# Thin Lenses. Physics 102 Workshop #7. General Instructions

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1 Thin Lenses Physics 102 Workshop #7 Name: Lab Partner(s): Instructor: Time of Workshop: General Instructions Workshop exercises are to be carried out in groups of three. One report per group is due by the end of the class. Each week s workshop session would typically contain three sections. The first two sections must be completed in class. The third section should be attempted if there is time. 1. A pre-lab reading and assignment section 2. Experiment section 3. Practice questions and problems This lab session is devoted to understanding how images are formed using lenses. Please make all distance measurements in centimeters and angle measurements in degrees. Also, please note that some textbooks interchange the definitions of the two focal points associated with a lens. Refer to your textbook if confusion should arise or ask your T.A. COMPREHENSIVE EQUIPMENT LIST 1. Convex lens 2. Optics bench with attached scale reading in millimeters 3. Illuminated object, to be mounted on optics bench 4. Viewing screen, scale in millimeters, mounted on optics bench with scale vertical 5. Convex lens on lens holder 6. Concave lens on lens holder PART I: Pre-lab reading assignment Objects In the context of this lab, an object is something that a lens makes an image of. Some objects are physical objects, things made of atoms. Other objects are images created by any number of various optical components, mirrors or lenses, which a lens can make an image of just as easily as it can make an image of a physical object. Physical objects are always in front of the lens, but the other kind of object can be either in front of the lens or behind it. If an object is in front of its lens, it is called a real object. If an object is behind 1

2 its lens, it is called a virtual object. Thus, physical objects are always real, but other kinds of objects can be either real or virtual. What it means to have an object in front of its lens is probably familiar to you. Think of looking at a bug with a magnifying glass. However, the idea of an object behind its lens most likely is confusing. You need a good example, so that you can see what it means. You will get that example at the end of this lab. In the meantime, you will have to be patient as the sections of this pre-lab reading assignment insist in drawing diagrams sometimes with the object of the lens in front of the lens and sometimes with the object of the lens behind the lens. Rays When something emits light, like the sun or a candle, we think of it as sending rays thin lines of light in all directions. Then, when some of the rays from the light source strike a physical object, the physical object sends rays out in all directions. If the physical object is in front of a lens, some but not all of the rays from the physical object strike the lens and are bent by the lens. Figure 1 shows rays sent out from the tip of an arrow, some of which strike a lens. There is a light source that is not shown. Also, all other points on the arrow send out rays, just as the tip of the arrow does, but the rays from those other points are not shown. Figure 1 CONVEX AND CONCAVE LENSES AND THEIR FOCAL POINTS Convex lenses bulge outward in the middle. Concave lenses are thinner at the middle than at the edges. See fig. 2 for side views of concave and convex lenses. Convex and concave lenses Figure 2: 2

3 Each lens has two focal points, a secondary focal point and a principal focal point. Both focal points are at the same distance from the lens but on different sides of the lens. Note that the secondary and principal focal points for a concave lens are on opposite sides of the lens compared to convex lenses. The distance from the middle of the lens to the focal points is called the focal length of the lens. For convex lenses, the focal length is positive, and, for concave lenses, the focal length is negative. Example: If a lens has a focal length of -340 millimeters, you know that the lens is concave, and the distance from the middle of the lens to the focal points is 340 millimeters. The line through the middle of a lens is called the axis of the lens. Figure 2 is drawn assuming that light rays approach the lenses from the left. Convex lenses always have their secondary focal points on the side of the lens from which the light rays approach, and concave lenses always have their secondary focal points on the opposite side of the lens. Rules for bending special rays through lenses To take at random any one of the infinite number of rays that strike a lens and determine exactly how it bends upon passing through a lens is very difficult. However, there are three rays of the infinite number that strike a lens for which it is relatively easy to determine how they bend. Table 1 gives the rules for drawing each of these three rays and for determining how they bend. After checking Table 1, look at the examples. Ray Incident ray is defined by: Exit ray is defined by: 1 Tip of object and secondary focal point Point where incident ray intersects lens and running parallel to the lens axis 2 Tip of object and running parallel to the lens axis Point where incident ray intersects lens and the principal focal point 3 Tip of object and Center of lens Exit ray is the same as the incident ray Here is a summary of how the focal points and the center of the lens are used to determine Rays 1, 2, and 3. Ray 1 The secondary focal point defines the incident ray (the other defining point is the tip of the object). Ray 2 The principal focal point defines the exit ray (the other defining point is where the incident ray meets the middle of the lens). Ray 3 The center of the lens defines the ray (the other point is the tip of the object). Example of Ray 2 using a concave lens and a real object This first example is done in steps, so you can see exactly how the lines are drawn. Figure 3 shows a concave lens, with its two focal points, and the real object, represented by an upright arrow. 3

