LAB REFLECTION FROM A PLANE MIRROR

Transcription

1 Name (printed) LAB REFLECTION FROM A PLANE MIRROR INTRODUCTION It s true that plane mirrors produce simpler images than curved mirrors, but they certainly aren t trivial. Ask a group of people who have never studied optics where they think the image produced by a plane mirror is located and you ll get three responses. Some will say, On the mirror surface, of course. Others will say, In front of the mirror s surface, of course. Still others will say, Behind the mirror s surface, of course. People tend to be emphatic about what they know to be true. But all three locations can t be right. This lab will lead you to the discovery of this image position. More important will be the discovery of two laws of reflection, one of which is a universal rule that applies to all reflections, not just those from flat surfaces. In the space below, write down where you think the image produced by a plane mirror is located. Include some explanation or evidence for your answer (even if it s just your intuition). Mirror Reflected Ray Reflected Angle Incident Angle Incident Ray Normal The ray that initially strikes a mirror is the incident ray. After it reflects it is then the reflected ray. At the point of reflection, a line drawn perpendicular to the surface of the mirror is the normal. The incident angle is always measured between the incident ray and the normal and the reflected angle is always measured between the reflected ray and the normal. Figure 1.6: Reflection terminology 1

2 PURPOSE To become acquainted with the qualitative and quantitative relationships which exist between the object and its corresponding image in a plane mirror. Specifically, you will discover: - the position of the image with respect to the object. - the relationship between the angle of incidence and the angle of reflection. PROCEDURE 1. Place cardboard under the next page of the handbook. 2. Place the reflective (back) edge on the line marked mirror on the next page. 3. Place the florist pin on the point marked object. 4. Place the straight pins on the points 1a and 1b. 5. Note how when you get your eye level with the paper and look across the page so that the straight pins are lined up, the image of the florist pin also lines up with them (Figure 1.7a). 6. Look into the mirror from the right side and place the straight pins so they line up with the image of the florist pin, as before (Figure 1.7b). Label these points 2a and 2b. 7. Repeat this for a third angle. Figure 1.7a: View into the mirror of the two stickpins lined up with the image of the florist pin. 8. For each of the three sets of points, draw a dashed line representing the path that light appears to be traveling from the image of the florist pin to the eye. (I ve done this for the first set of pins). 9. Use the intersection of these dotted lines to label the image position with the word image. 10. For each of the three sets of points, draw a solid line from the florist pin to the mirror (at the point the dashed line intersects the mirror) and then down to the straight pins. This represents the path that light actually takes as it goes from the object (florist pin) to the eye. Figure 1.7b: Placing the pins to locate an additional apparent path of light from the image to the eye. 11. Draw a perpendicular line (normal) to the mirror at the points where the solid lines touch the mirror. 12. Measure and record the three sets of incident and reflected angles. 2

3 LAB DRAWING Mirror 1a 1b Object DATA Record incident and reflected angles in the table below. Sighting i r

4 PLANE MIRRORS QUESTIONS AND PROBLEMS 1. How does the size of the incident angles compare to the corresponding reflected angles? 2. The image and object distances are the shortest distances from the object and image to the mirror. What are these distances (in cm) and how do they compare to each other? GET CHECKED BEFORE MOVING ON 3. A ray of light strikes a plane mirror at an angle of 28 from the mirror. What is the angle between the incident and reflected rays? Draw a diagram as part of your explanation. 4. Let s say you were standing 2.0 meters in front of a plane mirror and facing it. And let s say that a very attractive person is standing 4.5 meters behind you. You have your camera and want to take a picture, but you re too shy. So you decide to take a picture of the person s image and point your camera toward the mirror. What distance from you would you have to focus your camera in order to have the image of your crush in clear focus? Explain clearly and draw a diagram as part of your explanation. 5. There are five objects positioned in front of the plane mirror in the drawing to the right. Which images will the observer be able to see as he looks at the mirror? This will be a hard problem for most people. In order to say that a particular image can be seen, the most direct way is to show a light ray s path, starting at the object and then reflecting appropriately into the pupil of the eye. There are two other clever ways as well. Show appropriate rays to prove your conclusion. Plane Mirror A B C 4 D E

5 6. The astronauts put a corner-reflecting mirror on the moon. This is a combination of mirrors that are joined at 90 angles like the photo to the right (taken by David Angelo and Michael Barbaro, Class of 2004). The mirrors on the moon are designed to reflect laser beams sent from the Earth directly back along a line parallel to the path they came on. The perpendicular lines below represent two plane mirrors at right angles. Draw normals (I drew the first for you) in order to measure and draw angles. Complete the drawing and then bring in another incident ray in a direction that is different from the first, again completing the path, in order to illustrate this property of corner reflectors. 7. There are three objects positioned near a plane mirror in the drawing below. Which one(s) will the observer be able to see reflected in the mirror? a b c 5 GET CHECKED BEFORE MOVING ON

6 8. The cat below is sitting in front of a plane mirror. Draw two rays from the ear of the cat to the mirror and two rays from the foot of the cat to the mirror. Use the law of reflection to draw the four reflected rays. Finally, determine the position of the cat s image by tracing these back to the points from which they appear to emerge. Plane Mirror 9. The cat below is sitting in front of a convex mirror. Draw two rays from the ear of the cat to the mirror and two rays from the foot of the cat to the mirror. Use the law of reflection to draw the four reflected rays. Finally, determine the position of the cat s image by tracing these back to the points from which they appear to emerge. 6