College Physics -II (PHY205L) Baptist Health Science University Lab 10: Focal Length of Concave Mirror Purpose/objective: In this activity you can investigate the relation between object distance, image distance and focal length of a concave mirror. Equipment needed: 1. Computer 2. Paper, Pencil 3. Optic bench and accessories, two concave mirror, an illuminated object and a screen. Theory: Light rays that are parallel to the axis of a mirror, after reflecting through the mirror, converge to a point on the axis. This point is called the principal focus (F) of the mirror. The distance between the center of the mirror and the principal focus is called the focal length (f) of the lens. Focal length (f) If you imagine a sphere with one side of the mirror as part of it, the center of that sphere is called the center of curvature C of the lens. The distance between the center of curvature and the center of the mirror is called the radius of curvature R of the mirror. When an object is placed between the principal focus and the center of curvature, a real, enlarged and upside-down image is formed beyond C.5. What is the slope and Y-Intercept? 6. Using Y-Intercept, which is equal to 1/d, at 1/do=0. That means when the object is at infinity. In this situation, mirror equation, 1/do+1/d,=1/f, becomes 0+1/d,=1/f and therefore 1/d,=1/f (you do not need to show any work for it -it is just an information for you) 7. Focal length of the mirror, f= Y-Intercept= 8. Compare f obtained from (E, above) with average value of f obtained from table 1. Calculations/Checking your understanding 1. When a flower petal is 4.0 cm in front of a converging mirror, the inverted image is half the height of the petal. What is the focal length of this mirror? Show your work: 2. A statue that is 10.4 cm tall is placed 4.8 cm in front of a convex mirror. The image is 4.0 cm from the mirror. How tall is the image? Show your work: 3. A cell phone is placed 40 cm in front of a concave mirror with a focal length of magnitude 20 cm. How far is the image of this phone from the lens? Show your work:Figure 2 The distance between the object and the lens is called do and the distance between the image and the lens is called d. If f is the focal length of the lens, then a relation between f, do and d, can be written as 1/f=1 /d +1/d; Procedure: 1. Measure the values for five object distance and image distances. Table1. Trialsdo (cm)d, (cm) 1/ do (cm ]) 1/ d, (cm ]) 1/f=1 /d,+1/d|f (cm) Average Focal length for the data: 2. Draw a graph of 1/ d; (in Y-axis) vs 1/d. (in X-axis) as follows. (you can use excel to draw the graph) 3. Attach the graph (make sure that 1/d, is on Y-axis and 1/do is on X-axis.) 4. By using linear trend line using excel and using the display equation option, write the straight-line equation