1. A Young's double slit has a slit separation of 2.5 x10^-5 m on which a monochromatic light beam is directed. The resultant bright fringes on a screen 1.0 m from the double slit are separated by 2.3 x10^-2 m. What is the wavelength of this beam? (1 nm = l0^-9 m)

ans: 575 nm; use w/d = s/D where w=wavelength, d=slit separation, s=fringe spacing, and D=screen distance.

2. What is the minimum thickness of a soap bubble film (n = 1.46) on which light of wave length 500 nm shines, assuming one observes constructive interference of the reflected light?

ans: 86 nm; first, the wavelength in the soap is w/1.46. Then, since one reflection is "hard" (180 deg phase shift) and the other is "soft" (no phase shift), the added light path through the soap (2x thickness) must be 1/2 wavelength.

3. A silicon monoxide thin film (n = 1.45) of thickness 97.4 nm is applied to a camera lens made of glass (n = 1.58). This will result in a destructive interference for reflected light of what wavelength?

ans: 565 nm; both reflections are "hard," so 2x the thickness must be 1/2 wave, in the film. Then, w in air is 1.45*4*97.4.

4. In a Young's double slit interference apparatus, by what factor is the distance between adjacent light and dark fringes changed when the separation between slits is tripled?

ans: 1/3; use w/d = s/D as above.

5. In a Young's double slit interference apparatus, by what factor is the distance between adjacent light and dark fringes changed when the wavelength of the source is tripled?

ans: 3.0; use w/d = s/D as above.

6. Two flat glass plates are in contact along one end and are separated by a sheet of tissue paper at the other end. A monochromatic source of 490 nm illuminates the top plate. If 21 dark bands are counted across the top plate, what is the paper thickness?

ans: 4.9 x10^-6 m; each dark band signals one more wavelength in added path length (1/2 wave in spacing), so the thickness is 10 wavelengths.

7. A puddle of water (n = 1.33) is covered with a very thin layer of oil (n = 1.20). How thick is the oil in the region that reflects light with a wavelength of 550 nm?

ans: 229 nm; two hard reflections: the oil should be w/2 thick where w=550/1.20.