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Mirrors - Definition, Types, Laws, Reflection and Formula

A mirror is an object that reflects light in a uniform manner. Due to this, we can see a clear and crisp image of any object that is placed in front of it. Mirrors work by changing the direction of light beams that fall on them. The angle of incidence and angle of reflection is the same for a regular plain mirror. Mirrors are useful in a variety of situations. Humans have been looking at their image formed in a mirror for thousands of years. Clever placement of mirrors also allows us to see objects that are not visible from a certain angle. For example, we can see around the corner of a street by means of a mirror placed at the junction of two streets.

In earlier times, mirrors were made from highly polished stones like obsidian until humans discovered the element silica that is found in sand. This led to the making of highly effective reflecting surfaces.

Modern mirrors are made with a coating of silver or aluminium due to their high reflectivity.

Types of Mirrors

Today, several different kinds of mirrors are manufactured to cater to the different needs of the users. Some parameters on which mirrors can be classified are shape, reflective material, manufacturing method and application.

By shape, the mirrors can be distinguished into the categories of planar, concave or convex. These mirrors fall into the category of spherical mirrors. This means that the surface of such mirrors is part of the surface of a sphere. Concave mirrors are formed by polishing the inner surface of a spherical mirror. Concave mirrors have the property of converging the light beams that fall onto their surface. Thus, they can be used to concentrate light at a particular spot. They are also used in satellite communication to direct electromagnetic signals.

Convex mirrors are formed by polishing the outer surface of a sphere. These mirrors have the property of diverging the light beams that fall onto their surface. The image formed by such mirrors covers a large field of view, and therefore, they are used as rearview mirrors.

Reflection

Reflection is the characteristic property of a mirror. Reflection happens when light falling on any surface is returned by the surface without a significant loss of energy. If the surface absorbs all the energy of the light wave, then reflection cannot take place. An example of an object that absorbs all the light that falls on it is a black body. Ideal black bodies do not actually exist in nature because most of the surfaces reflect light in some form or the other. This is the reason we are able to see those objects. The only example of a near-perfect black body in the universe is a black hole. Black holes are entities where matter collapses to form a region of a monstrously large gravitational field. Nothing escapes from a black hole, not even light.

Reflection is also seen on the surface of a transparent medium like water. To observe images formed by such surfaces, the angle of view must be very steep as measured from the surface. That is why we can see the reflection of objects on the surface of a lake. But as we move our eyes from a far-away point to a point near us, the reflections cease to be observable.

The reflections produced by such transparent media are weak in intensity compared to a specular mirror because most light waves pass through the media and are refracted. Refraction is the change in the path of light that occurs because of different optical densities between two media.

Laws of Reflection

Laws of reflections govern the behaviour of electromagnetic waves when they interact with any reflective surface. For a surface to become specularly reflective, the surface must be very smooth. This means, in practice, the surface irregularities found on the surface of the material must be very small compared to the incident light's wavelength. If the surface irregularities are large, then reflection does not take place. In that case, the phenomenon is termed ‘diffusion’. Diffusion occurs when the light beams reflected from a surface are scattered in several different directions. This stops any image from being formed on the surface.

The laws of reflection are:

  • The incident ray, the reflected ray and the normal lie in the same plane.
  • Angles made by the incident ray and the reflected ray with the normal are equal to each other.
  • The incident ray and the reflected ray are always on the opposite side of the normal.
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