# Physics 109 - Index of Refraction

Today's lab begins with an Invention Activity designed to get you ready for calculations that will be done in the experiment below. Log on to http://tutor.phas.ubc.ca to begin the activity. Once completed, you can move on to the experiment.

This experiment is an introduction to optics and will explore Snell's Law, total internal reflection and Brewster's angle. The experiment involves making precise optical measurements with a laser and estimating the uncertainty in those measurements. These measurments and the data analysis allow you to measure the index of refraction n of plexiglass in three different ways.

CHECK ALIGNMENT

Check that the optical bench is aligned by setting the incident angle in air to 0 degrees and observe
the position of the final spot. Rotate the plexiglass through 180 degrees so that the incident angle
through plexiglass is again 0 degrees. The position of the final spot should not have moved.
If you feel that it is misaligned, contact your TA for help. DO NOT ADJUST ANY OF THE MIRRORS.

SNELL'S LAW

When entering the flat surface, the incident beam goes from air (index of refraction ~1) to plexiglass (index of refraction is n). By measuring the angle of the refracted beam and the angle of incidence, the index of refraction can be determined from

n = Sin(incident angle)/Sin(refracted angle)

Use an incident angle of 60 degrees for this measurement.
Record your value of n determined by this measurement, including an estimate of uncertainty.

CRITICAL ANGLE FOR TOTAL INTERNAL REFLECTION
With the beam entering the curved surface first, search for the critical angle of incidence at which you judge total internal reflection occurs. This is the incident angle beyond which the beam is reflected, but with no refracted beam. The index of refraction can be determined from this using:

Sin(critical angle) = 1/n

Record your value of n determined by this measurement, including an estimate of uncertainty.

### BREWSTER'S ANGLE

Brewster's angle is the angle of incidence at which the reflected beam is completely polarized. With the beam entering the flat surface first, determine the angle at which the reflected beam is completely polarized. At this angle, a polarizer intercepting the reflected beam can completely block it if it is oriented so that only passes the opposite polarization. The index of refraction can be determined from this using:

Tan(Brewter's angle) = n

Record your value of n determined by this measurment, including an estimate of uncertainty.

### Marking Scheme

Invention Activity: 2 marks

Hign quality measurment of index of refraction 3 different ways: 12 marks

Don't forget to describe your procedures as you go along through the experiment.