course Phy 232
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.
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I didn't actually have the lenses in my kit, so I just made my own by cracking open a surprisingly resilient christmas tree ornament of my own (since it says earlier that the ones in the kit were made from christmas tree ornaments), and I tried to find two bits equal in size, but it didn't break up evenly, so one is smaller than the other. Though, if they have the same curvature, I think it should be okay. I also used one of those red laser pointer toys that were trendy a few years ago instead of the flashlight like last time because it says 'laser pointer' in all the instructions.
Anyway, lens 1 was 69mm from the lens to the book, and lens 2 was 69.5mm to the book.
It's hard to break ornaments into usable pieces. Using a clear ornament you then have to fill the space between with water, or some other liquid, in order to get a lens that actually focuses the light.
However your kit should have included lenses; if you got the materials, as I know you did, you paid for the lenses, so if you send me your mailing address I'll send you a set. They're fairly decent lenses, could be fun to play with, and make good magnifying glasses.
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* 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.
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The dots were 2.8ish-3mm apart.
lens 1:
I put the book 158mm away, and for the first dot, it was 73.5mm. For the second dot (the lower one), it was also 73.5mm.
lens 2:
I put the book 159mm away, and for the first dot, it was 81mm. For the second dot, it was also 81mm.
According to my picture, the rays cross at 73.5mm for lens 1, and 81mm for lens 2. Lens 1 is closer to the predicted focal point. Lens 2 is a bit off.
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* 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.
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I used a flashlight 190cm away (I had the candles, but I don't like fire, and I figured the flashlight was about as bright as the candles).
Anyway,
Lens 1 = 62.6mm
Lens 2 = 63.4mm
These are close (kinda?), but a bit smaller than the original estimations.
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It looks like you got good results. See my note, and send me your address if you want me to send you a set of lenses.