Image Formation in a Plane Mirror
Simple Explanation
A plane (flat) mirror always forms an image that is: virtual (light rays only appear to come from behind the mirror, they don't actually meet there), upright, the same size as the object, and exactly as far behind the mirror as the object is in front of it. The image is also laterally inverted — left and right appear swapped.
Why Do We Need It?
Understanding exactly how a plane mirror image behaves explains everyday experiences — like why text held up to a mirror looks backward, and why you appear to be exactly as far "inside" the mirror as you are standing in front of it.
See It
A diagram showing an object in front of a plane mirror, with its virtual image the same distance behind the mirror, connected by dashed lines representing the apparent light paths
Worked Example
Find the position of a plane-mirror image
A person stands 1.8 m in front of a large plane mirror. Find how far their image appears to be behind the mirror, and their distance from the image.
Why Does This Work?
This rule follows directly from the law of reflection applied to every ray leaving the object — tracing the reflected rays backward (since they only APPEAR to come from behind the mirror) always shows they meet at a point exactly as far behind the mirror as the object is in front.
Real-Life Example
Why text looks backward in a mirror
Holding a page of text up to a mirror makes the letters appear reversed left-to-right (though not upside down).
This is the lateral inversion of a plane-mirror image — front-to-back is swapped by the reflection, which appears to us as a left-right reversal.
Practice
An object is placed 0.6 m in front of a plane mirror. Find the total distance between the object and its image.
MediumWhich of these correctly describes a plane-mirror image?
MediumCommon mistake
Calling a plane-mirror image "real" — since the light rays only APPEAR to come from behind the mirror (and never actually pass through that point), the image is always virtual, never real.
Quick Review
- A plane-mirror image is: virtual, upright, same size, and laterally inverted.
- The image is exactly as far behind the mirror as the object is in front.
- The image can never be projected onto a screen, since it is virtual.