University of Virginia
Physics Department

## Curved Mirrors

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.9

Objectives

Students will

• compare and contrast the properties and uses of concave and convex mirrors;
• compare and contrast real and virtual images;
• describe practical uses of curved mirrors.

Motivation for Learning

Driving Question

What images are produced when mirrors are curved?

Background Information

Curved mirrors are like plane mirrors in that they too have smooth, shiny surfaces that reflect light. The surface can either curve out (convex) or in (concave) as shown in Figure 1.

Convex mirrors are commonly found in cars as rear-view mirrors. They offer a wider view than a plane mirror, but the images are smaller than they would be in a plane mirror. To help drivers, these mirrors will frequently include the warning that "Objects in mirror are closer than they appear." The images produced are also called virtual images because they appear to form behind the mirror and cannot be projected on a screen.

Concave mirrors are commonly found as reflectors in flashlights and some telescopes. They are used in solar ovens, and though one cannot see the reflected electromagnetic rays, a satellite dish is essentially a concave mirror. When an object is beyond the focal point of a concave mirror, the image formed will be real and inverted. (A real image can be projected on a screen.) If the object is very far away and the light rays are parallel when reaching the surface of the mirror, the image will form at the focal point. If the object emitting the light rays is at the focal point, the reflected rays will not form a real image. Instead, the reflected rays leave the surface of the mirror parallel to one another. (This is the way a flashlight or car headlight is constructed.) A third possibility is if the object is between the focal point and the mirrored surface. In this case, the image will be virtual, larger, and right side up.

### Student Activity

Materials

• Convex mirror
• Concave mirror
• Plane mirror
• Birthday candles
• Modeling clay
• Meterstick
• Index card
• Matches

Procedure

These activities can be performed in small groups or as teacher-led demonstrations.

1. Hold the convex mirror in front of you. Move the mirror toward and away from you and observe the image formed. Hold a plane mirror in front of you. Move it toward and away from you to observe the image formed. Hold the concave mirror in front of you. Move it toward and away from you and observe the images formed. Compare the images with respect to size and orientation.
2. Measure the focal length of the concave mirror by focusing the sun's image on an index card. Reflect the sun's rays onto the card and move the mirror toward and away from the index card until a small dot is focused. Measure the straight-line distance from the card to the center of the mirror. This is the focal distance of the mirror, and the image is located at the focal point. (DO NOT LOOK DIRECTLY AT THE SUN OR FOCUS THE SUNLIGHT ON YOURSELF OR ANY OTHER PERSON!)

 Figure 2: One possible arrangement of the candle, mirror, and index card for examining real images.

3. Place a lit birthday candle mounted in modeling clay on a lab table. Hold the concave mirror a distance equal to the focal length from the candle. Dim the room's lights and hold the index card away from the mirror and candle to view the reflected light (figure 2). Compare the amount of light reaching the card when the mirror is in position and reflecting the candlelight to when there is no reflection of light and the candle sheds only direct light on the card.
4. Position the mirror so it is at a distance twice that of the focal length from the candle. Place the index card at a distance near the focal length from the mirror and move it towards and away from the mirror until an image of the flame forms on the index card. The image will be inverted. Continue to vary the distances among the candle, screen, and mirror to investigate the images formed.

Extensions

1. Research other uses of curved mirrors.
2. Build a solar oven.
3. Research satellite dish design and technology.

Students with Special Needs

Adapt as needed to children who have physical disabilities.

Click here for further information on laboratories with students with special needs.

Assessment

1. Accuracy of student data will provide information on student understanding.
2. Students predict images formed when a light source and mirror are in a variety of orientations.
3. Students predict position of light source when images and mirrors are in a variety of orientations.