PHY385 Module 9

                                                                    Student Guide

Concepts of this module

Polarization of light transmitted through a dichroic material (Polaroid film).

Malus's Law.

Polarization by reflection. Brewster's angle.

Quiz

Activity 1 - Calibration of the Light Sensor.

EQUIPMENT NEEDED:

-Optics Bench

-Light Source

- Diode laser

-Polarizers (2)

-Component Holders (3)

-Ray Table and Base

-Ray Table Component Holder

-Cylindrical Lens

-Crossed Arrow Target

-Slit Plate

-Slit Mask

-Light Sensor attached to a stand.

The PASCO Light Sensor CI-6504A is shown in Fig.1.

The sensor measures relative illuminance for a spectrum in a range of 320 nm - 1100 nm. In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. In SI derived units these are measured in lux (lx). Luminosity function approximates the response of the human eye to power of the light of different wavelengths.

The gain switch changes the maximum light intensity as follows:

1xGain, 500 lux;

10xGain, 50 lux;

100xGain, 5 lux.

The latter provides the greatest possible sensitivity if the light intensity is very low.

In some parts of this experiment the luminosity is low, thus the sensitivity must be the greatest possible.

Despite the fact that the illuminance is measured in lux, the value you will read is in volts. This conversion made by the interface, does not change any conclusion you can make basing on your measurements.

You must calibrate the sensor before using it for the first time.

Connect the DIN connector to the Light Sensor, and the interface connector plug into the A input of the Data Acquisition Device. The Data Acquisition Device is the computer interface.

On the desktop of your computer find an icon "2^nd year files" at the left bottom corner and click on it to turn on the data acquisition process. You will see numbers in volts. If you shade the photodiode aperture, the readings will still be non-zero and fluctuating very rapidly.

For each gain, find the zero by closing the aperture of the diode and measuring the offset of the sensor for this gain.

Activity 2 - Polarization of light transmitted through a dichroic material (Polaroid film).

Assemble the optical equipment as shown in Fig. 2.

For each exercise you will need just some of the shown elements of the optical system. The Light sensor must be mounted on the holder and its collimator with aperture must be directed towards the light source. The Crosse Arrow Target produces a narrow light beam, which is easy to detect.

You will need one polarizer for this exercise. The Polarizer B should be temporary removed from the Optics Bench.

Study the incandescent lamp in the Light Source and the Light Diode with the Light Sensor in two steps: first without, next with the polarizer in between the light source and the Light Sensor. The polarizer you are using is a film called Polaroid. Record your results for future calculations. Conclude on

(a) a possible kind of polarization of light of each source and

(b) on a coefficient of attenuation of the polarizer (the Polaroid film).

Activity 3 - Malus's Law

Use the incandescent lamp as a source for this experiment. Insert two polarizers between the source of the light and the Light Sensor. Assemble the pair of polarizers in such a way that the reading of the Light Sensor is at maximum. This corresponds to the parallel transmission axes of the two polarizers. Identify and record the positions of the transmission axes for your future measurements. If the Crassed Arrow Target dramatically reduces illuminance of the detector, remove it from the Optics Bench. In your notebook, prepare a table for values of measured angles θ between the axes of the two polarizers in the range of angles from 0 to 180^o; the measured illuminance for each angle in volts; and the (cos θ )². Slowly rotating one of the polarizers, record the step-by-step readings of the Light Sensor. Fill in the table and graph two functions: "illuminance vs. θ" and "illuminance vs. (cos θ)²". Using your results of the exercise 3, suggest an analytical expression for the relationship among the illuminance given by the primary beam of light; the illuminance, detected by the Light Sensor after passing the two polarizers; and the angle between the transmission axes of the two polarizers. It will be your discovery of Malus's law!

Activity 4 - Polarization by reflection. Brewster's angle.

The Brewster's angle of incidence of non-polarized light on the surface separating two media with different indices of refraction results in the fully polarized reflected light. In this case, the refracted and reflected rays are perpendicular to each other.

1. Using the cylindrical lens, assemble a setup as shown in Fig. 3.

2. Place a polarizer on the way of the reflected ray and the Light Sensor behind the polarizer.

3. Slowly rotate the polarizer to observe the change in the illuminance due to the change of the direction of the optical axis of the polarizer.

4. Give your conclusion on whether this position corresponds to the incidence under the Brewster's angle. Record the Brewster's angle in your notebook..

If the angle of incidence needs adjustments, get the maximum possible reading of the Light Sensor in the initial position of the lens, and then try to slightly change the angle of incidence and observe the change in the measured illuminance. After you find the perfect position of the lens, use the Brewster's angle to calculate the index of refraction of the lens and show your work.

Replace the incandescent lamp by the diode laser and repeat your measurements. What is the main difference in the results of experiments with the incandescent lamp and the laser in this exercise?

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This Student Guide was created by Natalia Krasnopolskaia in November 2014.