Concave and Convex Mirrors: An Up-Close Look at the Spoon
Mirrors can be flat or curved. When you see your image in a flat mirror, it is not distorted. When you look at your image in a curved mirror, light rays leave the mirror at a different angle from the approaching rays, so distortions result. Mirrors can be curved either inward or outward. Concave mirrors curve inward, like the interior of a bowl. Makeup mirrors are concave mirrors because they enlarge the image. Mirrors that curve or bulge outward are called convex mirrors. This type of mirror gives a wide field of view and is used in security mirrors in stores as well as the side mirror on cars and trucks. In this activity you will use the opposite sides of a shiny spoon to compare the images produced by concave and convex mirrors.
- Pick up the spoon by its handle and hold it so you are looking at the caved-in or hollowed-out side. You are looking into a concave mirror. Note whether your image appears larger or smaller.
- With the spoon held at a distance from your face, move a pencil point slowly from your face toward the bowl of the spoon.
- Note how the appearance of the pencil point changes as it moves closer to the bowl of the spoon.
- Turn the spoon around so you are looking at the back side of the bowl. You are looking at a convex mirror. Note your appearance in the spoon. Repeat the same process with the pencil point as you did with the concave side of the spoon.
- How did your appearance differ when you looked at the concave and convex sides of the spoon?
How did the appearance of the pencil point change as you moved it toward the spoon? Which side was inverted at one point and right side up at another point? Which side magnified the image?
- Answers will vary, but most students will indicate their image was upside down in the concave side and right side up in the convex side.
- As you moved the pencil toward the concave side of the spoon, the image started out upside down, but eventually flipped right side up and was enlarged. In the convex side of the spoon, the image of the pencil point remained right side up as you moved forward, but it did not enlarge as you got closer to the spoon.
Move the pencil point toward the concave side of the spoon until you get the pencil point into clear focus. Measure the distance from the spoon to the pencil. This is the focal point. Try and do the same for the convex side of the spoon. What do you discover?