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A Model of Microscope Optics |
Donald Cronkite (Hope College, Holland, MI)
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Every text has a picture of the lenses of a microscope, but how many people make sense of it? It's easy to make sense of microscope optics with this simple model made from household objects.
Materials
Two magnifying glasses,
about 3 inches ( cm) in diameter. Available at most drug stores for
less than $20 each, but often people have these lying around the house waiting
to be borrowed.
Two simple bookends I paid $2.50 for mine, but
go to a garage sale and find some. They need to be simple so you can
attach the magnifying glasses to them.
A flashlight You need a fairly powerful
one. Pull a single layer of facial tissue over the light end
and secure it with a tight rubber band. This gives a frosted look to the
illumination. A microscope illuminator with a frosted front is also
good.
Rubber Bands Thick ones are best. Mine are 1
inch thick. They should fit tightly with some stretching over the
bookends.
A thick book This will hold the specimen to be
viewed. Put a rubber band around the book to hold the pages tightly
closed. Then insert a transparency with a picture on it between the
leaves of the book and stand it up so that the light from the flashlight
passes through the specimen.
An overhead transparency Use a brightly colored image
that has a top and bottom so you can tell if the image is upside down or
not. I think it is fun
to use a picture of someone in the class. Best is to make the specimen 1
inch high. Then figuring out magnification is easy.
A large piece of paper for a screen. This is for
projecting images from the magnifying glass.
The Model
Secure the magnifying glasses to the book ends. Line things up in the order shown in the diagram below. It's important that there be a straight light path through specimen and the mirrors.
Illuminator
Specimen Holder
Objective
Ocular
Screen
Placement of Parts of the Model
1. Each lens has a focal length, the distance from the lens to the place where the lens focuses a beam of light at a point. To find the focal length of these lenses, shine the light through a lens and move the paper screen until the beam is focused to a tiny spot on the screen. It's best of the light is relatively far from the lens
2. Now set the ocular aside. Set the objective lens directly in the path of the light passing through the specimen so that light passes through the lens and on to the screen. Move the lens and its holder from very close to the specimen toward the screen. Watch for an inverted image to appear on the screen. When the inverted image is at its sharpest, note the position of the lens. [Both the specimen and the image will be outside the focal length of the objective lens, the specimen on the side nearest the light, and the image on the side away from the light and toward the screen.]
3. The image you see is inverted by the lens and is a "real image." It actually can be projected on a screen. Note that the image is larger than the specimen, but not too much so since the lens is only 2X or 3X. Note what happens when you move the screen back and forth after finding the image.
4. Now place the ocular lens in the light path and look through the lens from the side opposite the objective lens. Here you see the whole point of a compound microscope. You are using the ocular lens as a magnifier to observe and enlarge the image formed by the objective lens. Move the lens back and forth until you see the magnified image of the specimen.. This is a "virtual image." It is formed by your eye and brain looking through the lens, but it is not projected on a screen. For the virtual image to be seen, the real image has to be inside the focal length of the ocular lens.
Further activities
1. A modern microscope has many lenses glued together where we have used simple magnifiers for the objective and ocular lenses. Find out what chromatic and spherical aberrations are and how the complicated modern lenses deal with these aberrations.
2. Each of the magnifying glasses I used were 2X. That means they magnify an object to twice its usual height and width. Measure the specimen you are looking at and its real image on the little screen. Did the image confirm the power of the magnifying glass? Now look through the "ocular lens." That should magnify the first image another two-fold. Can you measure the second image to find out? Where do you need to put the ruler when you are measuring? You want to see the unmagnified ruler and the magnified image.
3. Obtain an ocular lens from a microscope. Be sure to instruct students on how to care for that lens before removing it. Look at the image from the first magnifying glass with the ocular lens. What similarities and differences do you see in the two images?
4. You can make this whole demonstration an inquiry by having the students figure out experimentally where each lens must be placed.
5. This model lacks a condenser, a lens or collection of lenses between the light and the specimen. Do some library research to find out what the condenser does.
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Donald Cronkite, Hope College,
Holland, MI 49423 cronkite@hope.edu Jewel Reuter, Archbishop Rummel Hiigh School, Severn Ave., Metairie,
LA 70001 |
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