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.
On the diagram, draw the path the Sun rays will likely take from the magnifying glass to the grass.
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?
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 imageto 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.
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?
Turn off the Parallel line and turn on the Line through focal point. Move the candle.
What do you notice about this line?
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?
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.
What do you notice about the orientation of the candle’s image on the right side of the lens?
What do you notice about the size of the image?
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.
Analyze: How is the image size related to the distance between the candle and the focal point?
(Activity A continued on next page)
Activity A (continued from previous page)
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 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.