Optics Lab: Take two lenses and make a telescope

Team___________      Name_________________________      Hour____

Skinny lens code _______
Fat      lens code _______
Your team has two lenses; one is skinny, one is fat. At the right, enter the code letter of the two lenses:

1. Find the focal length of each lens.

   Arrange the light bulb (object), the lens (one at a time), and a white 3×5 card as shown, on an "optical track." (An optical track can be nothing fancier than a meter stick on two pairs of legs, and clips for the items, like lenses, to ride on the track.)Adjust the distances until you are able to focus the image of the light bulb on the card.

   With the image of the light bulb (object) focused on the card, measure (for each lens) the object-to-lens distance do, and the lens-to-image distance di.

Skinny lens do=_______cmdi=_______cm
Fat     lens do=_______cmdi=_______cm

   With these data, calculate the focal length of each lens.

Equation that you used:

f skinny lens
the focal length
f fat lens
the focal length

2. Combination Lens
    Put the fat and skinny lenses together and use the same method as in (1) to find the focal length of the combination. (If one lens does not have a lens mount, have someone hold it next to the other lens.)

Combination lens do=_______cmdi=_______cmThe focal length is _______cm

Use the theoretical equation for the focal length of two lenses together to calculate what you would expect the focal length of the combination to be, based on their separate focal lengths as determined in (1).
Combination lens Theoretical focal length is _______cm

Are you satisfied that the experimental and theoretical values agree within experimental error? If not, explain, and discuss what effort you have made to correct the situation.


   You have already met Ferdinand. Use Ferdinand as the object that your telescope will be designed to view, magnified, of course. On a piece of tracing paper (translucent paper that allows you to draw on) 6cm by 6cm, make a black ink silhouette drawing of Ferdinand. Make him exactly 5 cm tall. This will be ho. Now wrap the width of the paper in the shape of a cylindrical tube, so that you can slide the "Ferdinand" over the light bulb.

Place the 'Ferdinand' over the bulb. Place the skinny lens 60cm to the right of Ferdinand.

Theoretical image distance, di is _______cm
Calculate, using the focal length of this lens as you measured it in (1), how far to the right of the lens you should place a white card if you expect a focused image of Ferdinand to appear on the card.

______Yes    ______No
Try placing a card this distance to the right of the lens. Is Ferdinand focused on the card?

Theoretical image height, hi is _______cm
The image is, of course, upside down. Calculate how tall the upside down Ferdinand should be.

Experimental image height, hi is _______cm
Measure the height of the image of Ferdinand.

4. Seeing an image in mid-air.

When you see the image of Ferdinand without the white card, put a check mark here
Take away the card. Place your eye at least l0cm to the right of where the card was (so your eye can focus). Look toward the lens and Ferdinand. Do you see the upside down image where the card had been? Keep trying this until you do.

5. Telescope

Estimate how much larger the image is now than before
Repeat (4), but in order to see the image closer, and therefore bigger, use the fat lens directly in front of your eye, so that the fat lens and your eye-lens form a combination lens. Approach closer to where the card had been. Do you now see the image of Ferdinand enlarged? About how much larger than before?

CONGRATULATIONS! You have built a telescope If you have time, you can draw a small bug on the reverse side of the "Ferdinand" tube, and look at it through your telescope, to see how much larger the telescope makes it seem to be.

Theoretical magnification is _______
Calculate the theoretical magnification of your "telescope."
    [Magnification = f(objective lens)/f(eyepiece lens)]

Does it appear to magnify this much?

6. Color
      Do you notice how horribly fuzzed out the colors are in the light part of the image? What is this due to?

Back to Job #1: Sally and Kenny go Wading
Back to Job #2: The famous fishing pole paradox
Back to Job #3: How a prism separates blue light from red light
Back to Job #4: The World's First Camera, or, How the Human Eye Works
Back to Job #5: The Telescope, or How the Dutch Lens Grinders Made Galileo Famous
Answers to "Job" problems
Back to Optics Main menu

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