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## Physics - Mirrors

You do not need to know much about flat mirrors. You should, however, know about the two types of spherical mirrors: a concave mirror and a convex mirror. To understand each type, imagine a sphere. A concave mirror would be on the inside of the sphere, and a convex mirror would be on the outside of the sphere. An important thing to know about spherical mirrors is that each one has a focal length.

Rule number 46:
The focal length of a mirror equals one-half times the radius of curvature.

At the end of the focal length sits the mirror’s focal point. In a convex mirror, the focal point is behind or inside the mirror. In a concave mirror, the focal point is in front of the mirror.

You might be asked about the size and location of an object’s image in a spherical mirror. First of all, remember that a mirror image can be located behind the mirror or in front of a mirror. An image that is located behind the mirror is called a virtual image. An image that is located in front of a mirror is called a real image. If an image is located behind the mirror, we say that its distance from the mirror is negative. If an image is located on the front side of the mirror, we say its distance from the mirror is positive. This concept about positive and negative signs goes also for focal lengths and focal points.

Rule number 47:
Images in mirrors: One over focal length equals one over object’s distance from the mirror plus one over distance between the image and the mirror, or 1/f = 1/o +1/i.

Now, what is the size of the image?

Rule number 48:
Magnification and mirrors: Magnification equals the distance between the image and the mirror divided by the distance between the object and the mirror. Do not forget that any image that appears behind the mirror (A virtual image) is upright, but any image that appears in front of the mirror (A real image) is inverted.