce the final pattern

16. Diffraction grating
There will be some arbitrary direction Θ measured from the horizontal where the waves must travel different paths lengths before reaching the screen

17. Diffraction grating
If the path length difference is some multiple of a wavelength all waves are in phase and constructive interference
Intensity drops off gradually as work out from center

18. Diffraction grating Constructive interference
dsinΘ = mλ where m = 0,1,2,3,…

19. Sample Problem
Monochromatic light from a helium neon laser (632.8 nm) is incident normally on a diffraction grating containing 6000 lines/cm. Find the angles at which one would observe the first order maximum, second order maximum, and so forth.