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Lab: Concave Mirror

Lab: Concave Mirror. By: Sarah Sultana, Qing Wang & Luohan Miao December 13, 2010 Westmount High School PHYS504-01 Mr. Wilder. Purpose.

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Lab: Concave Mirror

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  1. Lab: Concave Mirror By: Sarah Sultana, Qing Wang & Luohan Miao December 13, 2010 Westmount High School PHYS504-01 Mr. Wilder

  2. Purpose To find the focal point and the centre of curvature of concave mirror, to determine the relationship between the radius of curvature and the focal length of this mirror.

  3. Hypothesis • The radius of curvature should be twice the focal length.

  4. Materials • Concave mirror • Ray box • Blank sheet of paper • ruler

  5. Procedure • Draw a straight horizontal line passing through the middle of the paper to represent the principal axis. • Set up the apparatus as shown in the diagram below. Concave Mirror Ray box Principalaxis

  6. Procedure 3) Trace the outline of the concave mirror 4) Darken the room and adjust the ray box so that the five light rays are parallel to each other and the principal axis. 5) Trace the incident and reflected rays. 6) Mark the intersection point of the reflected rays. This is the focal point (F) of the mirror. (Refer to figure 1 for further explanation. ) 7) Using a ruler, measure the distance from the focal point to the vertex (V) of the concave mirror. This is known as the focal length of the mirror.

  7. Procedure 8) Adjust the ray box from 5 slits to a single slit and direct it towards the mirror. Adjust the direction of the single ray until the reflected ray coincides with the incident ray. Repeat step 8 and find the second ray. The point where the two rays meet is the center of curvature (C) of the mirror. 9) Using a ruler, measure the distance from the center of curvature to the vertex of the concave mirror. This is known as the radius of curvature. v 20.61cm

  8. Results Figure 1: Finding the Focal Point for a Concave Mirror 10.30cm

  9. Results Figure 2: Finding the Center of Curvature for a Concave Mirror 20.61cm

  10. Analysis • Analysis of the results • Errors and improvements • Laws used in the lab

  11. Analysis of the result 1) According to the result Figure 1 and 2 the radius of curvature is 20.63 cm and the focal length is 10.30cm. It is obvious that the radius of curvature is twice the focal length.According to the law of reflection, the angle of incidence is equal to the angle of refraction: O1=O2. Since angles O1 and O3 are alternate interior angles, O1=O3. So, O2=O3.Triangle ACF is therefore an isosceles triangle. As a result, it has two equal sides: CF=AF. When the size of mirror is smaller than the radius of curvature, angle CAV is equal to 90 degrees, which means triangle CAV is a right triangle therefore FA=FV. So, CF=FV. A O1 O2 Thus, the focal point is located at the half way point of segment CV. Since VF is the focal length(f) and CV is the radius of curvature(R), we can write: f=R/2. O3 V C F

  12. Errors and Improvement 2) In Figure 1, the 5 reflected rays do not exactly intersect at the focal point but form a shade on the principle axis and leads to the inaccuracy of the measurement of focal length. This is because the light beams are not sharp enough.These errors can be reduced by using sharper rays.

  13. Laws used in the lab 3)We have previously established that the law of reflection can be applied to curved mirrors. If you look closely at the reflective surface of a spherical mirror. It is actually linear. A single ray of light striking the mirror, only hits a small portion of mirror. if each sector is thought as being plane, than Snell’s law of reflection can be obeyed on curved mirrors as well.

  14. Conclusion • What happened in the experiment. • What does the data tell you about you experiment • Application of concave mirror • What did you learn

  15. Conclusion In this lab, the focal length was determined by directing 5 parallel rays towards the concave mirror. The radius of curvature was determined to be twice the focal length of the concave mirror using the ray box and the other equipments provided. As mentioned previously, the focal length is approximately 10.30cm according to Figure 1 and the radius of curvature is approximately 20.61cm according to Figure 2.

  16. Conclusion Concave mirrors can serve a wide variety of purposes in reality. They are used in cosmetics to enlarge areas of the face when applying make-up. They are also used as dentists' mirrors to magnify the image of a patients teeth to facilitate the examination.  It has been learned from this lab the relationship between the radius of curvature and the focal length of a concave mirror is always 2:1.

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