# Homework Help: Optics problems - real vs, virtual / thin film

1. May 13, 2012

### big_boi

1. The problem statement, all variables and given/known data

33. This experiment projects a red laser light beam of wavelength λ through a diffraction grating labeled 750 lines/mm onto a screen that is positioned a distance l from the diffraction grating. The diffraction grating has a distance d between adjacent lines. The distance between the central maximum and first order bright line is x.

In order for the bright lines produced on the screen to appear closer together, what change could be made in the above experiment?
a. Replace the red laser with a green laser.
b. Move the laser farther from the diffraction grating
c. Move the diffraction grating farther from the screen.
d. Replace the diffraction grating with one that is labeled 800 lines/mm.
e. Rotate the diffraction grating by a small angle within the same place

34. Which of the following could produce the largest real image of an object?
a. Convex mirror, f=-5 cm, with object 3 cm from center of mirror.
b. Concave lens, f=-5 cm, with object 3 cm from center of lens
c. Convex lens, f=5 cm, with object 3 cm from center of lens
d. Convex lens, f=10 cm, with object 12 cm from center of lens.
e. Concave mirror, f=15 cm, with object 10 cm from center of lens.

35. A thin film of oil on water causes us to seen a green reflection of light. Which of the following must be true?
i. The light source must be only green light.
ii. The oil must be of lower index of refraction than the water.
iii. Other colors present in the incident beam underwent destructive interference
a. i only
b. ii only
c. iii only
d. i and ii only
e. ii and iii only

2. Relevant equations

3. The attempt at a solution

33 is E? - all the others do not seem right
34 - i have no idea
35 - b or e - destructive interference?

2. May 13, 2012

### Mindscrape

I'm not going to answer myself, since that will not be helpful, but maybe I can ask questions of you that can guide you in the right direction.

33) You need a better reason than the others do not seem right. What is the equation for orders of diffracted light?

34) What happens in the convex lens case when an object is a distance less than the focal length? How about for concave? How does a mirror instead of a lens change the image? Use these answers to guide your answer. Or do a quick drawing of all the situations.

35) It's all about the wavelength of the light reflected at the surface of the thin film. If n2<n1 what is the reflected wavelength?