A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror.
When you focus on an object, a single point, your eyes are receiving light waves diverging from that point. This must be true for an object, or an image of an object, to be visible. To put it simply, if our eyes detect light waves diverging from a point, that point will be visible. As you will see, this is very important in terms of how mirrors work.
For plane (flat) mirrors, light is reflected according to the law of reflection. When the eyes receive these light waves, it looks as if the waves are diverging from behind the mirror, making it appear as if the object is behind the mirror as well. This type of image is called a virtual image, because light waves do not actually pass through that point, it only appears so. The distance between the object and the mirror is called the object distance and the distance between the virtual image and the mirror is the image distance. Notice that on plane mirrors, the object distance is equal to the image distance.
Shape of a mirror’s surface
In a plane mirror, a parallel beam of light changes its direction as a whole, while still remaining parallel; the images formed by a plane mirror are virtual images, of the same size as the original object (see mirror image).
- In a concave mirror, parallel beams of light becomes a convergent beam, whose rays intersect in the focus of the mirror.
- In a convex mirror, parallel beams become divergent, with the rays appearing to diverge from a common point of intersection “behind” the mirror.
- Spherical concave and convex mirrors do not focus parallel rays to a single point due to spherical aberration. However, the ideal of focusing to a point is a commonly-used approximation. Parabolic reflectors resolve this, allowing incoming parallel rays (for example, light from a distant star) to be focused to a small spot; almost an ideal point. Parabolic reflectors are not suitable for imaging nearby objects because the light rays are not parallel.