THIN LENSES AND CONCAVE MIRROR
Physics Lab II
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
3 right angle supports
Optical object box
Object screen
Lens holder
Screen and holder
Thin lens
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
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Lens position: _______ cm
Screen position _______ cm
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THIN LENSES AND CONCAVE MIRROR
Physics Lab II
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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THIN LENSES AND CONCAVE MIRROR
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
f
(n 1)
1
R1
1
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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Physics Lab II
THIN LENSES AND CONCAVE MIRROR
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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THIN LENSES AND CONCAVE MIRROR
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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THIN LENSES AND CONCAVE MIRROR
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