Investigation 21D: Build a microscope and a telescopeEssential question: How do a microscope and telescope work?

The basic microscope and telescope are each a compound optical device that uses two lenses. The object is imaged by the first (or objective) lens; this image is used as the object by the second (or eyepiece) lens. By choosing particular types of objective and eyepiece lenses, and by separating the two lenses by a suitable distance, both the microscope and the telescope can produce magnified images. Safety note: never look directly at the Sun with either a microscope or telescope!

Part 1: Build a simple microscopeA microscope creates a magnified image of an object that is held near to the objective lens. Which lenses create a microscope?

1. Use three lenses (illustrated above): two converging (convex) and one diverging (concave).2. Using two lenses at a time, determine which two lenses create a microscope. By generating a

magnified image of an object (graph paper). Hold the lenses over the graph paper on your desk. Keep the objective about 15 cm above the graph paper and the eyepiece lens close to your eye. Move the eyepiece lens to try and magnify the object.

When you succeed, have your partner use a meter stick to measure and record the approximate distances of the two lenses above the graph paper.

Objective lens Eyepiece lens

lens type (convex or concave)

position (m) (distance

from paper)

focal length(m)

lens type (convex or concave)

position (m) (distance

from paper)

focal length

(m)

lens separation

(m)

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3. Construct a microscope on the track. Using the approximate positions for the lenses and the object (the graph paper), construct

the microscope using the lenses, lens mounts, and the track. Instead of using the graph paper as an object, use written text on a piece of paper in a

circular lens mount. Adjust the lens positions by small amounts in order to increase the magnification of the

microscope—but without substantially distorting the quality of the image. Measure and record the final object distance and lens separation in the table below.

Objective lens Eyepiece lens

object position

(m)

lens position

(m)

focal length

(m)

object distance

(m)

lens position

(m)

focal length

(m)

lens separation

(m)

Questionsa. What kinds of lenses are used to make a microscope?

b. Which lens, if any, has a shorter focal length, the objective or the eyepiece?

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Part 2: Model the simple microscope

1. Enter your microscope’s parameters into the compound optics simulation.2. Make small modifications to the positions of the lenses to see what range of positions produces

the largest magnification.Note that if the image of your actual microscope was inverted, then your model should also have an inverted image!

Questionsa. Where is the object for the objective located relative to its focal point?

b. Where is the object for the eyepiece located relative to its focal point?

c. How is the separation of the lenses related to the focal lengths of the two lenses?

d. What is the magnification of your model microscope? How uncertain is your value?

e. Is the image produced by the eyepiece real or virtual? Why?

f. Is the image inverted or upright?

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Part 3: Build a simple telescope

1. Use the same three lenses from the previous part (two converging and one diverging lens). Holding two lenses at one time–one at arms length and the other close to your eye–determine which two lenses can create a telescope that magnifies a distant object (such as a far wall).*

Have your partner measure the approximate separation between the lenses in your hands.

Lenses that create a telescope: Lens 1: Lens 2:

Lens separation: meters

2. Construct your telescope on the track like the picture, using the distances above (to start). The object should be located in the distance, such as text or graph paper on a far wall.Make small adjustments to the lens positions in order to create a magnified and sharp image.Measure the positions of the lenses. As best you can, measure the distance to the far object.

Objective lens Eyepiece lensobject

distance(m)

lenstype

lens position

(m)

focal length

(m)

lenstype

lens position

(m)

focal length

(m)

lens separation

(m)

3. Devise a method to measure its magnification for a particular eye position.Possible method: Draw dry erase marks 3 cm apart on the eyepiece lens. Have your partner move pins on the far object to line up with these marks when you view the pins through the telescope. Measure the distance between the two pins. Record your data below.

* There are actually two ways to construct a telescope. If time permits, see if you can find a second method.

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Questionsa. What kinds of lenses are used to make a Galilean telescope? A Keplerian telescope?

b. Which lens (if any) has a shorter focal length, the objective or the eyepiece?

c. What is the magnification of your telescope?

d. Why is the diameter of a telescope’s objective usually much larger than the diameter of its eyepiece? [Hint: look through your telescope. How big does the objective lens appear when you are looking through the eyepiece?

Part 4: Model the simple telescope1. Using the compound optics interactive simulation, model the telescope you constructed by

entering the object position, lens focal lengths, and lens positions.2. Modify the positions of the lenses by small amounts to see where the magnification becomes

large.3. If the image in your actual telescope was inverted, then your model should also produce an

inverted image.

Questionsa. What position did you use for the object? Why?

b. What is the lens separation where you can obtain high magnifications?

c. How is this lens separation related to the focal lengths of the two lenses?

d. Write a multiple-choice question that tests the reader’s understanding of the difference(s) between a microscope and telescope.

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