# Thin-film equations

1. Apr 10, 2010

### r_swayze

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

White light illuminates an oil film on water. Viewing it directly from above, it looks red. Assume that the reflected red light has a wavelength of 615 nm in air, and that the oil has a thickness of 2.640e-7 m. What is the refractive index of the oil? Assume that the refractive index of water is greater than that of the oil.

2. Relevant equations

2nt = (m+1/2) λ

3. The attempt at a solution

2nt = (m+1/2) λ

n = 1/2 λ/(2t) = 1/2 (615e-9)/(2*2.64e-7)

n = 0.58239

I don't know why this answer is wrong. The problem seems to be straight forward, just plug in the numbers into formula right?

If m is greater than 0 then the answer n comes out higher then the refraction index of water, 1.33, so m has to be 0 I presume?

Edit: Never mind, figured it out.

Last edited: Apr 10, 2010
2. Jul 6, 2010

### Ravian

how you figured this out? can i know?

3. Jul 6, 2010

### MATLABdude

A refractive index less than 1 implies that the speed of light in that medium would be faster than the speed of light in vacuum!

To solve this example, you just need to consider the constructive interference condition, and couple that with Snell's Law.