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?

• 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.

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?

← Back to experiment

138 Free Printable Physics Lab Worksheets