4 Figure 3: Concave lens and real object (arrow) Add to Figure 3, the line that defines the incident ray for Ray 2 in Table 1. According to the directions for Ray 2 in Table 1, this must be the line that passes through the tip of the arrow and is parallel to the axis of the lens. The result is shown in Figure 4. Figure 4: The first step in constructing Ray 2 for the configuration of fig. 3. The line defining the incident ray passes through the tip of the arrow and is parallel to the axis. Next, add to fig. 4 the line defining the exit ray. According to the directions for Ray 2 in Table 1, this line is defined by two points: the point at which the incident ray intersects the lens and the principal focal point. The result is shown in fig. 5. Figure 5: The second step in constructing Ray 2 for the configuration of Figure 3. The line defining the exit ray is now added. Finally, one can trace how the incident ray is bent on passing through the lens by starting on the left with the line defining the incident ray, moving to the right, and switching to the line defining the exit ray upon passing the lens. See fig. 6. 4

5 Figure 6: The third step in constructing Ray 2 for the configuration of Figure 3. The bold line shows how the incident ray bends on passing through the lens. Note the ray is drawn so that it bends at the middle of the lens rather than at one of the curved surfaces. Example of Ray 1 using a convex lens and a virtual object In fig. 7 you see a Ray 1 from Table 1. Note that the object (represented by the arrow) is virtual, so it is located behind the lens. This in no way complicates the application of the rules in Table 1. Figure 7: Construction of Ray 1 for a convex lens with a virtual object. The line defining the incident ray is a straight line between two given points, the secondary focal point and the tip of the object. The line defining the exit ray is the straight line that intersects the incident ray in the lens and that is parallel to the axis of the lens. Example of Ray 3 using a concave lens and a virtual object For Ray 3, the tip of the object (represented, as usual, by an arrow) and the center of the lens defines both the incident and the exit ray. See Figure 8. Figure 8: Construction of Ray 3 for a concave lens with a virtual object. The ray is defined by the tip of the object and the center of the lens, and it does not bend. 5

6 Locating the image of an object Given a lens and an object, do the following to locate the image of the object. First, construct the three lines defining the incident rays for Rays 1, 2, and 3. Second, construct the corresponding three lines defining the exit rays. Third, the three lines defining the exit rays will intersect at a point. That point is the location of the tip of the image. The exit rays themselves may not intersect. It is the lines that define the exit rays that do intersect. If you look only at the exit rays, you may have to extend them forward or backward to find the intersection point. Figure 9: The lines that define the three exit rays intersect at a single point. That point is the location of the tip of the image. Figure 9 shows an example of using Rays 1, 2, and 3 to locate an image. For Rays 1 and 2, the lines defining the incident rays are solid (as is the object), and the lines defining the corresponding exit rays are dashed (as is the image). The line defining Ray 3, which does not bend, is solid and thicker than the other lines, to make it stand out. LENS FORMULAE The following two sections, on image location and image magnification, describe two lens formulae needed for this lab. Image location In the following, it is assumed that light rays travel from left to right, as is all diagrams in this lab handout d d f o i f - is the focal length of the lens. It may be positive or negative, depending on whether the lens is convex or concave. do - is the distance of the object from the lens. It is positive if the object is real (on the left side of the lens), and it is negative if the object is virtual (on the right side of the lens). di - is the distance of the image from the lens. It may be either positive or negative, and the rule for its sign is exactly the opposite of the rule for do. di is positive if the image is on the right side 6

