Passage II

Suppose light is shone on a thin film of oil floating on water. The reflected light will be almost as bright as the incident light (the light shone on the film) if the light undergoes constructive interference, and it will be much dimmer than the incident light if the light undergoes destructive interference.

Students conducted a series of studies to better understand the variables that affect interference.

Study 1

The students measured a small volume of a particular type of oil into a cylindrical tank of water, forming a film of depth D on the water's surface (see Figure 1).

They shone light at a frequency of 6.0 x 10¹⁴hertz (Hz) straight down onto the film and measured the brightness of the reflected light, B. They added more of the oil to the tank, measuring B and recording the total volume of oil in the tank after each addition.

The students used the known cross-sectional area of the tank and the total volume of oil in the tank to calculate D for each measured B. A graph of B versus D is shown in Figure 2.

Study 2

The students used different types of oil, each having a different index of refraction (a measure of the oil's tendency to bend light). For each oil, the students used the procedure from Study 1 to find D of the shallowest film that produced a maximum in B(see Table 1).

Study 3

The students used light of different frequencies and Oil W. For each light frequency, they followed the procedure from Study 1 to find D of the shallowest film that produced a maximum in B (see Table 2).