Concave and Convex Mirrors

Sometimes you see images that are very distorted. Look into both sides of a polished metal spoon. The images you see are quite different from the image formed by a plane mirror. Each side of the spoon produces a different image because each side is curved differently. The curved surface of the spoon changes the way light is reflected.

Concave Mirrors When the inside surface of a curved mirror is the reflecting surface, the mirror is a concave mirror. Figure 3A shows how a concave mirror reflects light rays that are parallel to the optical axis. The curvature of the reflecting surface causes the rays to come together. The point at which the light rays meet is called the focal point.

Look again at your reflection in the bowl of a spoon. The upside down image you see is a real image. A real image is a copy of an object formed at the point where light rays actually meet. Unlike a virtual image, a real image can be viewed on a surface such as a screen.

Concave mirrors can form either real or virtual images. The type of image formed depends upon where the object is in relation to the mirror. Figure 3B shows how a concave mirror forms a real image. When the object is farther from the mirror than the focal point, the reflected rays meet in front of the mirror. Figure 3C shows how a concave mirror forms a virtual image. When the object is closer to the mirror than the focal point is the reflected rays spread out and appear to come from behind the mirror.

Concave mirrors are often used in automobile headlights and flashlights to direct the illumination from a single light bulb into a beam. If the bulb is placed at the focal point of a concave mirror, the reflected light rays will be parallel to one another. This results in a brighter beam of light.

Figure 3 Concave mirrors can form either real or virtual images. A When parallel incoming rays strike a concave mirror, they are reflected through the focal point. B Concave mirrors form real images when the reflected light rays converge. C Concave mirrors form virtual images when the reflected rays appear to come from a point behind the mirror. Interpreting Diagrams What determines the type of the image formed by a concave mirror?

d

Table of Contents