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Optical Instruments
LBS 267L
Purpose
The purpose of this experiment is to understand and explore optical instruments by assembling various optical elements and
observing the effect. These optical instruments are important in every day life.
Theory
Various combinations of lenses can produce different effects. A distant image can be magnified (telescope). A nearby image
can be made to appear enlarged on a screen some distance away (projector). A nearby image can be enlarged (magnifier and
microscope). All these systems of lenses are governed by the lens equation
1 1 1

 
p q f
and the corresponding definition of magnification
q
m

p
where p is the object distance, q is the image distance, and f is the focal length.
Equipment Needed
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PASCO Optical Bench with Light Source
Component Holders
75 mm Focal Length Convex Lens
150 mm Focal Length Convex Lens
Viewing Screen
Cross Arrow Target
Variable Aperture
LBS 267L
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Optical Instruments
The Projector
Setup
Set up the optical bench as shown below in Figure 1. Place the light source, crossed arrow target, variable aperture, 75 mm
lens, and the viewing screen as shown. The variable aperture is attached to the same component holder as the 75 mm lens.
Theory
When an object is placed between the focal point and twice the focal point of a converging lens, a real, inverted, magnified
image is formed. If a viewing screen is placed at the location of the image, the image will be focussed on the screen. In this case, the
lens functions as a projector.
Figure 1. Schematic drawing showing the experimental setup for a projector.
Procedure
1. Place the object such that f < p < 2f. Describe the resulting image.
2. Draw a ray diagram of this arrangement.
Questions
1. What happens if p < f?
2. What happens if p > 2f?
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Optical Instruments
The Magnifier
Setup
Setup the optical bench as shown below in Figure 2. Remove the light source. Place the 75 mm lens and the viewing screen as
shown.
Figure 2. Schematic drawing showing the experimental setup for a magnifier.
Theory
When an object is located between a converging lens and it focal point, a virtual, magnified, upright image is formed. Because
the image is virtual, it cannot be projected on a screen but can be viewed by an observer.
Procedure
1. Place the object (the viewing screen so the p < f. Position the object so that the magnification is a maximum and the image is
clearly focused. Describe the resulting image.
2. Draw a ray diagram of this arrangement.
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Optical Instruments
Question
1. Does the lens equation put any limit on the magnification that a lens can produce?
The Telescope
Setup
Set up the apparatus as shown in Figure 3. Remove the light source from the optical bench. Place the 150 mm lens such that it
is just inside the sum of the two focal lengths.
Figure 3. Schematic drawing showing the experimental setup for a telescope.
Theory
The object is at a large distance so the incident rays are parallel. Thus an image is formed by the first lens at its focal point.
This image is real and inverted. This image becomes the object for the second lens. When the object for the second lens is placed just
inside its focal length, the second lens will function as a magnifier.
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Optical Instruments
Procedure
Choose a distant object to image. Place the two lenses at a distance slightly smaller than the sum of the two focal lengths. Describe
the resulting image.
Draw a ray diagram of this arrangement.
Question
What is the angular magnification of this telescope?
The Microscope
Setup
Set up the apparatus as shown in Figure 4. Remove the light source from the optical bench. Use the 75 mm lens as the
objective lens and the 150 mm lens as the eyepiece.
Figure 4. Schematic drawing showing the experimental setup for a microscope.
Theory
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Optical Instruments
In this case, the objective lens serves as a projector. The eyepiece functions as a magnifier. The first lens has its object placed
such that f < p < 2f. This placement produces a real, inverted, magnified image. This image then becomes the object for the second
lens placed such that p < f.
Procedure
1. Place the object about 110 mm from the objective lens. Move the eyepiece until a clear image of the viewing screen is obtained.
Describe the resulting image.
2. Draw a ray diagram of this arrangement.
Question
1. Why does the magnification increase as the object is moved closer to the objective lens?
LBS 267L
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Optical Instruments