A convex lens is a transparent instrument that uses refraction to bend and focus light from an object, forming a sharp image. We'll first investigate how the lens produces an image from a point source of light (a lamp). Select Screen at upper right, which makes the object a lamp and gives a black screen that can be dragged around. Select Many rays at upper left to see how the lens bends some of the rays of light from the lamp. With the lamp positioned far to the left of the lens, you should see that the rays that go through the lens converge to a point. If the screen is placed where the beams converge, the image on the screen will be in focus (it will be a small dot of light since that is what the object looks like in this case). The screen is then at the focal plane. As the lamp is moved closer to the lens, the distance between the focal plane and the lens _____. remains the same. decreases. Correct When the lamp is closer to the lens, the rays going into the lens are diverging more quickly than when the lamp is further away. Thus, after going through the lens, they aren't converging as quickly, so it takes a longer distance for them to focus. Part B The horizontal blue line through the middle of the lens is called the optical axis . As the lamp is moved above or below the axis, keeping the horizontal distance to the center of the lens constant, how does the horizontal distance from the lens to the image change? It decreases. It increases. It does not change. Correct This explains why we refer to a focal plane. All objects with the same horizontal distance to the lens will be focused by the lens on the same
plane. You can see this by selecting 2nd Point and placing one of the lamps above original lamp. Part C Do the rays that go through the lens always converge on the right side of the lens (forming a real image), regardless of the position of the lamp?
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