NORTHERN ILLINOIS UNIVERSITY PHYSICS DEPARTMENT Physics 211 E&M and Quantum Physics Spring 2018 Lab #7: Reflection & Refraction Lab Writeup Due: Mon/Wed/Thu/Fri, March 26/28/29/30, 2018 Background Light we all take it for granted since it is always there and a basic part of life, in fact there would be no life as we know it without light. However, do you know anything about the properties of this basic necessity? Does light really behave as we assume it does? Isaac Newton published the first extensive study of light in his Opticks in 1704. Newton envisioned light as made of corpuscles or small particles. The Dutch scientist Christiaan Huygens described light as waves in the late 1600 s. In fact, both Newton and Huygens were correct light is both a particle (photon) and a wave! Particle physics tells us that photons of light serve the function of communicating the electromagnetic force between charged particles. All waves transport energy, this is true for sound, water and earthquake waves in addition to light. The energy of a single photon is directly proportional to the frequency of the wave based on the formula E = hf (where h is Planck s constant). Frequency is inversely proportional to wavelength, so higher frequency is proportional to shorter wavelength. Geometric optics describes the propagation of light as rays and describes the interaction of visible light with reflecting, transparent and semi-transparent materials. The purpose of this lab is to observe and measure the reflection and refraction of light rays using mirrors and various transparent materials. 1. Overview Snell s law states that the angle of incidence θ 1 is equal to the angle of reflection θ 1 in reference to a line normal (perpendicular) to the surface at the ray s point of contact as shown below. Note that this applies to reflected rays:
Snell s law also addresses refraction of light, which occurs when light travels through a (semi) transparent material. The light rays will actually bend as they travel through any (semi) transparent material and the degree of bending is proportional to the density of the material. The materials density to light rays is proportional to the index of refraction, n. Snell s law of reflection and refraction is: n 1 sinq 1 = n 2 sinq 2 In the example above, n = 1.3 for water and n = 1.0 for air. The ratio of the index of refraction of the two materials is proportional to the velocities of the light rays in the two materials: n n v v 1 1 2 2. This means that the light ray slows down to a velocity less then c inside a material, which is the reason the ray bends in the material. All angles are measured from the normal line drawn perpendicular to the surface at the point of incidence. Note that as the light slows down after entering the material it bends towards the normal line. The index of refraction of a material is equal to the ratio of the speed of light in vacuum to the c speed of light in the material: n v Materials may have both reflections and refractions occurring at exterior and interior surface points as pictured below (left): In prisms the angle of deviation is defined as the angle between the direction an incident light ray would travel if it wasn t refracted and the light ray that exits the prism after refracting at both surfaces, as shown above (right). 2
We see white light when all visible wavelengths are present our eyes can t distinguish the individual colors (wavelengths). Prisms can separate white light into constituent colors because the wavelength of the individual colors has a different angle of deviation through the prism. The two refractions through the prism separate the white light by wavelength, which corresponds to color in the visible part of the spectrum (400-700 nanometers). The velocity of light is proportional to wavelength because the frequency of the light wave remains constant. The light slows down inside the prism, but to different velocities based on v = fλ. 2. Procedure Apparatus includes:. Pasco (OS-8470) Light Box with single & 4 slit filter, red & blue colored filter. Semi-circular plastic, equilateral plastic triangle, mirror on stand. Protractor. Protractor paper. Ruler. Brown cork board & pins. CARBON DI-OXIDE acetate A. Activity #1 Snell's Law of Reflection You will be using a light box, specifically the narrow end of the box in which you will insert slit patterns and filters adjust the collimator lens to generate parallel light rays. You should put an unused plastic filter in the large end of the light box to block the light. 1. Orient the protractor paper so the 0-180 degree line is on the horizontal axis and attach to the cork board with pins. 2. Setup the light box with a single slit filter and corresponding light ray exactly along the 0-180 deg line. Place the mirror on the polar graph paper at any angle between 70 and 80 degrees (making an angle of 20-10 degrees with the light ray). 