How Do Mirrors Magnify an Image?

Written by laurel brown
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How Do Mirrors Magnify an Image?
Ray diagram for an object between the centre of curvature (C) and the focal point (F)

Most mirrors that we deal with on a regular basis reflect images that are the same size, as the original object. The size of the mirror image depends on merely the object's distance from the reflecting surface. There are, however, some mirrors that change the size of the reflected image as a result of their optics.

There are three basic classes of mirrors: plane, convex and concave. Plane mirrors have a flat reflective surface, and are the type of mirrors most familiar in our daily lives. Convex mirrors have a reflective surface that curves outward and reflects images that are smaller than the original objects. The final type of mirror is the concave mirror. Concave mirrors also are curved, but the reflective surface bends inward to make the mirror appear to have a shape like the inside of a bowl. This surface shape allows for reflections to be magnified or diminished, inverted or right-side up.

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Important Terms

The principal axis of a concave mirror is a straight line that runs perpendicular to the very centre of the reflective surface. The focal point of a concave mirror is that point where light rays that travel toward the mirror, parallel to the principal axis, reflect through. The light focuses at this point as a result. The centre of curvature of the mirror is the point that would be the centre of the sphere if the mirror were entirely spherical and not just a segment of a sphere. Incident rays are rays of light travelling to the mirror, while reflected rays are rays of light bouncing off the mirror surface.

Concave Mirror Reflection

Incident light rays that pass through the focal point reflect parallel to the principal axis. Light rays that travel to the mirror on paths parallel to the principal axis produce reflected rays that pass through the focal point. Using these two rules, it is possible to construct ray diagrams to find the size of the reflected image.

You can use the ray diagram to find out what the reflected image will look like. Draw two light rays from a vertical line representing your object: one incident ray is parallel to the principal axis and one travels through the focal point. Next draw the reflected rays: The incident ray parallel to the principal axis reflects through the focal point and the incident ray through the focal point reflects parallel to the principal axis.

The place where the two reflected rays intersect shows the location and size of the reflected image.

How Do Mirrors Magnify an Image?
Ray diagram for an object between the centre of curvature (C) and the focal point (F)

Magnification

If your original object lies between the centre of curvature and the surface of a concave mirror, it will be magnified. The image also will be upright if the object is between the focal point and the mirror. If the object is between the focal point and the centre of curvature, the image will be inverted. Objects located beyond the centre of curvature create reflected images smaller than the original.

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