GIZMOS Student Exploration Ray Tracing (Lenses)

Exam (elaborations) GIZMOS Student Exploration Ray Tracing (Lenses) Student Exploration: Ray Tracing (Lenses) Vocabulary: concave lens, convex lens, focal point, image, magnification, real image, refraction, virtual image Prior Knowledge Questions (Do these BEFORE using the Gizmo.) Agnes is trapped on a desert island with nothing but a magnifying glass. She wants to use the glass to focus sunlight and start a fire. She holds the glass above some dry grass as shown at right. 1. On the diagram, draw the path the Sun rays will likely take from the magnifying glass to the grass. 2. A magnifying glass is an example of a convex lens —a lens that curves outward on both sides. Why is a convex lens useful for starting fires? Because the lens can direct the light to a point, it focuses all the light there so that point heats up, and can catch fire. Gizmo Warm-up The Ray Tracing (Lenses) Gizmo shows light rays passing through a lens. The light rays are bent by refraction as they pass through the lens and form a focused image to the right of the lens. To begin, turn on the Colorize lines checkbox. Under Show lines, turn off the Central line and the Line through focal point so that only the Parallel line is showing. 1. The blue dots in front of and behind the lens are the focal points of the lens. Move the candle on the left back and forth and up and down. What is always true about the light ray that emerges from the right side of the lens? It always goes through the focal point 2. Turn off the Parallel line and turn on the Line through focal point. Move the candle. What do you notice about this line? It is always parallel to the principal axis GIZMOS Student Exploration Ray Tracing (Lenses) 2019 Activity A: Real and virtual images Get the Gizmo ready:  Turn on the Parallel line, Central line, and Line through focal point.  Move the candle to -24 on the central axis, with the focal point at -12. Introduction: A convex lens is called a “converging lens” because it focuses light rays into a point. A real image is formed where the light rays emitted from a point converge on the other side of the lens. If you placed a sheet of paper at the image, a focused image would be projected onto the paper. Question: How do lenses create images? 1. Observe : In its current configuration, the distance from the candle to the focal point is 12 units and the distance from the focal point to the lens is also 12 units. A. What do you notice about the orientation of the candle’s image on the right side of the lens? It’s inverted. B. What do you notice about the size of the image? The image is slightly smaller. 2. Investigate : Complete each action listed in the table below, and describe how that action affects the image to the right of the lens. Return the candle and focal point to their original positions (-24 for the candle, -12 for the focal point) after each action. Action Effect on image Move the candle to the left. Image gets smaller and moves toward lens. Move the candle to the right. Image gets bigger and moves away from lens. Move the left focal point to the left. Image gets bigger and moves away from lens. Move the left focal point to the right. Image gets smaller and moves toward lens. 3. Analyze : How is the image size related to the distance between the candle and the focal point? As the distance between the candle and focal point increases, the image goes smaller and moves towards the lens, and vice versa. 2019 (Activity A continued on next page) 2019 Activity A (continued from previous page) 4. Explore : Move the candle to -12 and the focal point to -24. Shorten the candle to see the whole image. What do you notice when the candle is between the focal point and the lens? The image is bigger, upright and further from the lens than the candle. The image you see is called a virtual image because no actual light rays are focused there. The dotted lines represent locations from which light appears to be coming to an observer located left of the lens. An observer would perceive a magnified virtual image of the candle. This is what happens when you look at an object through a magnifying glass. 5. Investigate : Return the candle to -24 and the left focal point to -12. Under Show specific line segments, turn off Apparent light lines. On the menu at lower right, select Concave lens. A concave lens curves inward on both sides. A. What do you notice about the three lines after they pass through the concave lens? The light rays separate from each other. Because a concave lens causes light rays to spread apart, it is also called a “diverging lens.” B. Turn on Apparent light lines. Is the image of the candle a real image or a virtual image? Explain. (Hint: Recall that a real image forms where light rays are focused.) Virtual image because the light went through the lens and the image is where the apparent light lines are. C. Move the candle back and forth. No matter where the candle is placed, what is true about the image? The image stays virtual. D. What would an observer see if she looked at the candle through the concave lens? The observer would only see the smaller image through