Plane Mirrors

A plane mirror is a flat, smooth reflecting surface. When light from an object falls on it, the rays bounce back according to the laws of reflection. When I extend these reflected rays backward, they appear to come from a point behind the mirror. That point is where the image is formed.

So the image is not actually behind the mirror — it only looks like the rays are coming from there. This is why the image is called a virtual image.

• A plane mirror always forms a virtual image (cannot be obtained on a screen).
• The image is upright (erect).
• The image is the same size as the object.
• The image is formed at the same distance behind the mirror as the object is in front.
• The left and right sides of the image are interchanged — this is called lateral inversion.

Whenever I draw a ray diagram for a plane mirror, I use these terms:

• Object: The source of light rays.
• Incident rays: Rays coming from the object towards the mirror.
• Reflected rays: Rays bouncing back from the mirror.
• Virtual image: The point behind the mirror from which the reflected rays seem to come.

When reflected rays diverge, my eye extends them backward in straight lines. These backward extensions meet at a point behind the mirror. The brain assumes that light always travels in straight lines, so it thinks the rays came from that point.

That point is the virtual image. This explains why the image is seen behind the mirror even though no light actually comes from there.

This is one of the most common and interesting effects of plane mirrors. The left side of the object appears on the right side of the image, and the right side appears on the left.

This happens because the image is formed behind the mirror by reversing the direction perpendicular to the mirror surface. The depth direction flips, and that causes the left–right switch.

The field of view is the region visible in the mirror. The size of the mirror affects how much of the surroundings I can see. A wider mirror gives a larger field of view.

When I move away from the mirror, more of the surroundings come into view because my eye subtends a wider angle at the mirror.

If two plane mirrors face each other, light reflects back and forth several times. This can create multiple images. The number and arrangement of these images depend on the angle between the mirrors.

When the two mirrors are parallel, an infinite sequence of images is formed, each fainter than the previous because some light is lost in each reflection.

Some simple places where I notice the effects of plane mirrors:

• The reflection in a bathroom mirror.
• Rear-view mirrors in vehicles (plane or slightly curved).
• Decorative panels using mirror tiles.
• Periscopes and simple optical setups.

Even though these are everyday situations, the same clear rules of image formation apply every time.