A Convex Mirror: Always a Reduced Virtual Image
1 · Predict
A convex (bulging outward) mirror has a negative focal length by convention, the same way a diverging lens does. Does it ever form a real, enlarged image?
- No — like a diverging lens, a convex mirror always forms a reduced, upright virtual image, regardless of object distance.
- It can form a real image if the object is close enough.
- It always forms an enlarged image.
2 · Set Up
- Open the convex-virtual preset and press Reset. The mirror's focal length is fixed at −15 cm.
- Enable the image-distance and magnification readouts.
- Set the object distance for each trial and record the image distance and magnification.
3 · Collect Data
| Object distance d_o (cm) | Image distance d_i (cm) | Magnification m |
|---|---|---|
| 30 | ||
| 60 | ||
| 90 |
Plot magnification m (y-axis) against object distance d_o (x-axis) for your three trials. Does m ever exceed 1?
4 · Analyze
- 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.
- Explain why a convex mirror's negative focal length guarantees a virtual, reduced image at every object distance — mirroring (pun intended) the diverging-lens experiment's behavior exactly.
5 · Extend
- Convex mirrors are used for store security mirrors and passenger-side car mirrors ('objects in mirror are closer than they appear') precisely because they show a wide field of view at reduced size. Explain, using your magnification data, why that wording on car mirrors is necessary.
- Compare this experiment's magnification values to the concave-real experiment's, at similar object distances. Explain why a convex mirror's reduced image is fundamentally different from a concave mirror's enlarged (or inverted) one.
The Physics Behind This Experiment
Convex Mirror Imaging
A convex (diverging) mirror has negative focal length by convention. Substituting f < 0 into the mirror equation always produces a virtual image with 0 < m < 1, exactly like a diverging lens.