Spherical mirrors are widely used in various optical instruments such as telescopes, microscopes, and cameras. However, the aperture of a spherical mirror is often much smaller than its radius of curvature. In this article, we will explain why this is the case.


One of the main reasons for using a spherical mirror with a small aperture is to minimize optical aberrations. Optical aberrations are deviations from ideal imaging performance, which can result in blurred or distorted images. Spherical mirrors with large apertures are more prone to optical aberrations because they are more sensitive to misalignments and imperfections in the optical system.


One common type of optical aberration is spherical aberration, which occurs when light rays passing through different parts of a spherical mirror converge at different points. This results in a blurred image with a loss of sharpness and contrast. Spherical aberration is more pronounced in spherical mirrors with large apertures because the light rays passing through different parts of the mirror are more spread out.


By using a spherical mirror with a smaller aperture, the amount of spherical aberration can be reduced. This is because the light rays passing through the mirror are more collimated and converge at a more precise point. Additionally, smaller apertures allow for a more precise shaping of the mirror surface, which can further reduce spherical aberration.


Another benefit of using a spherical mirror with a small aperture is an increased depth of field. Depth of field refers to the range of distances in an image that appear sharp and in focus. A smaller aperture increases the depth of field by reducing the amount of light that enters the system and by increasing the distance over which the light is in focus.


In summary, the use of a spherical mirror with a small aperture is beneficial for reducing optical aberrations, such as spherical aberration, and increasing the depth of field. By minimizing aberrations and increasing the depth of field, spherical mirrors with small apertures can produce sharper and more precise images in a variety of optical applications.

The point of using an aperture is much smaller than radius of curvature of the mirror is to restrict the incoming light where rays almost come to a single focal point.