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course phy 122
3/10 4
Experiment 27: Thin Lenses (revised version)Using lenses constructed from broken pieces of clear Christmas ornaments we determine focal points and focal lengths of two concave and two convex lenses.
You will receive two small plastic concave lenses.
Using a permanent marker or some other means, place two small dots on each lens, with one dot about 2 mm above the center and the other 2 mm below the center (maybe 3 mm for the larger lenses). You will end up with a top and a bottom dot, separated by about 4-6 mm.
Support the lens by the handle (e.g. use two blocks or books to sandwich the handle) in such a way that it lies in a vertical plane, perpendicular to your table top.
We begin by determining the focal length of each of the convex lenses.
• First estimate the focal length of each lens by placing the screen 'behind' the lens and moving the laser back and forth in front of the lens, as you did with the circular lens in the preceding experiment.
• The focal distance is the screen distance at which, if you manage to keep the direction of the pointer consistent, the dot on the screen remains stationary.
• Measure the distance of this point behind the lens.
• Now determine the focal distance using the two dots on each lens.
• Place the screen at about twice the distance behind the lens as your estimated focal point; measure and note this distance.
• Keeping the laser pointer in the horizontal direction, direct the beam through the top dot on the lens and onto the ruler on the screen; note as accurately as possible the vertical position of the beam on the ruler.
• Repeat for the bottom dot, again being sure to keep the pointer horizontal so the incoming beams for both measurement are parallal.
• Measure the vertical distance between the dots, then using this distance and other observed distances construct an accurate picture of the paths of the beams.
• From your picture determine the distance from the lens at which the beams cross.
• This distance should be the focal distance.
Data collected:
The following data is for the 15 cm lens. The distance from the lens to the screen is 16 cm, so twice that distance would be 32 cm. I learned by reading the experiment that the focal distance is found when a laser is placed on the first dot and subsequently on the second dot and they show the same number on a vertically placed ruler. I initially found that when I shot the laser through the first dot that the reading was 2.5 cm and the second reading was 1.7 cm. I read on to know that I would have to adjust my laser position in order for the vertical line position to become equal. I measured again and found that the number on the vertical line was 2.5 for both dots, but then I had to move the laser from 45 cm to 54.6 cm to bring the laser into focus. The 45cm and 54.6 positions indicate the distance from the laser pointer to the screen when the laser was placed on the vertical ruler position 2.5cm from both dots.
The following data is for the 30 cm lens. The distance from the lens to the screen is 16.6cm, so twice that distance would be 33.2 cm. I learned by reading the experiment that the focal distance is found when a laser is placed on the first dot and subsequently on the second dot and they show the same number on a vertically placed ruler. I found that the number on the vertical line was 2.9 cm for both dots, and then I had to move the laser from 50.1 cm to 135.8 cm to bring the laser into focus. This was a little tricky with the 30 cm lens and I struggled finding the distance for the lower dot. I had to refocus my each time I refocused the laser in order to maintain the same position on the lens. (I had a hard time seeing the dot ant the 135.8 cm position) The 50.1 cm and 135.8 cm indicate the distance from the laser pointer to the screen when the laser was placed on the vertical ruler position 2.9 cm from both dots.
The following data is for the 10 cm concave lens. The distance from the lens to the screen is 7.5 cm, so twice that distance would be 15 cm. I learned by reading the experiment that the focal distance is found when a laser is placed on the first dot and subsequently on the second dot and they show the same number on a vertically placed ruler. I found that the number on the vertical line was 3.5 cm for both dots, and then I had to move the laser from 34.9 cm to an unknown distance to bring the laser into focus. I moved back as far as I could from the lens and could never get the laser pointer to line up on 3.5 cm from the second dot. I was unable to find focus on the second distance.
• To check the focal distance, place the screen at this focal distance.
• Determine whether horizontal rays through the two dots on the lens strike the screen at the same vertical position.
• If this is the case, as it should be, then you have determined the focal distance.
• Now place a burning candle as far away as possible from the lens. The candle flame should be at the same height as the lens, which might require that you elevate the lens slightly. The room should be dark. Move the screen behind the lens until a sharp image of the candle flame appears on the screen. As long as the candle is 2 meters or more from the screen, the distance between the lens and the image will be within experimental error of the focal distance. Compare this distance to the focal distance you determined earlier.
I completed this experiment with all three lenses with the following results: (first space indicates the lens size, second space indicates the distance from candle to lens, and third space indicates the distance from the lens to the book)
15cm, 200 cm, 16cm
30cm, 200 cm, 37 cm
10 cm, 200 cm, unable to determine focal point with this lens. The only thing projected onto the screen was a small black circle.
• You can also determine the focal distance on a sunny day by forming a concentrated image of the sun on your screen. This part is optional and you don't need to report it but it's worth a couple of minutes, so some time when the sun is out do it.
• The image of the sun will shrink to a very small and sharply defined disc at the focal point.
• Do not attempt to look at the sun through this lens. If you do, and if your eye is anywhere near the focal point, you will burn out parts of your retina and see dark blotches for the rest of your life.
• Don't leave the point image of the sun on its target for long or you risk setting it on fire.
• Resist the temptation to start fires or to focus the sun on small defenseless insects.
• Look through the lens at your finger.
• Move your finger close to the lens, then further from the lens. Note and describe what you see.
• What does the image of your finger look like when your finger is at the focal distance?
• What happens to the image of your finger if you move back and forth across the focal point?
With the 30 cm lens as I move my finger closer to the lens, it appears sharper and clearer. As I move my finger away from the lens, it becomes larger and less focused. When my finger is at the focal distance, it is sharp and clear. As I move my finger back and forth across the focal point it is clear and then blurry, clear and blurry, this trend continues.
With the 15 cm lens, the results are the same as with the 30 cm lens.
With the 10 cm concave lens, as I move my finger closer to the lens, it seems to be magnified, as I move it away it appears to shrink, but the clarity remains the same. When my finger is at the focal distance, it is sharp and clear as it was previously. As I move my finger back and forth across the focal point my finger appears to be magnified and the shrunk, magnified and then shrunk, this trend continues.
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&#This lab submission looks very good. Let me know if you have any questions. &#