← Waves

Lenses and mirrors (every paper)

Draw the ray diagram FIRST — the image position, size, and orientation all come off it. Real/inverted/diminished (or the opposite three) is the standard answer shape.

The two rays that locate any image

A ray from the object PARALLEL to the axis refracts (or reflects) THROUGH the focal point.
A ray from the object THROUGH the focal point (or aimed at it) emerges PARALLEL to the axis.
Where the two rays cross (or appear to cross, extended backward) is the image.
Describe every image with THREE properties: real or virtual, upright or inverted, magnified or diminished.

Exam-style A candle in front of a convex lens forms an inverted, diminished image on a screen Explain how the image could be made larger while still forming on the screen. A real image forms on a screen only when the object sits OUTSIDE the focal length — the two constructed rays actually cross on the far side. Moving the object CLOSER to the focal point (but still outside it) moves the image further from the lens and makes it LARGER — a real image can still form on the screen. Moving the object inside the focal length would make the rays diverge instead of cross, producing a virtual image the screen cannot catch. ✓ Move the object closer to F (without crossing it) — the image grows and stays real, so it still projects onto the screen. ✕

Real image ≠ upright image

A real image (one a screen can catch, formed where light rays ACTUALLY cross) is usually inverted for a single converging lens or concave mirror with the object outside the focal length. Students often assume "real" implies "upright" — state all three properties independently, since none of them implies another.

A fast check before describing an image

Have I stated all THREE properties (real/virtual, upright/inverted, magnified/diminished), not just one or two?
Did I draw both construction rays before describing the image, rather than guessing its position?

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