# Interference of light in thin films

jerry222
Homework Statement:
Suppose we have a thin oil film (n=1.5) in air. The light strikes the film vertically. We use a spectrometer to see that an intensity maximum at wavelength of 600nm and a minimum at 300nm. And there is no minima inbetween. How thick is the film?
Relevant Equations:
\delta=(2pi/lamda)
Phase difference is $\phi=\frac{2pi}{\lambda}* \Delta+\pi$

Phase difference, max: $\Delta \phi=2pim=\frac{2pi}{\lamda_{max}}*2nd$

Phase difference, max: $\Delta \phi=2pim=\frac{2pi}{\lamda_{min}}*2nd+pi$

Flim thickness: $d=100nm$

Set the equations equal to each other i got a d=-100nm which is strange. Can someone help?

Homework Helper
Gold Member
2022 Award
Homework Statement:: Suppose we have a thin oil film (n=1.5) in air. The light strikes the film vertically. We use a spectrometer to see that an intensity maximum at wavelength of 600nm and a minimum at 300nm. And there is no minima inbetween. How thick is the film?
Relevant Equations:: ##\delta=(2\pi/\lambda)##

Phase difference is ##\phi=\frac{2pi}{\lambda}* \Delta+\pi##
Phase difference, max: ##\Delta \phi=2pim=\frac{2pi}{\lamda_{max}}*2nd##
Phase difference, max: ##\Delta \phi=2pim=\frac{2pi}{\lamda_{min}}*2nd+pi##
Flim thickness: ##d=100nm##

Set the equations equal to each other i got a d=-100nm which is strange. Can someone help?
I tried fixing up your LaTeX but it still isn’t making much sense:
Phase difference is ##\phi=\frac{2\pi}{\lambda}* \Delta+\pi##
Phase difference, max: ##\Delta \phi=2\pi m=\frac{2\pi}{\lambda_{max}}*2nd##
Phase difference, max: ##\Delta \phi=2\pi m=\frac{2\pi}{\lambda_{min}}*2nd+\pi##
Flim thickness: ##d=100nm##

Don't you mean Phase difference is ##\pi##?
Anyway, a difference is unsigned, so in the equation it's ##\pm\pi##.

jerry222
I tried fixing up your LaTeX but it still isn’t making much sense:
Phase difference is ##\phi=\frac{2\pi}{\lambda}* \Delta+\pi##
Phase difference, max: ##\Delta \phi=2\pi m=\frac{2\pi}{\lambda_{max}}*2nd##
Phase difference, max: ##\Delta \phi=2\pi m=\frac{2\pi}{\lambda_{min}}*2nd+\pi##
Flim thickness: ##d=100nm##

Don't you mean Phase difference is ##\pi##?
Anyway, a difference is unsigned, so in the equation it's ##\pm\pi##.
yeah where pi is the phase change due to refraction and reflection

Homework Helper
Gold Member
2022 Award
yeah where pi is the phase change due to refraction and reflection
So which sign should you select?

jerry222
So which sign should you select?
ah i got -100nm, the correct answer is 100...i just set those two eaquations equal to each other, that s what i wanted to ask...