Lecture 17: Chapter 35 (Diffraction)

Thin film interference

Consider two identical microscopic slides in air illuminated with light from a laser. The bottom slide is rotated upward so that the wedge angle gets a bit smaller. What happens to the interference fringes?

1. spaced farther apart
2. spaced closer together
3. no change

Ans: A.

Thin film interference

Two identical microscope slides in air illuminated with light from a laser are creating an interference pattern. The space between the slides is now filled with water (n = 1.33). What happens to the interference fringes?

1. spaced farther apart
2. spaced closer together
3. no change

Ans: B.

Diffraction

The diffraction pattern below arises from a single slit. If we would like to sharpen the pattern, i.e., make the central bright spot narrower, what should we do to the slit width?

1. narrow the slit
2. widen the slit
3. enlarge the screen
4. close off the slit

Ans: A.

Diffraction

Imagine holding a circular disk in a beam of monochromatic light. Diffraction occurs at the edge of the disk. The center of the shadow is

1. darker than the rest of the shadow
2. a bright spot
3. bright or dark, depending on the wavelength
4. bright or dark, depending on the distance to the screen

Ans: B.

Diffraction?

Consider a beam of coherent monochromatic light, like a pencil shaped beam from a laser. At a certain time and position, the cross section of the beam is found to be a perfect circle of diameter D. However, it is moving in free space. Will this beam show a diffraction pattern at a screen some distance away later on?

1. Yes, sure.
2. No way.

Ans: A.