A perfectly flat piece of glass (n = 1.50) is placed over a perfectly flat piece of black plastic (n = 1.20) as shown in the figure a. They touch at A. Light of wavelength 600 nm is incident normally from above. The location of the dark fringes in the reflected light is shown on the sketch of figure b. (a) How thick is the space between the glass and the plastic at B? (b) Water (n = 1.33) seeps into the region between the glass and the plastic. How many dark fringes are seen when all the air has been displaced by water? (The straightness and equal spacing of the fringes is an accurate test of the flatness of the glass.) Glass =1.50 Plastic n=1.20 (a) (b) A B

A perfectly flat piece of glass (n = 1.50) is placed over a perfectly flat piece of black plastic (n = 1.20) as shown in the figure a. They touch at A. Light of wavelength 600 nm is incident normally from above. The location of the dark fringes in the reflected light is shown on the sketch of figure b. (a) How thick is the space between the glass and the plastic at B? (b) Water (n = 1.33) seeps into the region between the glass and the plastic. How many dark fringes are seen when all the air has been displaced by water? (The straightness and equal spacing of the fringes is an accurate test of the flatness of the glass.) Glass =1.50 Plastic n=1.20 (a) (b) A B