3. Center the mirror at the point the incident ray hits the mirror, mark this spot on your graph paper, and trace the position of the mirror. Draw the line normal to the mirror at the point the incident ray strikes the mirror, then measure the incident and reflected angle in degrees from the normal line. In your lab notebook, write down your measured Incident angle and Reflected angle. Cell Phone Picture #1: Take a cell phone picture of your drawings. All cell phone pictures should be of good quality (not blurry or distorted): 3
B. Activity #2 Virtual Images using Plane Mirrors 4. Place the mirror about 4 cm behind the pin and trace its position. Observe the reflection of the pin in the mirror it should appear behind the mirror. 5. Aim a single ray of light at the mirror at an angle such that the reflected ray from the mirror hits the pin (real pin, not the image in the mirror) mark the mirror position and draw the reflected ray only. 6. Now move the light box to another angle such that the reflected light ray strikes the pin and draw the reflected light ray, repeat for several positions of the light ray on either side of the pin. 7. Remove the mirror and extend all the reflected rays you drew to behind where the mirror was. Do they meet at a single point? 8. Replace the mirror at its previous position do the extensions of the reflected rays you drew pass through the reflection of the pin? Measure the distance from the mirror to the actual pin and compare it to the distance from the mirror to the point your lines converge at behind the mirror they should be the same. 9. Since there are no actual light rays going through the mirror, the reflected image of the pin is a virtual image that is produced by the eye/brain visual system based on the location that would produce the reflected rays we see in the mirror. Cell Phone Picture #2: Take a cell phone picture of your drawings and measurements. Make certain picture is of good quality and readable. C. Activity #3 Lateral Inversion in a Plane Mirror 10. In your PreLab activities you predicted how the word CARBON DI-OXIDE, would appear in a mirror, the reflection appears to exhibit lateral (horizontal in this case) inversion because the reflected words are backwards and the non-symmetric letters are reversed. However, is this really what a mirror does? 11. Write the word CARBON DI-OXIDE on a piece of clear acetate (clear tape) and look at its reflection in a mirror and you should see that when you are able to read the word on the acetate, the reflection is not reversed. Plane mirrors reflect exactly what is in front of them, except they invert it front to back as seen below. Looking from the side we can see our image faces in the opposite direction. 12. Pair up with one of your partners and shake each other s right hand. Your partner s right hand will be on your left, this is not what you see when looking at your reflection in a mirror your right hand appears on the right, but if you put yourself in 4
the place of your reflection you are reaching out your left hand! The key is don t put yourself in the place of your reflection!! 13. (Light Box) Place a 4 slit filter in the light box. Place a blue filter in front of the leftmost slit (as viewed from the front) and a red filter over the rightmost slit (the other 2 slits will be covered by the frames of the colored filters). Rotate the mirror to any angle as shown below and draw the resulting reflected red & blue rays on the picture below. Are the reflections reversed or not? Your answer depends on what perspective you take when you look at the mirror from behind the light box, the incident blue light comes out of the right side of the light box and the reflected blue ray comes from the right side of the mirror. Only if you put yourself in the mirror would you perceive that the blue reflected ray is coming from the left side of the mirror!! Draw the reflected rays and locate the virtual image of the red and blue light on the above diagram and include the entire diagram in your lab notebook. 14. (Pasco (OS-8470) Light Box) Rotate the slit/filter wheel to the colored filters. Rotate the mirror to any angle as shown below and draw the resulting reflected blue, green & red rays on the picture below. Are the reflections reversed or not? Your answer depends on what perspective you take when you look at the mirror from behind the light box, the incident red light comes out of the right side of the light box and the reflected blue ray comes from the left side of the mirror. Only if you put yourself in the mirror would you perceive that the blue reflected ray is coming from the right side of the mirror!! Draw the reflected rays and locate the virtual image of the red and blue light on the above diagram and include the entire diagram in your lab notebook. D. Activity #4 Refraction 15. NOTE: When using the Pasco OS-8470, use the PLASTIC SEMI-CIRCULAR piece instead of the glass for steps 16-19. Orient the protractor paper so the 0-180 degree line is on the horizontal axis. Align the flat side of the semi-circular plastic (SCP) piece along the 90/270 degree line. 5
