Follow these steps to solve this problem: A very thin oil film (noil=1.25) float
ID: 250708 • Letter: F
Question
Follow these steps to solve this problem: A very thin oil film (noil=1.25) floats on water (nwater=1.33). What is the thinnest film that produces a strong reflection for green light with a wavelength of 500 nm?
1.Which of the following equations should be used to find the minimum thickness t of the oil film that satisfies the conditions of the problem? Let be the wavelength of light in air and n the index of refraction of the thin film
. a. 2t=mn(m=0,1,2,...) for constructive interference.
b.2t=(m+12)n(m=0,1,2,...) for destructive interference.
c.2t=(m+12)n(m=0,1,2,...) for constructive interference.
d. 2t=mn(m=0,1,2,...) for destructive interference.
2. What is the minimum (non-zero) thickness t of the film that produces a strong reflection for green light with a wavelength of 500 nm?
Explanation / Answer
1.When light reflected from oil and water interfere constructively, strong reflection occurs;
If t is thickness of oil, extra distance travelled by light in reflacting from water is 2t, this distance has to be a multiple of wavelength for constructive interference.
Also in material of index n , wavelength decreases by factor of n
these conditions gives us that we should use (a) equation(i.e. 2t = m/n) for solving the problem.
2.For minimum thickness t we put m = 1 => t = /2noil = 500/2.5 = 200nm