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Comparing images: mirrors vs lenses, virtual vs real

A recurring Excellence task: compare the image produced by TWO different optical devices (or the same device under different conditions), naming genuine similarities AND differences.

How to structure a comparison of two images

• State each image's THREE properties separately first (real/virtual, upright/inverted, magnified/diminished) — you cannot compare what you have not each fully described.
• Identify genuine SIMILARITIES: e.g. both a concave mirror's and a convex mirror's virtual images are upright and formed "behind" the reflecting surface (where reflected rays appear to diverge from).
• Identify genuine DIFFERENCES: e.g. a concave mirror's virtual image (object inside the focal length) is MAGNIFIED, while a convex mirror's virtual image is always DIMINISHED, regardless of object position.
• Where relevant, explain WHY the difference occurs — a convex mirror always diverges reflected rays (so its image is always virtual, upright, diminished), while a concave mirror's behaviour depends on where the object sits relative to the focal point.

Worked example Comparing a concave mirror's virtual image with a convex mirror's A concave mirror forms a virtual image when the object is placed inside its focal length. Compare the similarities and differences with a virtual image formed by a convex mirror. SIMILARITY: both images are VIRTUAL (light rays only appear to diverge from behind the mirror, not actually cross there) and both are UPRIGHT. DIFFERENCE — MAGNIFICATION: the concave mirror's virtual image (object inside F) is MAGNIFIED, whereas a convex mirror's image is always DIMINISHED, no matter where the object is placed. DIFFERENCE — WHY: a convex mirror always diverges reflected rays regardless of object position, so it can ONLY ever produce a virtual, upright, diminished image — a concave mirror only produces a virtual image (and only when the object sits inside the focal length); outside the focal length it produces a real, inverted image instead. ✓ A complete comparison states real similarities AND real differences, and explains the difference in terms of how each mirror TYPE behaves — not just a list of adjectives for each image with no connecting explanation. ✕

Comparing images without stating all three properties for each first

A comparison that jumps straight to "they are different sizes" without first stating each image's full real/virtual, upright/inverted, magnified/diminished description leaves the comparison incomplete — always establish the full description of BOTH images before comparing them.

Property	Concave mirror (object inside F)	Convex mirror (any position)
Real or virtual	Virtual	Virtual
Upright or inverted	Upright	Upright
Magnified or diminished	Magnified	Diminished

Practice for this standard covers the whole topic — flashcards and quiz.