Plane Mirrors

Have you ever wondered where exactly at the back of a mirror is your image located or wonder why the image you see in the mirror looked exactly as you? Why when you raise your right hand while facing the mirror, your image is raising the left hand? This type of mirror that we usually use is called plane mirror. Plane mirror has flat reflective surface. It can reflect a beam  of light in only one direction.

Image can be real or virtual. When all light rays pass through and separate from the image point, a real image is formed. A real image can be projected on  screen. When most if not all light rays do not pass through the image point but only appear to separate from the image point, a virtual image is formed. This type of image cannot be projected on screen.

Let’s see how light travels and reflects on a plane mirror to analyze and determine what type of image appears in a plane mirror.

In a plane mirror, the characteristics of an image are:

  1. the same size as the object or the magnification is 1, m=1
  2. the distance of the image, s_i is equal to the distance of the object from the mirror, s_o;
  3. upright or virtual; and
  4. laterally inverted

But how do the image form and where? To illustrate the formation of image in a plane mirror, we use a ray diagram. It is a diagram illustrating the path of light after it strikes an object and how it is reflected on the mirror.

To show the image formation in a plane mirror through a ray diagram, we use an arrow as the light from the object traveling to the mirror which reflects from the mirror to the eye. Then, we extend the reflected rays of light through the mirror using broken lines to show where the image is located and how it looks like.

Another characteristic of the image formed in a plane mirror is the lateral inversion. This means that the image is reversed along the normal in the mirror and is located at the same distance perpendicularly behind the mirror as the object is in front of it.


Example:

Locate the image of the object which is in front of the mirror as shown in the figure below.

Steps:

  1. Draw a perpendicular line from the object to the mirror.
  2. Extend the line using broken lines to a point or distance from the mirror which is the same as the distance from the object to the mirror.
  3. Draw a ray of light from the object to the mirror.
  4. The mirror reflects the light represented by the reflected ray which is moving towards the direction of the eye.
  5. Extend the reflected rays using broken line to the point where the image is located.
  6. Draw the image.