7 of the lens (a real image), and di is negative if the image is on the left side of the lens (a virtual image). Image magnification The magnification, m, of a lens is how many times taller the image is than the object. By this definition, hi m ho m - is the magnification. The magnification is negative if the image is up-side-down compared to the object; otherwise the magnification is positive. ho - is the height of the object. In ray diagrams, ho is always oriented upward, and its value is always positive. hi - is the height of the image. hi may be either positive or negative. In ray diagrams, it is positive if the image is oriented upward, and it is negative if the image is oriented downward. m can also be calculated from the object and image distances. d m d i o di and do are as described in the previous section and so can be either positive or negative. Locating objects on the optical bench Lenses and the small optical screen used on the optical bench in this lab are mounted on a small metal bracket that slides back and forth on the optical bench. On the foot of the small metal bracket called a component carrier is a white tic mark that can be used to locate the position of the component carrier with respect to the millimeter graduated scale on the optical bench. The idea is to be able to quickly and easily determine the distances between the components (lenses, screen, illuminated object) mounted on the optical bench. lens, mounted on component carrier component carrier white tic mark actual location location of of lens component carrier Figure 10: A component carrier holding a lens on the optical bench The problem is that the location of the optical component mounted on the component carrier is not the same as the location of the white tic mark, as fig. 10 illustrates. Therefore, be careful to determine the correct location of the component and not the location of the component carrier. The correct location of components depends a little on the component. See Table 2. 7

8 Component Reference point on component Offset from tic mark Lens center of lens 6 mm white screen front face of screen 5 mm illuminated object face of illuminated object 0 mm (no tic mark...) Table 1: Optical component reference points for determining location on the optical bench. PART II: Pre-lab assignment 1. In the context of thin lenses, what is the difference between real and virtual objects? 2. Compare the locations of secondary and principal focal points for convex and concave lenses. 3. A lens forms an image of an object. The object distance is 80 mm, and the image distance is -120 mm. Find the focal length of the lens. What type of lens is this? 4. Use a straight edge to draw a ray diagram showing how a Ray 1 bends when it passes through the lens in the diagram below. The result should show the bending ray the way Figure 9 does (the bent ray is dark and with arrows showing its direction). 5. Use a straight edge to draw a ray diagram showing how a Ray 2 bends when it passes through the lens in the diagram below. The result should show the bending ray the way Figure 9 does (the bent ray is dark and with arrows showing its direction). 8

9 6. Use a straight edge to draw a ray diagram showing how a Ray 3 bends when it passes through the lens in the diagram below. The result should show the bending ray the way Figure 9 does (the bent ray is dark and with arrows showing its direction). 7. Use a straight edge to draw a ray diagram using Rays 1, 2, and 3 to locate the image for the diagram below. PART III: Ray diagrams for convex lens In order to understand how the images are formed in the next part, you are to construct ray diagrams that approximately represent the situations that you will be working with when you do your next part. Measuring focal length Drawing ray diagrams Figure 11 Follow the instructions below and make all necessary diagrams in the space provided below the instructions. 1. Add to the diagram an upward-pointing arrow, representing an object, that is 20 mm high and located a distance 2f to the left of the center of the lens. Figure 11 shows how the diagram will look after you have added the arrow representing the object. 9

10 2. Use the ruler and a sharp pencil to draw Rays 1, 2, 3 and locate the image. Refer to part I of this hand out if necessary. d o 2f 3. Locate the object at about 3.5f instead of 2f in the space below and repeat steps 1 and 2. d o 3.5f 4. Locate the object at about 5f instead of 2f in the space below and repeat steps 1 and 2. d o 3.5f 10

11 PART IV: Measuring focal length of convex lens Identify the convex lens using your sense of touch. Follow the instructions below to fill out row one of table Place the illuminated object at one end of the optics bench so that the object is 100 mm from the left end. 2. Place the convex lens approximately 240 mm in front of the illuminated object (distance between object and lens: do 240 mm). 3. Record the exact value of do in Table 3. Since the object is real, do is positive. 4. Turn off your desk lamps (if you had them on!). Place the viewing screen on the optics bench on the side of the lens opposite the illuminated object, and move the screen back and forth until you obtain the sharpest image. 5. Record the distance from the lens to the image in Table 3 as di. Since the image is real, di is positive. 6. Calculate f and md (md is the magnification calculated using object and image distances) from do and di, and record the results in Table 3. Refer to part I for the necessary formulae. 7. Measure the object height and the image height (include the correct signs), and record the values as ho and hi in Table 3. (Take the height to be the distance between the base of the vertical arrow and its tip. The height is positive if the arrow points upward, and the height is negative if the arrow points downward.) 8. Calculate mh (mh is the same magnification but calculated from object and image heights) from ho and hi, and record the result in Table Repeat steps 1 8 with object distances d o of approximately 420 mm and 350 mm. d o (mm) d i (mm) f (mm) m d from d o & d i h o (mm) h i (mm) m h from h o & h i Table 2: Focal length of the convex lens 10. The three values of the focal length should be approximately the same. To measure how well they agree, calculate the average value and a percentage discrepancy. f240 mm f420 mm f600 mm faverage 3 f average = mm 1 2 ( fbiggest fsmallest) Discrepanc y 100% f Discrepancy = % average 11

