Reflection of Light and Image Formation

Light always follows the law of reflection, whether the reflection occurs off a curved surface or off a flat surface. The task of determining the direction in which an incident light ray would reflect involves determining the normal to the surface at the point of incidence. For a concave mirror, the normal at the point of incidence on the mirror surface is a line which extends through the center of curvature. Once the normal is drawn the angle of incidence can be measured and the reflected ray can be drawn with the same angle. This process is illustrated with two separate incident rays in the diagram at the right. Concave mirrors are capable of producing real images (as well as virtual images). When a real image is formed, it still appears to an observer as though light is diverging from the real image location.

A light bulb placed in front of a concave mirror at a location somewhere behind the center of curvature (C), will emit light in a variety of directions, some of which will strike the mirror. Each individual ray of light which strikes the mirror will reflect according to the law of reflection. Upon reflecting, the lightwill converge at a point. At the point where the light from the object converges, a replica, likeness or reproduction of the actual object is created. This replica is known as the image. Once the reflected light rays reach the image location, theybegin to diverge. The point where all the reflected light rays converge is known as the image point. Not only is it the point where light rays converge, it is also the point where reflected light rays appear to an observer to be diverging from. Regardless of the observer's location, the observer will see a ray of light passing through the real image location.

If the light bulb is located at a different location, the same principles apply. The image location is the location where reflected light appears to diverge from. By determining the path which light from the bulb takes after reflecting from the mirror, the image location can be identified. The diagram below depicts this concept.

You might notice that while the same principle applies for determining the image location, a different result is obtained. When the object is located beyond the center of curvature (C), the image is located between the center of curvature (C) and the focal point (F). On the other hand, when the object is located between the center of curvature (C) and the focal point (F), the image is located beyond the center of curvature (C). Unlike plane mirrors, the object distance is not necessarily equal to the image distance. The actual relationship between object distance and image distance is dependent upon the location of the object.