Lab 2 Procedure

To verify Fresnel's equations you will need to measure the intensities for two polarizations (parallel and perpendicular) of a light beam incident on a glass plate for a range of incident angles. A helium neon laser will be used as the light source, and a photometer with a fiber optic input will be used to measure the intensity.

Apparatus to measure Fresnel coefficients

The glass plate is placed on an angular translator, with a vernier scale, so that the incident angle can be accurately measured. A polaroid is used to linearly polarize the incoming light so that it is parallel or perpendicular to the plane of incidence.

Set the glass plate in the holder at a right angle to the laser.
Set the polarizer at 0 degrees. Measure the intensity of the straight through beam, with the plate still perpendicular to the laser.
Rotate the glass plate and measure the intensity of the reflected ray. Note: You will be seeing two reflected beams, one from the front surface of the glass and one from the back surface. You only want the front surface reflection.
Set the polarizer to 90 degrees and measure the intensity of the straight through beam. If the intensity differs from the intensity for the beam at 0 degrees, the correct for this factor in your measurements.
Take measurements at the same angles as you did with the polaroid at 0 degrees. It is possible to be very exact because of the vernier scale on the glass stand.

Record the intensities for each polarization and then plot the ratio of the parallel / perpendicular intensities versus the incident angle. Determine Brewster's angle from the plot. A typical plot is shown below.

PROCEDURE In this experiment you will verify Fresnel's Equations, and find the Brewster angle for a glass plate. You will also need to measure the refractive index of the glass plate. The refractive index can be measured with an Abbe Refractometer, using the procedure outlined below. In order to become familiar with the apparatus and its capabilities, you are also asked to measure the refractive indices of two liquids for which the refractive index has been recorded in the CRC Handbook of Physics & Chemistry.
The Abbe Refractometer
1. Calibrating the Refractometer Before any measurements can be taken, the refractometer must be calibrated with the standard specimen supplied with the instrument. Use the Prism Lock Knob to unlock the Incident Prism and lift it out of the way. Apply a drop or two of Napthalene Bromide (supplied with the instrument) to the polished face of the Refracting Prism. Lay the polished face of the standard sample on the refracting prism surface, with the Napthalene bromide acting as a "glue" to ensure proper contact between the sample and the prism face. Open the Reflecting Mirror for the incident white light.

	When you look into the Eyepiece you will see a circle with crosshairs and, below that, a scale illuminated with a green light. The scale is graduated on the bottom for refractive indices from 1.300 to 1.700 and on the top scale for % sugar solution. Set the refractive index scale to the number marked on the sample. Turn the Dispersion Correction Knob until the colors disappear and a distinct white/black boundary can be seen in the crosshairs. Using the supplied screwdriver, adjust the Adjustment Screw until the white/black boundary is lined up exactly in the center of the crosshairs.

2. Measuring the Refractive Index of a Liquid Open the Incident Prism (with the Prism Lock Knob) and place a few drops of the liquid to be tested on the polished surface of the Refracting Prism. Close the Incident Prism and lock it into place with the knob, so that the liquid is evely distributed on the face of the refracting prism. Close the reflecting mirror and open the light shield for the incident prism. Adjust the dispersion, using the Dispersion Correction Knob, until a clear light/dark boundary is seen. Center the boundary in the crosshairs of the telescope, using the Refractive Index Measurement Knob, and read the refractive index from the lower scale (the Condensor can be adjusted if the light on the scale is insufficient for easy reading). If you are measuring the concentration of a sugar solution, this can be read directly from the upper scale. Measure the refractive indices of the two liquids provided. 3. Measuring the Refractive Index of a Solid Place the glass plate on the Refracting Prism using a few drops of Napthalene Bromide. Adjust the dispersion, using the Dispersion Correction Knob, until a clear light/dark boundary is seen. Center the boundary in the crosshairs of the telescope and read the refractive index from the lower scale. Measure the refractive index of the glass plate that you will be using to measure the Fresnel reflection coefficients.
Fresnel's Equations and Brewster's Angle
To verify Fresnel's equations you will need to measure the intensities for two polarizations (parallel and perpendicular) of a light beam incident on a glass plate for a range of incident angles. A helium neon laser will be used as the light source, and a photometer with a fiber optic input will be used to measure the intensity. Apparatus to measure Fresnel coefficients The glass plate is placed on an angular translator, with a vernier scale, so that the incident angle can be accurately measured. A polaroid is used to linearly polarize the incoming light so that it is parallel or perpendicular to the plane of incidence. Set the glass plate in the holder at a right angle to the laser. Set the polarizer at 0 degrees. Measure the intensity of the straight through beam, with the plate still perpendicular to the laser. Rotate the glass plate and measure the intensity of the reflected ray. Note: You will be seeing two reflected beams, one from the front surface of the glass and one from the back surface. You only want the front surface reflection. Set the polarizer to 90 degrees and measure the intensity of the straight through beam. If the intensity differs from the intensity for the beam at 0 degrees, the correct for this factor in your measurements. Take measurements at the same angles as you did with the polaroid at 0 degrees. It is possible to be very exact because of the vernier scale on the glass stand. Record the intensities for each polarization and then plot the ratio of the parallel / perpendicular intensities versus the incident angle. Determine Brewster's angle from the plot. A typical plot is shown below.
A Typical Plot of the Data Matt Bravander, Alex Dzierba, March 2000