REFLECTION AND REFRACTION OF LIGHT

Transcription

1 PHYSICS LAB REFLECTION AND REFRACTION OF LIGHT Printed Names: Signatures: Date: Lab Section: Instructor: GRADE: PHYSICS DEPARTMENT JAMES MADISON UNIVERSITY Revision Spring 2002 REF 45

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3 PHYSICS 150 LAB INSTRUCTIONS REFLECTION AND REFRACTION OF LIGHT 1 Purpose: To observe the behavior of reflected and refracted light rays and to determine that such observed results confirm the stated laws of reflection and refraction. Equipment: Plane Mirror on Block, Laser, Corkboard, 5 Pins, Paper, Ruler/Straightedge, Plastic Block, Semi-circular Plastic Block and Protractor. Background: Ray tracing is a useful method for analyzing the behavior of light. According to the ray model, a source of light consists of rays, pencil beams of light whose thickness is arbitrarily small. These beams travel in perfectly straight lines as long as they remain in a medium with constant optical properties. The lasers that we will use in the lab mimic the ideal behavior of a light ray. You will observe however that the laser beam spot is not arbitrarily small so one must assume that the laser beam is made up of many parallel rays. In addition you will observe that the beam spot increases as the beam travels further from the laser. This can be explained by assuming that the rays are not exactly parallel. For our purposes we can assume that the laser is an adequate approximation to a light ray. Light rays travel in a straight line until they encounter a change in medium then the rays are reflected and refracted. This model clearly explains many phenomena. Determining the paths that light rays travel (ray tracing) is useful for illustrating and predicting various optical phenomena. The ray model is however incomplete. The model is extremely useful when considering, for example, systems of lenses. The model however does not properly account for the wave characteristics of light. Law of Reflection. A ray incident on a surface will be reflected, as shown in figure 1, at an angle equal to that made by the incident ray. θ = θ i f θ f θ i 1 K.L. Giovanetti Copyright: James Madison University REF 47

4 Law of Refraction. A ray that passes from one optical medium into another optical medium will be refracted, as shown in figure 2. Law of refraction or Snell's law states sinθ 1 = 2sinθ 2 n1 n where n 1 and n 2 are the indices of refraction and theta the angle of incidence and the angle of refraction. θf θi Most light sources emit light in all directions. Direct sources, for example light bulbs, produce the light. Indirect sources, for example a hand, merely scatter the light. The result is the same light appears to be coming from the source. We model extended sources, light bulb and hand, by imagining they are composed of many point sources. Each point emits light rays in all directions. In describing the transmission of light through optical systems we distinguish between object points and image points. An object point is the point source of light before the light enters the optical system. An image point is the source point after the optical system. In this lab we will use the head of a pin as an approximation to an object point. We will then observe the apparent location of the pin head as the image point. In this lab you will be constructing diagrams on paper. You must be sure that your work is neat. Use a straight edge for all straight lines. You will need to determine the path of a laser beam and represent that path on a sheet of paper. You will be expected to try methods for finding the laser path and accurately representing it in your diagram. Part I. Reflection Draw a straight line on a plain sheet of paper then place the mirror mounted on the wooden block directly on the line. The line will represent the mirror in the diagram that you will be drawing in this part of the lab so be sure the mirror is directly over the line. Shine the laser beam so that it is reflected from the mirror. Trace the path of the incident laser beam and the reflected laser beam. Trace in total three incident and their corresponding reflected beams. For clarity be sure that all three incident beams hit the mirror at the same point and choose significantly different angles. Remove the mirror and analyze. Draw a line perpendicular to the mirror surface that passes through the intersection point of the three incident beams and the mirror. Measure and label the three angles of incidence and the three angles of reflection. Tabulate results in a spreadsheet. At what level do the results agree with the laws of reflection. After completing part I you may decide to repeat the measurement if you feel you could improve your results. REF 48

5 Part II. Image On a new sheet of paper draw a straight line across the center to represent the mirror. In this part of the lab the resulting diagram will extend behind the mirror. Place the paper on the cork board and place the mirror directly on the line. Place a pin about 8 cm in front of the mirror. This represents our object. Shine the laser so that the pin splits the beam in the center and then the beam hits the mirror. This represents a ray coming from the object. Trace the incident and reflected ray. Repeat this so that you have five or six rays emerging from your object and striking the mirror. Make sure you cover a large angular range. Remove the mirror. Extend the reflected rays as dashed lines back through the mirror until they meet. Place a pin at this location. Now return the mirror to its original position and compare the location of the pin viewed in the mirror with the pin you just place in the corkboard. Do they appear to be in the same location? Briefly explain what you have demonstrated with this diagram. Part 1. (repeat for block surface.) Repeat part 1 using a block in place of the mirror. (Three reflected rays drawn and measured.) Part III. Refraction Use the laser, a plastic block and a plain sheet of paper to construct a diagram of how a light ray incident on a plastic block is refracted. Show only the air/plastic surface, the normal to the surface the incident and the refracted ray. Repeat this process for two rays at different angles of incidence. Carefully measure the angles and tabulate in spreadsheet. Do your results agree with the law for the refraction of light? Construct a diagram for light incident on a plastic block with an angle of incidence of roughly 30 o search for all the rays that are generated by the front and back surface. You should find at least five. Are any of these rays parallel? Part IV. Total Internal Reflection You should have a semi-circular plastic block. By aligning the laser perpendicular to the circular part of the surface at the point of entry the beam continues on a straight path. Why? If you rotate the block such that the center of the circle that corresponds to the circular surface remains fixed, the laser will remain perpendicular to the surface and so the beam will continue to pass into the block along a straight path. We will use this block to study total internal reflection as light leaves the block (plastic/air surface). Use the diagramming techniques developed above to find refracted and reflected rays from the plastic/air surface. Begin with a small angle of incidence. Now increase the angle of incidence and observe what happens to the refracted and reflected ray. Find the angle of incidence where the onset of total internal reflection occurs. Mirror - 3 rays drawn very neatly. Use ruler. REF 49

6 Mirror - table of measured angles and comparison. Mirror and pin - 5 rays, show image and object. Block - 3 reflected rays drawn very neatly. Use ruler. Block - table of measured reflection angles and comparison. Block - 3 rays refracted. Block - table of measured angles and comparison. Compare n sinθ. Block - find 5 rays. Total internal reflection - describe. Total internal reflection - find angle and compare with theory. REF 50