Link+to+Wangene's+Lab













__**Telescope/spyglass project**__

A telescope is a device which takes an object at infinity (a star or the moon) and creates a highly magnified virtual image that is viewed by the eye. Because converging lenses create images that are small (fractional magnification) and inverted when the object is placed a long distance away, a second lens must be used. This second lens (the eyepiece) uses the image created by the first lens (the objective) and creates an image of the image (a secondary image). The secondary image appears on the same side of the lens as the object (like with a magnifying glass), and so it remains inverted (relative to the object) but is magnified. The objective lens typically has a long focal length (giving the largest possible first image) and the eyepiece typically has a short focal length (giving the largest magnification for the secondary image).

Can you explain why the lenses are separated by approximately the sum of their focal lengths? Think about the location of the first image and how that image should be placed to create a virtual image with the second lens. The lenses are separated by approximately the sum of their focal lengths because the images are created approximately at the focal length for a far away object. Unfortunately, the object seen through the lenses is not actually at infinity, so the position of the image may be a slightly different length away than the length of the focal length itself.

Build a telescope. Use a 40cm focal length lens for the objective, and use a second lens of your choice for the eyepiece. Measure its focal length using any technique you like. You should also confirm the focal length of the 40cm lens (don’t trust the labels).

Determine the approximate magnification of the telescope. Compare this magnification to an expression for the magnification you find for a telescope online.

Modify the telescope by swapping in a different focal length lens for the eyepiece. How must you modify the telescope to accommodate the new eyepiece? How does this change affect the approximate magnification? Repeat the switch for several eyepieces. Make a graph showing how the magnification and the length of the telescope depend on the focal length of the telescope.

The distance between the objective and the "third lens": 67 cm The distance between the "third lens" and the eyepiece: 30 cm The focal length of the third lens: approximately 10 cm
 * Eyepieces || Focus ||
 * 1st eyepiece || 45 cm ||
 * 2nd || 10 cm ||
 * 3rd || 11 cm ||
 * 4th || 9 cm ||
 * 5th || 6 cm ||

A shorter focus results in a shorter telescope lens (higher magnification), whereas a longer focus results in a longer telescope (lower magnification). I found the magnification of the telescope containing 2 lenses-- which resulted in an optimized magnification for the 10 cm third lens.

One deficit of the telescope is that it inverts the image, making it difficult to interpret what your eye sees. One way to modify the telescope is to introduce a third lens between the objective and eyepiece that flips the orientation of the image before it is magnified by the eyepiece.

A second technique, one employed by Galileo, is to use a diverging lens for the eyepiece as opposed to a third lens between the objective and the eyepiece. Select various diverging lenses and describe how they may be used to make a telescope. What are the advantages of using a diverging lens as opposed to a third lens.

I did not use a diverging lens for my eyepiece, but I suspect that the focal length will be behind the lens itself. The diverging lens will only cause the image to be filtered through the lens once, meaning its resolution will most likely be clearer. The diverging lens also would probably cause light to be directed to the eye from a different angle.

For your write-up, choose your favorite combination of lenses for the telescope and describe its properties.

The telescope combination I used had a 45 cm objective lens, a 10 cm "third lens" and a 20 cm eyepiece. This resulted in a working telescope, due to the positioning of the lenses. The resolution wasn't extremely clear, but the image is highly magnified at a rather long distance away. I learned that the virtual image created is rather far away (either above or below) the actual position of the object. The distance between the lenses were quite larger than I expected them to be, based on the focal lengths I had previously measured. In conclusion, this lab taught me that the image seen by a telescope is not, in fact, a real image, but the magnified virtual image of an object at a far away distance. The rays from a real object would hit the lens from a distance, creating the impression that they had come from a real and central point. The lens from a telescope give the impression that the object seen is in a different position and at a higher magnification than its actual location.



Make a ray diagram to scale with an object located a distance of 5x the focal length of the objective lens away. Be sure to indicate the location of (a) the first image from the objective and (b) the image the eyepiece makes of that image. You may need to tape several sheets of paper together to accomplish this.