University of Puget Sound Introductory Physics Laboratory
10. Optical instruments
Name:____________________
Date:___________________
Objective
1. To construct two simple optical instruments, the microscope and the
telescope, and to verify the principles of their operation.
2. To practice the ray diagram approach to analyzing optical systems.
Equipment
Optical rails, lens mounts, cardboard screen, light sources, lenses and mirrors.
Introduction
With knowledge of how single lenses and mirrors redirect light rays, we can now
understand the operation of optical instruments composed of two or more
optical elements (lenses or mirrors). Today you will analyze the principles of
operation of both microscopes and telescopes, and construct simple versions of
each instrument.
The Microscope
Microscopes are used to make small, nearby objects appear larger. A basic
microscope consists of two converging lenses separated by an appropriate
distance. The lens nearest the object to be viewed is called the objective; the lens
you look into is called the eyepiece.
objective
lens
fo
eyepiece
lens
fe
fe
fo
object
(arrow)
optical
axis
intermediate
image
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image
Last week you explored how a single converging lens redirects light rays. The
microscope simply combines two lenses to produce a final image that is highly
magnified. We can think through how two lenses will act by drawing out a ray
diagram. In the above ray diagram, the object (drawn as an arrow instead of a
paramecium) is located just beyond the front focal point fo of the objective lens.
A real, magnified intermediate image is formed behind the objective lens. To
find this intermediate image on our picture we draw a few useful light rays.
Three light rays have been traced from the tip of the object arrow headed
towards the objective lens. There's light going all directions from the tip of the
arrow, but we draw these three because we know how they get deflected going
through the lens. What's special about how these three rays are deflected?



The ray going parallel to the optical axis (the center line of the lens
system) gets bent and passes through the rear focal point, also labeled fo.
The light ray going through the front focal point gets bent on passing
through the lens and comes out parallel to the optical axis.
The light ray traveling through the center of the lens doesn't get bent at
all.
These three rays intersect at the tip of the intermediate image (which is an image
formed by the objective but becomes the object for the eyepiece). All of the other
rays of light that leave the tip of the arrow and go through the lens also converge
of the tip of the intermediate image. Pretty amazing!
In a microscope the eyepiece is used as a magnifying glass to form a final image,
which is simply a magnified image of the intermediate image. Check out the
three rays drawn leaving the intermediate image and make sure they follow the
same rules outlined above for the objective ray diagram. Imagine the light
leaving some other spot on the object arrow; say half way down from the tip to
the base. Where will these light rays end up on the intermediate and final
images? What works for one point works for a set of points.
As an exercise, draw a ray diagram for an object (use an arrow) that is located
two focal lengths away from a converging lens. Make your drawing on graph
paper and trace out the ray diagram to scale. To get started, draw a line for the
optical axis, a picture for the lens, and mark the position of the focal point on
each side of the lens to set a length scale. Add your diagram to your lab
notebook. Where is the image located? How big is the image? This
configuration is particularly useful for determining the focal length of a lens.
How?
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Microscope construction
Use an arrow light source as your initial object and relatively short focal length
lenses (we have approximately 6 cm and 10 cm focal length lenses available) for
both the objective and the eyepiece. Set them up on the optical rail and measure
the focal lengths of all of your lenses using the trick you learned above. Note the
focal lengths of your lenses below, accurate to 0.5 cm. You'll need the longer
focal length lens later on for your telescope.
focal lengths
Set up a ground glass viewing screen on the opposite side of the objective lens as
the arrow light source. Move the arrow source around in the vicinity of the front
focal point and follow the position of the intermediate image by adjusting the
position of the viewing screen. Find an object position that produces a
magnified, real image of the arrow source on a viewing screen. Now set up your
eyepiece on the opposite side of the viewing screen. While looking through it,
adjust its position until you can see a final image in focus.
If you now remove the viewing screen, you should have a working microscope.
Without the ground glass screen the light will probably be too bright to look at
directly. Try projecting your final image onto the viewing screen, placed at the
position of the final image. Fine-tune the position of the eyepiece and the screen
to produce a final image in focus. What is the magnification of your microscope
as it is presently configured?
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To look into your microscope we need a dimmer object, and to operate at higher
magnification we need a smaller object. Turn the arrow light source off and tape
a piece of paper over it with some (small) object drawn on it. Illuminate your
object by shining a bright tungsten lamp directly on it (and use a large cardboard
screen to shield other groups from the glare of this source.) Fine-tune the
location of the objective, and the positioning of your eye until the image is clearly
visible. The greater the magnification, the more delicate this fine-tuning process
will be. Short focal length lenses are hard to adjust, and their images may appear
very distorted. It may help to place an adjustable aperture at the location of the
intermediate image to control the field of view and make the correct eye position
easier to find. Try to observe a small three-dimensional object (pencil tip,
thumbnail, etc.). Notice how shallow the depth of field has become. Is the image
erect or inverted? Estimate the magnification.
The Refracting Telescope
A telescope is used to make a distant object appear larger and/or brighter. As
with the microscope, a basic telescope employs an objective and an eyepiece, but
their focal lengths and separation are chosen differently. Reflecting telescopes
use a concave mirror as an objective; refracting telescopes use a converging lens.
A Keplerian telescope uses a converging lens eyepiece of short focal length along
with a longer focal length converging objective lens. The distant object will be
well beyond the focal length of the objective; in this case the objective forms a
real image near its rear focal point. This image is inverted and reduced in size.
To make this intermediate image as large as possible, we need a long focal length
objective lens. On a separate sheet of graph paper draw a ray diagram for a
Keplerian telescope. How far apart should the lenses be? What will be the
magnification of the telescope (hint: think about similar triangles on your ray
diagram)?
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lens separation? magnification?
Telescope Construction
Mount one of the long focal length lenses on the optical rail for use as the
objective. Find the real image of some bright, distant object using the viewing
screen (maybe some object out the window). Use a shorter focal length lens as a
magnifying glass to observe the intermediate image on the viewing screen.
When you remove the screen, you should have a working telescope. Fine-tune
the eyepiece position to bring the image in focus. Is the final image inverted?
Try both the 25 cm and 50 cm focal length lenses as objectives. You can wheel
your telescope into the hall for more distant viewing. Try all three shorter focal
length lenses as eyepieces. For the three eyepieces and one of the objectives,
measure the magnification of your telescope, by measuring the size of both the
object and image. Also measure the separation between the lenses.
fe (cm)
fo(cm)
distance between lenses (cm)
magnification
How do your results compare to your predictions based on the ray diagram
analysis?
Before you leave:
Explain to your instructor how you constructed your ray diagram for the
Keplerian telescope.
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