A Concave Mirror: Real Images Like a Converging Lens

1 · Predict

A concave mirror focuses light like a converging lens. Does moving the object farther from a concave mirror move its real image closer to the mirror's focal point, the same way it does for a lens?

• No — a farther object moves the image farther from the mirror.
• Mirrors and lenses follow completely different imaging rules.
• Yes — the same 1/f = 1/d_o + 1/d_i relationship applies, so a farther object gives an image closer to the focal length.

2 · Set Up

1. Open the concave-real preset and press Reset. The mirror's focal length is fixed at 15 cm.
2. Enable the image-distance and magnification readouts.
3. Set the object distance for each trial and record the image distance and magnification.

3 · Collect Data

Plot image distance d_i (y-axis) against object distance d_o (x-axis) for your three trials.

4 · Analyze

1. For one trial, compute d_i from 1/f = 1/d_o + 1/d_i using f = 15 cm, then m = −d_i/d_o. Compare both to the table.
2. Explain why the mirror equation is mathematically identical in form to the thin-lens equation — both use the same 1/f = 1/d_o + 1/d_i relationship, even though a mirror reflects light rather than refracting it.

5 · Extend

1. A shaving or makeup mirror is concave, held closer than its focal length to give an enlarged virtual image (like the magnifier experiment). This experiment instead keeps the object beyond f, giving a real, inverted image. Explain when you'd want each configuration.
2. Every image in this experiment has negative magnification. Explain, using a concave mirror's actual reflecting geometry, why a real image formed by a concave mirror is inverted, just like a converging lens's real image.

The Physics Behind This Experiment

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