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Concave & Convex
A concave mirror, or converging mirror, is a curved mirror that bulges inward(away from the incident light). Concave mirrors reflect light inward to one focal point, therefore they are used to focus light. Concave mirrors show different image types depending on the distance between the object and the mirror. These mirrors are called “converging” because they tend to collect light that falls on them, refocusing parallel incoming rays toward a focus. This is because the light is reflected at different angles, since the normal to the surface differs with each spot on the mirror.
The image location and size can also be found by graphical ray tracing. A ray drawn from the top of the object to the surface vertex(where the optical axis meets the mirror) will form an angle with that axis. The reflected ray has the same angle to the axis, but is below it. A second ray can be drawn from the top of the object passing through the focal point and reflecting off the mirror at a point somewhere below the optical axis. This ray will be reflected from the mirror parallel to the optical axis. The point at which the two rays meet is the image point corresponding to the top of the object. Its distance from the axis defines the height of the image, and its location along the axis is the image location. The mirror equation and magnification equation can be derived geometrically from these two rays.
In their most basic form, telescopes use a set of curved mirrors to collect and focus light to increase the size of an object being viewed.
We have written many articles about concave mirrors for Universe Today. Here’s an article about the telescope mirror, and here’s an article about the astronomical telescope.
If you’d like more info on the Concave Mirror, check out NASA’s The Most Dreadful Weapon, and here’s a link to Build a Telescope Page.
We’ve also recorded an entire episode of Astronomy Cast all about the Telescope. Listen here, Episode 150: Telescopes, The Next Level.
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