Optics

Plane mirror

A plane mirror makes an upright, same-size virtual image as far behind as the object is in front.

Plane mirror — interactive Optics simulation. A plane mirror makes an upright, same-size virtual image as far behind as the object is in front. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.

Plane mirror

A plane mirror makes an upright, same-size virtual image as far behind as the object is in front.

Reflection at a flat surface follows the law of incidence. Image distance equals object distance behind the mirror, giving unity magnification. The image is virtual because reflected rays do not actually converge there—the brain extrapolates backward along dashed extensions.

m = +1
Image distance = object distance

Investigation brief

Plan the question before you open the lab

The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.

Driving question

An ordinary flat (plane) mirror is the everyday case of mirror imaging. As you move an object farther from a plane mirror, does its virtual image's size change?

Predictions to weigh

The image shrinks as the object moves farther away.
No — a plane mirror's image is always the same size as the object (magnification exactly 1), no matter the distance.
The image grows as the object moves farther away.

Variable roles

What you set:

Object distance d_o (cm)

What you measure:

Image distance d_i (cm)

How the investigation runs

Open the plane-mirror preset and press Reset.
Enable the image-distance readout.
Set the object distance for each trial and record the image distance.

Governing equation

Plane Mirror as f → ∞ — dᵢ = dₒ·f/(dₒ − f)

A plane mirror is the special case of the mirror equation with infinite focal length: 1/∞ = 1/d_o + 1/d_i forces d_i = −d_o exactly, with magnification always 1 — an unmagnified, upright virtual image.

What the printable worksheet asks students to work out

For one trial, confirm d_i = −d_o exactly (a plane mirror is the f → ∞ limit of the mirror equation). Compare to the table.
Explain why a plane mirror's image always sits exactly as far behind the mirror as the object sits in front of it, and why magnification is always exactly 1.

Where this shows up beyond the lab

A plane mirror can be thought of as a curved mirror with infinite focal length (perfectly flat = zero curvature). Explain, using 1/f = 1/d_o + 1/d_i with f → ∞, why this forces d_i = −d_o exactly.
As you walk toward a plane mirror, your image walks toward you at the same rate — the image distance always exactly matches your own distance from the mirror's surface. Why does this make plane mirrors uniquely simple compared to curved mirrors and lenses?

AP Physics 2 — Unit 13: Geometric Optics
General High School Physics — Light & optics
NGSS High School Physics — Wave properties
Middle School Physical Science — Waves meeting materials

Welcome to Plane Mirror
Select the object
Press Play
Image behind the mirror
Open the Properties panel
You did it!

Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.

Open Interactive Lab →

Download lab sheet →

The Mirror Equation