The Plane Mirror: A Special, Invariant Case

1 · Predict

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?

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

2 · Set Up

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

3 · Collect Data

Plot image distance d_i (y-axis) against object distance d_o (x-axis) for your three trials. Is the line straight through the origin with slope −1?

4 · Analyze

1. For one trial, confirm d_i = −d_o exactly (a plane mirror is the f → ∞ limit of the mirror equation). Compare to the table.
2. 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.

5 · Extend

1. 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.
2. 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?

The Physics Behind This Experiment

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