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# Mirrors

When you look at a smooth surface, you don't actually see light rays. Instead, you are seeing images. To understand where the images come from, you must look at light rays, though.

The diagram shows an object J in front of a flat mirror. Light from it bounces off the mirror at N and ends up at R. Our eyes perceive the light to have come from N and beyond the mirror from O. With just one ray, though, the image cannot be placed. Our eyes can just figure out the image is somewhere between O and N. We need another piece. Another ray coming out of J that bounces off of A enter the eyes as U. This completes the image by placing it at G, where the two assumed lines of origins of the light rays intersect. An image formed like this from flat mirrors is called a virtual image, because there aren't really light rays crossing there; our brains just think that there are light rays there.

## Curved Mirrors

All images formed from a flat mirror are virtual. When we curve mirrors, things start getting really interesting. Simplicity dictates that we utilize the two basic forms of curved mirrors: concave and convex. A concave spherical mirror is a reflecting surface that is located on the inside of a hollow sphere.

The imaginary line going through the center of the sphere and the middle of the mirror is called the optic axis. Light rays that come into the mirror parallel to the axis are reflected toward a point called the focal point. It is always halfway on the axis between the center of the sphere and the edge. The distance from the mirror to the the focal point is called the focal length.

### Convex Mirrors

Convex mirrors are just like concave mirrors except for the reflecting surface is on the outside of the sphere. This makes all the difference.

The optic axis is there, but this time, the focal point and the center of the sphere are both behind the mirror. Light rays coming parallel to the optic axis now reflect, and appear as if they're coming from the focal point behind the mirror.

### Concave Mirrors

Concave mirrors can produce both virtual images or real images (place where real light rays do meet), depending on where you are standing. At a large distance, the image seen is real. After you pass the center point of the sphere, the image you see becomes virtual. Once the rays start becoming divergent, there is no way from them to meet again to create a real image. You can however, see a virtual image, where they appear to come from. Likewise, because the light reflecting from a convex mirror is divergent, all you see is a virtual image.

## Finding Images

Locating the image of an object from the mirror is relatively simple. Any single point on an object gives off light rays in all directions. Three of these rays are particularly easy to draw, and will give the position of the image. The first ray is drawn through the radius of the sphere so it bounces back on itself. The second approaches the mirror parallel to the axis, and is reflected through the focal point. The third ray crosses the focal point on the way to the mirror, and is reflected parallel to the axis. The place where the three meet is the position of the real image; or, the place where the three meet after you extend them behind the mirror is the position of the virtual image.

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This ThinkQuest Project last updated 7/31/97 by

Stan Seibert, Brett Bennett, and Jur Jang