Optical Bench Polarization and Brewster s Angle When light is reflected from a surface it can become polarized. If light is reflected at a sharp angle from a surface (such as light from a sunset reflecting from the surface of a lake, the polarization can be dramatic. This is why polarizing sunglasses block horizontally polarized light in order to reduce sun glare. The angle at which polarization is maximized is called Brewster s angle. Equipment: Optical Bench Cautions: This equipment is delicate. Everything should go together with the lightest of touches. Do not force anything! Procedure to set up the Optical Bench 1. Place the optical bench on the table with the zero point of the scale to the left. 2. Place Light Source with filament (vertical notch reference along bottom edge) at 20 cm on the Optics Bench shining toward the right end of the bench. 3. Find one of the taller component holders with a solid flat foot 4. Mount the Slit Plate on the foot side of a Component Holder. The foot will point toward the light source. 5. Mount the Slit Mask on the other side of the same Component Holder (taller frame with solid flat foot). 6. Place the Component Holder so that its vertical plate is at the 30 cm mark on the Optics Bench and its foot points toward the light source. 7. Attach the round Ray Table to the Ray Table base with the protractor side (not the grid side up. 8. Mount the Ray Table and Base to the optical bench to the right of the component holder so that the center of the Ray Table is at the 40 cm mark on the bench and the downhill edge of the Ray Table is closest to the Light Source. 9. Turn the round ray table so that the zero degree Normal Line is parallel to the length of the Optics Bench. 10. Connect the Light Source power supply and turn the Light Source on. 11. Align the Slit Plate and Slit Mask so that light from just one slit shines along the center NORMAL line of the Ray Table. This alignment requires that one slit of the Slit Plate be centered in the Light Source beam, and that the Slit Mask opening is centered on that one Slit Plate slit, and that the Ray Table is turned to align the Normal Line with the light ray. One other adjustment that may be necessary is some slight rotation of the knob on top of the Light Source, which rotates the lamp and filament inside to fine tune the position of the light ray. 12. Mount one Polarizer on a solid-foot Component Holder with its zero degree mark at the top, 13. Position the holder on the Optics Bench between the Light Source and the Slit Plate. 14. Mount the second Polarizer on the Ray Table Component Holder (the shorter frame with diamond hole in foot); on the side of the Holder frame away from the foot. This Polarizer must also be oriented with zero degrees at the top. 15. Position the second Polarizer and Holder on the edge of the Ray Table farthest from the Light Source. TOPS Optical Bench Polarization and Brewster's Angle.doc Page 1
Now you will observe the effect of passing light through two polarizers at different angles. 1. Look through the polarizer mounted on the ray table back toward the light source (you may have to move your head back and forth a bit to find the spot at which the light passes through all the assembled components. You are looking for the image of the filament itself. Observe the brightness of the filament image. Don t look at the filament for too long to avoid afterimages. Note: you can discern 6 levels of relative brightness (bright, dim, medium, medium bright, medium dim, or no light at all). Please use these terms while recording your observations. Write your observation here (hint: this one is bright!) 2. Leave the second Polarizer in place on the Ray Table, but reorient the Polarizer by rotating it counterclockwise so the 30 degree mark is at the top. Continue in 30 degree steps up to and including 180 degrees orientation, and observe the brightness of light transmitted through the polarizers set at each angle and record the relative brightness of light below: Relative Brightness at 30 : Relative Brightness at 60 : Relative Brightness at 120 : Relative Brightness at 150 : Relative Brightness at 180 : Now you have some experience with determining the brightness of the transmitted light. You will use this experience in the next procedure 3. Remove the first Polarizer and Holder from its place between the Light Source and Slit Plate. Put this Polarizer and Holder away carefully. Next, you will observe the effect on polarization of light passing through a lens. 