course Phy 202
Good work on this experiment.
Experiment 28Thin Lenses (revised v
· 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. For the small convex lens I measured a focal length to be around 12.5 cm. For the large convex I measured a focal length of around 16.7cm. I measured these by putting the case behind the lens. I used the laser beam to aim. I took measurements when the laser was stationary on the case.
· Place the screen at about twice the distance behind the lens as your estimated focal point; measure and note this distance. It is around 10 cm.
· 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. The vertical distance is around 4.5 cm.
· From your picture determine the distance from the lens at which the beams cross. I observed that the rays from the two dots converged in front of the lens around 7 cm. I noticed that the rays converged in front of the lens around 4.5 cm
· Determine whether horizontal rays through the two dots on the lens strike the screen at the same vertical position. The horizontal rays do seem to strike the screen at the same vertical position.
· 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. Three rays are depicted from the tip of the flame: a ray parallel to the central axis of the lens, a ray passing through the center of the lens, and a ray striking the lower part of the lens at such an angle that the transmitted ray ends up parallel to the axis of the lens.
· Compare this distance to the focal distance you determined earlier. The distance is very similar to the one that I calculated earlier. It actually is less than a cm off.
· Move your finger close to the lens, then further from the lens. Note and describe what you see. Looking at my finger through the lens it appears to grow larger and larger as I move it away from the lens until it reaches the focal distance, where it becomes infinitely large and therefore impossible to see. Beyond the focal distance my finger looks blurry.
· What does the image of your finger look like when your finger is at the focal distance? It becomes infinitely large and therefore impossible to see.
· What happens to the image of your finger if you move back and forth across the focal point? It forms a blurry image.
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