A mirror that cannot reflect is called a “non-reflective mirror”. However, this term is somewhat of an oxymoron since the definition of a mirror is an object that reflects light. So, a non-reflective mirror is not really a mirror at all.

A non-reflective mirror, also known as an anti-reflective mirror, is an optical surface that has been specially designed to minimize the amount of light reflected from its surface. The primary goal of a non-reflective mirror is to reduce the amount of glare and unwanted reflections that can interfere with the clarity and contrast of an image.

Non-reflective mirrors are commonly used in a variety of applications where image quality is critical. For example, they are often used in microscopes, telescopes, and other optical instruments to improve image contrast and clarity. They are also used in photography, where they help to reduce unwanted glare and reflections.

There are several methods used to create non-reflective mirrors. One common method is to apply a thin coating of a material with a low refractive index, such as magnesium fluoride, onto the surface of the mirror. This coating helps to reduce the amount of light reflected from the surface by minimizing the difference in refractive index between the air and the mirror’s surface.

Another method is to use a series of thin layers with varying refractive indices to create a so-called “interference coating”. This type of coating can be engineered to reflect a very specific wavelength of light, while transmitting other wavelengths. The result is a mirror that appears to be transparent at certain angles, but reflects light at other angles.

Overall, non-reflective mirrors are an important tool for improving image quality and reducing unwanted reflections in a variety of optical systems.