Rotate the slit/filter wheel to the single slit and align the single slit light ray along the 0/180 degree line so the light ray hits the exact center of the flat side of the SCP and there is no reflected ray, which occurs when the incident light ray is perpendicular to the flat side. Trace in the position of the SCP and the light ray. Figure 1 Alignment of light ray along 0-180 deg line centered on the base of the SCP 16. Rotate the flat side of the SCP clockwise 10 degrees (to the 75 degrees line on the paper), keeping the ray on the center of the flat side of the SCP. Note that the light ray that exits the SCP is perpendicular to the circular side. Open up Excel and make the following table below: Measure the refracted angle and put that value in the Excel table. Note: You need to make sure that the incident light ray is on the 0-180 degree line and centered on the flat side of the SCP during each measurement. Take a cell phone picture (Cell Phone Picture #3) of your setup when you are at an angle of 60 degrees. NOTE: All angles are measured from the line normal to the surface. 6
17. Move the SCP so the incident light ray strikes at a 20 degree angle at a point approximately midway between the center and the edge of the flat base, similar to below. On your protractor graph paper, trace the position of the SCP and mark the points of entry & exit of the light ray. Remember to draw in the lines normal to the surface at both points of entry & exit. Using a protractor, measure the angle of refraction as the light ray enters the flat base and notice that there is also an angle of refraction as the ray leaves the semi-circular side. Record the angle of incidence and both angles of refraction. Take a cell phone picture (Cell Phone Picture #4) of your drawings and make certain you record the angles in your lab notebook. 3. Questions 1. For your lab writup, put your 4 (or more if you wish) cell phone pictures and tables at the appropriate places along with answers to all of the questions (in blue) above. Make certain to write a short explanation (or figure caption) for each cell phone picture. When inserting pictures, use word to crop the images so that only the important parts of the images show (do not show the whole notebook page unless your drawings take up the entire page). 2. Put the Cell Phone Picture #1 in your lab writeup (Part 3) and comment on your measurments. 3. Describe how Snell s law of reflection creates virtual images that appear to be behind plane mirrors. Explain how our eyes use the reflected rays to determine the location of a virtual image. To help you illustrate your explanation, include the Cell Phone Picture #2 (Part 9) (and answer the questions in Parts 7 & 8). 4. Include in your lab writeup the cell phone pictures of your lab notebook drawings for Parts 13 & 14, and comment on your results. 5. For Part 16: a. Put the Excel table along with the Cell Phone Picture #3 in your lab writeup. b. In your Excel table, which of the following quantities were relatively constant: the difference i r, the ratio i / r, or the ratio sin( i) / sin( r)? 7
c. Make an Excel plot of i verses r and include it in your lab writeup. Is the plot linear? What useful information does the plot give? d. Make an Excel plot of sin( i ) verses sin( r ) and include it in your lab writeup. Is this plot linear? Fit the plot with a straight line and put the equation of the fit on the plot. What does this slope represent? How much does the slope differ from the average value of sin( i) / sin( r) in your Excel table? Which value should you report for the index of refraction in a journal article: the slope or the average value of sin( ) / sin( )? e. Explain why there was no refraction as the light ray exited the circular side of the SCP. 6. For Part 17: Put the Cell Phone Picture #4 along with your 3 measured angles into you lab writeup. Why did the light ray refract when it exited the SCP on the circular side? Which way did the light ray bend (toward or away from the normal)? Using the index of refraction you measured in Question 5(d) above, verify that all the refracted angles satisfy Snell's law. i r 8