1. Place the Cylindrical Lens on the Ray Table with the label Cylindrical Lens up and the curved face away from the Light Source. Adjust the Cylindrical Lens so that the flat surface facing the Light Source is centered and aligned on the Component line of the Ray Table so that the light beam remains aligned with the normal line. 2. Observe the brightness of the light ray transmitted through the lens with the Polarizer oriented at zero degrees, again at 45 degrees, and once more at 90 degrees, and record your observations here: Make sure that you look through the bottom half of the polarizer so you are seeing light that passes through the lens. TOPS Optical Bench Polarization and Brewster's Angle.doc Page 2
TOPS Physics 3. Rotate the Ray Table 70 degrees clockwise (light ray falls along the 70 degree line). Move the Component holder with the polarizer 40 degrees counterclockwise around the edge of the Ray Table to position it in line with the transmitted ray. 4. Observe the brightness of the light ray transmitted through the lens with the Polarizer oriented at zero degrees, again at 45 degrees, and once more at 90 degrees, and record your observations here: Now you will observe the effect on polarization when a beam of light is reflected from a surface. 1. Move the ray table counterclockwise 140 degrees so that the incoming ray sits on the other 70 degree mark on the Ray Table 2. Align the polarizer in front of the reflected ray from the cylindrical lens. It should be somewhere near the other 70 degree mark on the table. 3. Observe the intensity with the Polarizer oriented at zero degrees, again at 45 degrees, and 4. Rotate the Ray Table 10 degrees clockwise (light ray falls along the 60 degree line). 5. Move the Polarizer 10 degrees clockwise (to the 60 degree position). 6. Observe the intensity with the Polarizer oriented at zero degrees, again at 45 degrees, and 7. Rotate the Ray Table 10 degrees clockwise (light ray falls along the 50 degree line). 8. Move the Polarizer 10 degrees clockwise (to the 50 degree position). 9. Observe the intensity with the Polarizer oriented at zero degrees, again at 45 degrees, and TOPS Optical Bench Polarization and Brewster's Angle.doc Page 3
TOPS Physics 10. Rotate the Ray Table 10 degrees clockwise (light ray falls along the 40 degree line). 11. Move the Polarizer 10 degrees clockwise (to the 40 degree position). 12. Observe the intensity with the Polarizer oriented at zero degrees, again at 45 degrees, and 13. Rotate the Ray Table 10 degrees clockwise (light ray falls along the 30 degree line). 14. Move the Polarizer 10 degrees clockwise (to the 30 degree position). 15. Observe the intensity with the Polarizer oriented at zero degrees, again at 45 degrees, and 16. Turn off the Light Source and put the materials away carefully, unless the Teacher instructs you to leave specific items in place for the next class. Data Analysis: You will now examine the data collected to see if there are any trends. The first set of data with two polarizers should be fairly straightforward. Answer the following questions after referring to the data taken for two polarizers: At what polarizer angle(s) was the light brightest? At what polarizer angle(s) was the light least bright? TOPS Optical Bench Polarization and Brewster's Angle.doc Page 4
Next, you ll examine the data collected when the light passed through and was refracted by the cylindrical lens. Look at your data and answer the following question: Did the angle of light transmission affect the brightness of the image at any angle of refraction or polarization? Finally, you will look at the relationship between polarization and angle of reflection You have a lot of data for this, so you ll tabulate it below. You might find it to be more convenient to use numbers to represent the observed brightness of the filament. 0=dark, 1=dim, 2=medium dim, 3=medium, 4=medium bright, 5=bright Incident Angle 70 60 50 40 30 0 Polarizer Angle 45 90 The definition of Brewster s Angle is the angle of reflection at which the light is completely polarized parallel to the reflecting surface. At angles less than or greater than Brewster s angle, the light is only partially polarized. Examining the data in the table above, make an estimate of Brewster s angle for the reflections. It is the incident angle at which the difference between light intensity at 0 degrees and 90 degrees polarizer angle is greatest. Discuss your estimate below: TOPS Optical Bench Polarization and Brewster's Angle.doc Page 5