Physics Lab II THIN LENSES AND CONCAVE MIRROR
Name: ______Partner: ______Partner: ______
PURPOSE: To apply the thin lens equation and the similar relation for a spherical mirror. OBJECTIVE: To determine the focal lengths of a thin converging lens and a concave spherical mirror. APPARATUS: Optical bench Object screen 3 right angle supports Lens holder Thin lens Optical object box Screen and holder Concave mirror
DISCUSSION:
Converging Lenses The approximate focal length of a thin converging lens is the distance from the lens to a screen on which is formed the real image of a very distant object, such as the landscape seen through a window of the laboratory. The glass lens which is thicker at its center than its edge is the converging lens. DO NOT EVER TOUCH OPTICAL SURFACES WITH YOUR FINGERS.
When shorter object and image distances are measured on the optical bench, the thin lens equation (derived in your text) may be solved for the focal length.
Concave Mirrors When a real object is placed on one side of the principal axis of a concave spherical mirror at the center of curvature, a real, inverted image of the same size is formed on the other side of the principal axis at the center of curvature.
PROCEDURE: PART A 1. Place the white screen in its holder in one right angle clamp on the optical bench and the converging lens in a lens holder in another clamp. 2. CAREFULLY orient the optical bench so that light entering a laboratory window will pass through the lens toward the screen. 3. Adjust the distance between lens and screen until the image of the most distant object is sharply focused on the screen. Record lens position ______cm and screen position. ______cm. PART B 1. Mount the optical object box in one right angle clamp at one end of the optical bench. 2. Mount the converging lens in a lens holder in a right angle clamp at about the 35 cm mark of the optical bench. 3. Mount the white screen in its holder in a right angle clamp and adjust the position until a sharply focused image of the front of the optical object box is obtained on the screen. 4. Determine the positions of object, lens, and screen. (Note that for these optical object boxes, the right angle clamp index is not located beneath the object.)
Object’s position: ______cm Lens position: ______cm Screen position ______cm
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5. Measure the length of some straight part of the object and the length of the corresponding part of the image.
Length of the object’s part: ______cm Length of the image part: ______cm
6. Leaving object and screen fixed, move the lens until a sharp image is obtained for a different position of the lens. Record this conjugate focus position.
Conjugate position: ______cm
7. Turn off the lamp.
PART C 1. Place the concave mirror in a lens holder in a right angle clamp. Mount it on the optical bench. 2. Hold the cardboard containing the metal screen against the front of the object box with the white side away from the box. The metal screen is the object. 3. Adjust the mirror position until a sharp image of the metal screen is formed on the white board beside the metal screen. Record the position of screen and mirror.
Screen position: ______cm Mirror position: ______cm
CALCULATIONS
Part I. 1. From your result of Part A above find the focal length of the converging lens.
Focal length: ______cm
2. What approximation or assumption do you use? ______
Part II. 1. From the results of Part B and the thin lens equation find the focal length of the converging lens. Find percent difference from the result of Part A. Show your work.
Calculations:
Converging lens focal length: ______cm
Percent difference (show how you got it):
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2. Calculate the magnification from the results of step 5 in Procedure, Part B. Calculate also by the formula for magnification in terms of object and image distances.
Magnification from the object and image lengths: ______Formula:
Magnification from the object and image distances: ______Formula:
3. Make a scale drawing and by ray tracing deduce the focal length of the converging lens.
4. What is the relation between image and object distances for conjugate focus positions? Discuss fully. ______
5. If the lens is equiconvex and has index of refraction 1.50, find the magnitude of its radii of curvature. In order to do this you will use the Lensmaker’s Equation:
1 1 1 (n 1) f R1 R2
In this equation f is the focal length of the lens, n is the index of refraction, and R1 and R2 are the radii of curvature of the front and back surface of the lens. If the lens is equiconvex, then R1 = R2.
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Write down the Lensmaker’s Equation for R1 = R2 = R (simplify it where possible):
Radius of the lens curvature: ______cm
Part III. 1. Using your data from Procedure, Part C, calculate the focal length of the concave mirror.
Focal length of the mirror: ______cm
2. Find the radius of curvature of the concave mirror. Hint: what is the relationship between the radius of curvature and the focal length for a spherical mirror?
Radius of the mirror curvature: ______cm
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RAY DIAGRAM RULES (Do NOT return this and next page with your lab report -- keep them for review and study): (1) From the object point draw a line parallel to the principal axis to represent an incident ray. From the point (of incidence) where this ray strikes the lens, draw a line through the proper one of the focal points to represent the ray of light emerging from the lens.
(2) From the object point draw a line through the other focal point to the lens. From the point of incidence draw a line parallel to the principal axis.
(3) From the object point draw a line through the vertex and extend it straight on.
Where these three emergent rays intersect is a real image point, where they appear to have intersected is a virtual image point.
These are illustrated in the following figures. The numbers beside the rays identify them with the above rule having the corresponding number.
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