1. Lecture 21 Wave Optics-2 Chapter 22
PHYSICS 270
Dennis Papadopoulos
March 31, 2010

2. White light passes through a diffraction grating and forms rainbow patterns on a screen behind the grating. For each rainbow,
the red side is farthest from the center of the  screen, the violet side is closest to the center.
the red side is closest to the center of the screen,  the violet side is farthest from the center.
the red side is on the left, the violet side on  the right.
the red side is on the right, the violet side on  the left.
STT22.3

3. Two slit and five slit diffraction
653 nm with 150 slits

5. Single Slit Diffraction
a<l

6. Pair wavelets with extra distance traveled l/2
Angles of dark fringes
Pair wavelets with extra distance traveled l/4
etc

7. Positions on the screen
Angles of dark fringes
Positions on the screen

8. The figure shows two single-slit diffraction patterns
The figure shows two single-slit diffraction patterns. The distance between the slit and the viewing screen is the same in both cases. Which of the following could be true?
STT22.5
Answer: A
The wavelengths are the same for both; a1 > a2.
The wavelengths are the same for both; a2 > a1.
The slits and the wavelengths are the same for both; p1 > p2.
The slits and the wavelengths are the same for both; p2 > p1.

9. The figure shows two single-slit diffraction patterns
The figure shows two single-slit diffraction patterns. The distance between the slit and the viewing screen is the same in both cases. Which of the following could be true?
STT22.5
The wavelengths are the same for both; a1 > a2.
The wavelengths are the same for both; a2 > a1.
The slits and the wavelengths are the same for both; p1 > p2.
The slits and the wavelengths are the same for both; p2 > p1.

10. Light decides to take the path of “least time” which turns out to be given by the reflection law.
But how does it find this path?
Does it check all other paths? Does it smell the nearby paths and checks them against each other?
The answer is in a way yes. The “smelling instrument” is the wavelength and the decision process is interference.

11. In fact light follows all possible paths that first reflect on the mirror and reach point P.
However we must keep not only the amplitude but also the phase and then add the result as vectors
Light goes from S through the mirror to P with the same amplitude through all paths but with different phase since the lengths of the paths are different

12. S P

13. What is the minimum size
of a mirror to give reflection? y S P L D

14. The ends of he mirror not important

15. y L
Dy ->Minimum size of the mirror
Notice that l≈.3-.6 microns

20. The Diffraction Grating
Suppose we were to replace the double slit with an opaque screen that has N closely spaced slits. When illuminated from one side, each of these slits becomes the source
of a light wave that diffracts, or spreads out, behind the slit. Such a multi-slit device is called a diffraction grating. Bright fringes will occur at angles θm, such that
The y-positions of these fringes will occur at

27. rays L D

28. Single slit diffraction

31. When laser light shines on a screen after passing through two closely spaced slits, you see
a diffraction pattern.
interference fringes.
two dim, closely spaced points of light.
constructive interference.
Answer: B

32. When laser light shines on a screen after passing through two closely spaced slits, you see
a diffraction pattern.
interference fringes.
two dim, closely spaced points of light.
constructive interference.

34. Circular-Aperture Diffraction
Light of wavelength λ passes through a circular aperture of diameter D, and is then incident on a viewing screen a distance L behind the aperture, L>>D. The diffraction pattern has a circular central maximum, surrounded by a series of secondary bright fringes shaped like rings.
The angle of the first minimum in the intensity is
The width of the central maximum on the screen is

35. Tactics: Choosing a model of light
l≈500 nm
L≈1 m