Physics 7 E Prof D Casper Admin Website

Physics 7 E Prof. D. Casper

Admin • Website updated • Draft formula sheet posted • Register i. Clickers at https: //www 1. iclicker. com/register-clicker/ • Reminder: Programmable/graphing calculators are NOT allowed • Chapter 35 HW due Monday morning • Review session: Saturday, 2 – 4 pm, ICS 174 Monday Tuesday Wednesday Thursday Friday Today Ch. 35 HW Due Final (8 am) Saturday Review ICS 174 2 -4 pm

Interference in Thin Films A bit similar to yesterday’s mirror problem: One source produces two rays that can interfere with each other There is nothing new involved here, just a combination of things we learned long ago!

What’s Going On Here? In this case, there are reflections from two different surfaces spaced a short distance apart (the thickness of the film) The two rays will have a phase difference and can interfere constructively or destructively

First Source of Phase Difference: Distance •

Phase Shift from Travel Distance •

Second Source of Phase Different: Reflection •

Phase Shift from Reflection There are two reflections; either one (or both!, or neither!) can introduce an inversion! If the “top” material is air (usual case), then the first reflection (at point b) will have an inversion. The reflection at point d will depend on the indices of the film and whatever is on the other side.

Constructive or Destructive? •

Putting it All Together The exact relation we need to solve depends on whether we are interested in constructive or destructive interference, and whethere is a net phase shift between the two reflections. Far better, in my opinion, to remember the basic principles instead of trying to memorize four slightly different equations.

Example: A Soap Bubble in Air • n=1. 33 n=1

Another Example: Non-Reflective Coating • n=1. 40 n=1. 60

Another Example: Air Wedge Is there an overall phase shift from reflection here? Yes – the reflection from the glass/air boundary is not inverted, but the one from the air/glass boundary is inverted. Q: Is the point of contact (at far left) bright or dark? A: Dark (why? )

Q 35. 6 An air wedge separates two glass plates as shown. Light of wavelength strikes the upper plate at normal incidence. At a point where the air wedge has thickness t, you will see a bright fringe if t equals A. /2. B. 3 /4. C. . D. either A. or C. E. any of A. , B. , or C.

A 35. 6 An air wedge separates two glass plates as shown. Light of wavelength strikes the upper plate at normal incidence. At a point where the air wedge has thickness t, you will see a bright fringe if t equals A. /2. B. 3 /4. C. . D. either A. or C. E. any of A. , B. , or C.

Air Wedge •

“Newton’s Rings” A similar interference pattern occurs when a planoconvex lens is placed on a flat surface

Thank you!