12 11. The magnifications should all be different, but the same magnification calculated from the d o, d i and from the h o, h i should be reasonably close. To see how accurate that is, complete Error! Reference source not found. using the following formulae. m average = ½(m d + m h ) Discrepanc y 1 2 ( m m d m average h ) 100% d o (mm) m average Discrepancy % % % PART V: Ray diagrams for measuring concave lens focal length 1. On the first ray diagram in the next page, containing only a convex lens and, on the left, a real object, construct the image of the object. 2. By counting squares in the first ray diagram, determine the values for do, di, and f, and write them into the places provided in the first ray diagram. (Each square 10 mm x 10 mm). 3. Copy the image you just constructed to the second ray diagram on page 16. The copied image must be in the same location and must have the same height and orientation as the image in the first ray diagram. You will use this second ray diagram to determine what happens to the image when a concave lens is inserted between the convex lens and its image. 4. The image you just copied onto the second ray diagram is now a virtual object for the concave lens. Construct the concave lens s image of this virtual object. Refer to part I if necessary. Use Rays 1, 2, and By counting squares in the second ray diagram, determine the values for do, di, and f, and write them into the places provided in the second ray diagram. Be sure to measure relative to the concave lens and to include the correct signs. 12

13 d o = mm d i = mm f = mm Use concave lens 10 mm 10 mm squares Second ray diagram d o = mm d i = mm f = mm Use convex lens 10 mm 10 mm squares First ray diagram principal secondary principal secondary 13

14 PART VI: Measuring concave lens focal length 1. Overview Just read steps a d; do not do anything. a. You will set up the convex lens to form a real image. b. You then insert the concave lens between the convex lens and its image, making the image a virtual object for the concave lens. c. By locating the virtual object and its image with respect to the concave lens, you can determine the focal length of the concave lens. d. By measuring the height and orientation of the virtual object and its image, you can determine the magnification of the concave lens. 2. With the illuminated object at the left end of the optics bench, place the convex lens on the optics bench about 170 mm to the right of the illuminated object. 3. One person in your group uses the screen to locate the image as carefully as possible. This image will be the virtual object for the concave lens. Record its location on the optics bench here. Position of virtual object on optics bench: mm The only purpose of the convex lens is to create the virtual object. From now on, you ignore the convex lens but work with the properties of the virtual object it created. 4. Measure the height of the virtual object and enter it into the first row of Table 5 in the ho column. 5. Place the concave lens on the optics bench about 100 mm to the left of the virtual object. 6. Determine the object distance of the virtual object with respect to the concave lens and enter it into the first row of Table 5 in the do column. Since the object is virtual, do is a negative number. 7. Use the screen to locate the image of the virtual object. Do this as carefully as possible. 8. Determine the image distance with respect to the concave lens and enter it into the first row of Table 5 in the di column. The image is real, so di is a positive number. 9. Finally, determine the image height and enter it into the first row of Table 5 in the hi column. 10. From the measurements recorded in the first row of Table 5 calculate f, md, and mh. d o (mm) d i (mm) f (mm) m d from d o & d i h o (mm) h i (mm) m h from h o & h i Table 3: Focal length of the concave lens 11. Move the convex lens to about 400 mm in front of the illuminated source and repeat steps 4 10 and fill out row 2 in Table Move the convex lens to about 440 mm in front of the illuminated source and repeat steps 4 10 and fill out row 2 in Table The three values of the focal length should be approximately the same. To measure how well they agree, calculate the average value and a percentage discrepancy. 14

15 faverage = mm Discrepancy = % 14. A value of the magnification md calculated from do and di should be very close to the corresponding value of mh calculated from ho and hi. To see how accurate that is, complete Table 6 using the following formulae. maverage = ½(md + mh) m average Discrepancy First Measurement % Second Measurement % Third Measurement % Table 6 15

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