Optics

Spherical Mirrors: How are they made?

A spherical mirror is any reflecting surface in the form of a spherical cap.

To make a concave mirror, curve the reflecting surface of the mirror inward. This modifies several characteristics of the mirror and the image formed by it.

Features of concave mirrors.
1. The center of curvature (the center of the sphere to which the mirror surface belongs) which, in the plane mirror, was infinitely far away, is now closer and in front of the mirror.
2. The field of view, that is, the size of the scene seen by the observer, decreases in relation to the plane mirror.
3. The image distance increases from the plane mirror.
4. The height of the image increases in relation to the plane mirror. This is why many makeup mirrors are concave.

Features of convex mirrors.

To produce a convex mirror, we curve the surface of the mirror outwards. This causes the following changes to the mirror and the image it produces:

1. The center of curvature is now behind the mirror’s reflecting surface.
2. The field of view increases in relation to the flat mirror. That’s why this type of mirror is used in some car mirrors, as well as in stores, providing a wider viewing angle.
3. The image distance decreases with respect to the plane mirror.
4. The height of the image decreases in relation to the plane mirror.

Gaussian Sharpness Condition

The image of an object, formed by a spherical mirror, is not clear, because each point of the object corresponds to several points of the image.
Under certain conditions, spherical mirrors provide images whose lack of sharpness is not so perceived by the human eye, that is, spherical mirrors under these conditions are almost stigmatic.
These conditions are called Gaussian conditions:

• the mirror must have a small aperture angle (< 10º)
• the incident rays must be close to the principal axis;
• incident rays must be slightly inclined with respect to the main axis.

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