The graph of the sine curve below is of electromagnetic energy that represents red light.
what function accurately represents the sine curve for red light?

a. f(x) = sin pi over 640 x

b. f(x) = sin 640πx

c. f(x) = sin 320πx

d. f(x) = sin pi over 320 x

The answer is D. Suppose a function y=sin(ax), then the period of y is 2pi/a. According to the graph, the period of f(x) is 640, so 2pi/a=640, s=2pi/640, or pi/320, so choose D. To verify this is true, plug in some x values on the graph and check if f(x) is correct. For instance, when x=320, f(x)=0.