Lab & Mirrors First, since the equipment we are using today is small, make sure that every person in your group gets a chance to see each individual experiment. Also, since much of our lab today is qualitative, be sure to include in your report sketches or diagrams of everything you observe, and answers to all questions. Finally, please use the handles on the lenses and mirrors instead of touching them directly. 1) The Law of Reflection Place the ray box on top of a piece of paper so that you will have a surface upon which to trace the rays. Using the appropriate slit forming plate at the end of the ray box farthest from the bulb, project a single, narrow light ray upon the plane mirror at an angle. Trace the incident ray, the reflected ray, and the mirror surface. Draw a line ( the normal) perpendicular to the mirror at the point where the ray is reflected. Measure the angle of incidence and the angle of reflection, and verify the law of reflection. Now verify the same law the old-fashioned way (without the ray box): Place the mirror on another piece of paper which has cardboard underneath it. Place a pin about 2 inches in front of the mirror, and use a straight edge to draw a sighting line aimed at the pin's image. The sighting line is the path of the reflected ray of light. The line from the pin to the mirror is the path of the incident ray. Draw the normal and verify the law of reflection. Keep this set up for the next part. 2) Image Location Use the sighting line you just drew in the previous step and another sighting line drawn from a second position. Extend both sighting lines behind the mirror until they intersect. This is the position of the image. How does the image position compare with the object position? Now, as a visual check on your results, place a second pin at the image position. Part of the second pin should now be visible above the mirror. If the second pin is at the correct image position, it will appear continuous with the image of the first pin, no matter what viewing position you choose. Verify that this is so. 3) Multiple Images Place two mirrors (use one of the mirrors attached to the ray box) so that they form a right angle between them, then place a pin at the halfway point between the mirrors as shown. You should see three images of the pin. Now try placing the mirrors at a variety of angles, such that the angle between them when divided into 360" yields an integer. By simple observation, develop a formula relating the number of images of the pin to this integer.4R ianlnMirrr Aim a single ray of light at a plane mirror. When the mirror is rotated through a certain angle, is the reflected ray rotated through the same angle? Take measurements to determine how these angles are related. |:') . This technique is often used by scientists to exaggerate or amplify slight movements within measuring equipment such as meters. 5) ggncave Spherical Mirrgr Select the semi-circular concave mirror. Aim a set of four parallel rays into the center of the inside curve of the mirror so that the rays are parallel to the axis of ----------------------------------------- symmetry of the mirror. Trace the incident and reected rays, and note where the reflected rays meet. This point is called the focal point (or focus) of the mirror. Carefully measure the distance from the mirror to the focal point. This distance is called the focal length, f. If the focal point appears blurred and broad, with too many rays overlapping through it, block the outer rays as they leave the light box and use only the central ones. The fact that the outer rays do not meet exactly at the focal point is referred to as spherical aberration, and can be corrected by using a parabolic mirror instead, as we will see in part 7. Now, set the semi-circular mirror on a piece of paper and trace the inside reflecting surface. Move the mirror around the curve and continue tracing until you have a complete circle. Measure the diameter of this circle in several directions and calculate an average diameter. What is the radius of the circle? How does the radius, R, compare with the focal length, f, you found a few moments ago? LConxexitmr Project a number of parallel rays to strike the outside surface of the semi-circular mirror, parallel to its axis. Trace the mirror position and ray paths and indicate the ray directions with arrow heads. Where do the diverging rays appear to come from? Locate this point by drawing the diverging rays backward through the mirror position. The point they come from is called the focal point (or focus), and the distance of this point from the mirror is the focal length, f. How does this focal length compare with the concave side of the mirror? By how much do the results differ, and why? Why do you think that convex mirrors are used as rear vision mirrors in cars? 7] Parabolic Reflector A parabola is an unusually shaped curve that can be produced by graphing y = x2, among other quadratic algebraic functions. It is also the shape of the path followed by a projectile in the absence of air resistance. Aim a set of four parallel rays into a parabolic reector along paths parallel to the axis of symmetry of the mirror. Notice the sharpness of the focal point, in comparison to the spherical mirror. Move the light box sideways keeping the rays parallel to the axis of symmetry. What do you notice about the position of the focal point? What shaped mirror would be used to produce sharp images of stars that are very far away (ie incident rays nearly parallel)? Why do radar antennae, radio-telescopes and car headlamp reectors have a parabolic shape rather than a spherical one? In all of the previous examples, where would you put the receiving or transmitting device