A large mirror makes a room look larger because it creates an illusion of depth. It tricks the eye into thinking there is more space than there really is. The optical illusion occurs as the mirror reflects light and color across the room. Our eyes perceive the bouncing reflections as increased volume, making the room look bigger.
Physics can explain why a large mirror makes a room look larger. According to the law of reflection, an angle approaching a mirror will equal the angle that it reflects. Scientifically put, the angle of incidence equals the angle of reflection. A larger mirror has more surface area to allow for more angles of incidence. With more angles of incidence, there will be more angles of reflection—the mirror will reflect whatever amount that approaches it, so a larger mirror will reflect a bigger amount and make a room look larger.
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Light is the visible form of approaching and reflecting angles, since nothing can be seen without it. As light approaches the mirror at a specific frequency, the atoms in the mirror glass absorb it and shoot the light back at that same frequency in all directions. The waves of light shooting away from the mirror overlap with the light approaching it, and the non-overlapping portion is formed by the angle of reflection. This angle equals the angle of light that approached the mirror.
If more light is available to bounce off of the mirror, then more light will reflect across the room. Light will also bring in color to brighten up the space. It will make the room look larger because there is more for the eye to see. A large mirror will amplify this effect with its size.
Using a large mirror to make a room look larger is a useful interior decorating technique. It is effective to use a large mirror combined with as much indoor and outdoor lighting as possible to open up a small room. This allows the illusion to occur successfully during both daytime and nighttime. Placing a large mirror where it will reflect light from a window is an efficient way to utilize natural lighting during the day. At night, the mirror will reflect artificial light within the room itself.
There are alternative methods of using a large mirror to make a room look larger according to preference—a large mirror can decorate a room in many forms. For example, furniture with mirrored surfaces is a way to make a room look larger in place of a standard wall mirror.