Image formation by Plain Mirror

Images in a plane mirror are the same size as the object, located behind the mirror, and oriented in the same direction.

To understand how this happens, consider the figure below. Two rays emerge from point P, strike the mirror, and reflect into the observer’s eye.

If the reflected rays are extended backward behind the mirror (see dashed lines in the figure below), they seem to originate from point $\mathit{Q}$. This is where the image of point $\mathit{P}$ is located. If we repeat this process for point $\mathit{P}\text{'}$, we obtain its image at point $\mathit{Q}\text{'}$.

Locating an Image in a Plane Mirror

The law of reflection states that the angle of incidence equals the angle of reflection. In the above figure, triangles $\mathit{PAB}$ and $\mathit{QAB}$ are congruent due to this law. Thus, the distance PB from the object to the mirror equals the distance $\mathit{BQ}$ from the mirror to the image. In general, the object distance (denoted as ${\mathit{d}}_0$) is the distance from the mirror to the object, and the image distance (denoted as ${\mathit{d}}_{\mathit{i}}$) is the distance from the mirror to the image.

Multiple Images

If an object is situated in front of two mirrors, you may see images in both mirrors. In addition, the image in the first mirror may act as an object for the second mirror, so the second mirror may form an image of the image. If the mirrors are placed parallel to each other and the object is placed at a point other than the midpoint between them, then this process of image-of-an-image continues without end, as you may have noticed when standing in a hallway with mirrors on each side. This is shown in the figure below, which shows three images produced by the blue object. Notice that each reflection reverses front and back, just like pulling a right-hand glove inside out produces a left-hand glove (this is why a reflection of your right hand is a left hand). Thus, the fronts and backs of image $1$ and $2$ are both inverted with respect to the object, and the front and back of image $3$ is inverted with respect to image $2$, which is the object for image $3$.

Infinite reflections may terminate. For instance, two mirrors at right angles form three images, as shown in part (a) of the figure below. Images $1$ and $2$ result from rays that reflect from only a single mirror, but image $1$,$2$ is formed by rays that reflect from both mirrors. This is shown in the ray-tracing diagram in part (b) of the figure below. To find images $1$ and $2$, you have to look behind the corner of the two mirrors.