51 Reflection and Refraction Name(s): Date: In this lab, we will investigate the behavior of light when it reflects from an optical surface, such as a mirror, and when it passes from one optical medium to another. Reflection Imagine a small object a distance, i, from a mirror. Distance i is known as object distance. The object emits an infinite number light rays in all directions. Some will strike the mirror, and some will not. Figure 1 shows three particular rays: 1, 2, and 3, which will strike the mirror and reflect from it. Each ray reflects from the mirror so that its angle of incidence 0; (the angle between the incoming ray and the line perpendicular to the mirror [normal] at the point where the ray hits the mirror) equals to the angle of reflection 0, (the angle between the reflected ray and the normal). This is known as the Law of Reflection, or: airled nollsoo eat of paper place 0; = Or. WO quotit savor yor ent notenamib a slov - 12AFigure 1 Object Observer ! Normal 2 Mirror Image In Figure 1, an observer is located at some position in front of the mirror. An interesting question arises: is there a ray of light emitted by the object which after the reflection enters the observer's eye? The answer is yes! Ray 1 does just that, and therefore allows the observer to see the object. Where does the object appear to be located to the observer? It appears to be located behind the mirror. To the observer, ray 1 came from the mirror; therefore the source of this ray appears to him to be behind the mirror at the point in the figure which is labeled "image". The image is located a distance o from the mirror. For a flat mirror, i = o. The image is called "virtual" since it does not emit any real light rays. It is just what the observer sees. Note that rays 1, 2, and 3 (and in fact all rays reflected from the mirror) appear to come from the image. Regardless, where the observer is located with respect to the mirror, he will see the image of the object at the same location behind the mirror. Refraction When a ray of light moves through empty space, its speed is c=300,000 km/s. However, if the ray moves through a transparent medium like water, glass, diamond, etc., the effective speed of a light ray is less, say v. The particular value of v depends on the type of medium. The ratio of c to v, i.e. c/v is called n - the index of refraction. The index of refraction, n, is a dimensionless number, most often larger than 1. 2 202A53 Figure 2 Normal Medium 1 n1 Medium 2 n2 Refer to Figure 2. A ray of light moves through medium 1 with index of refraction n1. It approaches a boundary between medium 1 and medium 2 (index of refraction n2) with an angle 0;. After passing the boundary, the ray changes its direction because the effective speed of light in medium 2 is different than in medium 1. In medium 2, the angle between the ray and the normal is Ot - the angle of transmittance. The two angles and the indices of refraction are related through Snell's law: n1sin 0; = n2sin 0t- Part I. Reflection From a Flat Mirror Procedure 1. On a white sheet of paper placed on the cork board, draw a length-wise line which divides the sheet into two equal parts. This line is the optical axis. 2. Draw a line perpendicular to the optical axis, which again divides the sheet into two equal parts. Call this line and mm line. U.OF 3. Place the mirror on the mm line in an upright position. PHYS 202A54 4. Stick a yellow pin into the paper somewhere on the optical axis and in front of the mirror. This pin acts as an object. Stick a second pin (silver) into the paper so that it is to the left from the yellow pin and closer to the mirror. This pin, together with the yellow pin, defines a path of a ray that will strike the mirror (See Figure 3 below). Figure 3 Mirror mm Silver Pin > Yellow Pin Observer Optical Axis 6. Look into the mirror. You will see two images: one of the yellow pin, and the second of the silver pin. Move your head until the two images line up, i.e. one image is in front of the other. If this is so, stick the third pin (silver) so that it lines up with the two images in the mirror. 7. Remove the silver pins, mark the points, and repeat steps 5 and 6 for a silver pin to the right of the yellow pin. 8. Do you understand what you have done? Explain. 9. If yes, remove all the pins from the paper. Draw the ray diagram which shows the paths of the two rays of light before and after reflection, normal lines at the points where the two rays strike the mirror, and the position of the image (see Figure 1). 10. Using a protractor, measure the angle of incidence and reflection for the two rays and compare these angles for each ray separately. Report the angles on your drawing. 4 PHYS 202A55 11. Using the straight edge measure the object and image distances. Report their values on your drawing. Part II. Refraction Procedure 1. On a new white sheet of paper placed on a cork board, put a rectangular piece of glass. Outline the shape of the glass with a pencil. 2. Stick a yellow pin near the top left edge of the glass and the second pin (silver) closer to the glass and to the glass' center (see Figure 4). Figure 4 do anig owl love Yellow Pin Silver Pin Glass conve Observer 3. Look through the glass. You should see two images of the pins (yellow and silver). Move your head so that the two images line up. Place two pins (silver) which line up with the two images in the glass. grilweis woy no caulay 4. Remove the pins and the glass from the paper. 5. Draw the ray diagram on your paper. Draw the normal lines at the two points where the ray crosses the boundary. 6. With the protractor, measure the angle of incidence and transmission for each boundary. Report the values on your drawing. 20 HYS 202A56 7. Use Snell's law to calculate the index of refraction of the glass. Do the calculation for each boundary and report these two values and their average. Assume that index of refraction of the air is 1. Part III. More Refraction 2003000 Procedure 1. On another new white sheet of paper placed on a cork board, put a triangular piece of glass. 2. Stick two pins (yellow and silver) as shown in Figure 5 below. Figure 5 Silver Pin 45 Yellow Pin Observer Glass 90 3. Look through the glass. Again you should see two images. Place two silver pins so that they are lined up with the images. 4. Remove the pins and glass and complete the ray diagram. 5. Measure the angles of incidence and transmission for each boundary. Report the values on your drawing. 6. The triangular piece of glass is made from the same material as the rectangular piece of glass; therefore, the indices of refraction for the two pieces are the same. 7. Using Snell's Law, calculate all of the expected angles in the ray diagram. Compare these values with those that you measured in step 5. 6 S 202A 2024