The Image Walk

Why it matters

A projector, a camera, a photocopier and a magnifying glass are the SAME convex lens — the only thing that changed is where the object stands.

The idea in plain words

The five cases every board exam asks. Tap each landmark.

u vs F and 2F decides everything

Object at 30 cm from a convex lens, f = 10 cm. Which case, and where is the image?

Real, inverted, diminished image at 15 cm (between F and 2F) ✓

• case 1: beyond 2F — image between F and 2F: real, inverted, SMALLER — the camera
• case 2: at 2F — image at 2F: real, inverted, SAME SIZE — the photocopier
• case 3: F to 2F — image beyond 2F: real, inverted, LARGER — the projector
• case 4: at F — rays emerge parallel: image at infinity — the searchlight
• case 5: inside F — image on the SAME side: virtual, upright, magnified — the magnifying glass
• 2F = 20 cm, so u = 30 cm is beyond 2F → camera case
• 1/v = 1/10 − 1/30 = 2/30 → v = 15 cm

Predict first

An object slides from far away TOWARDS the focus F of a convex lens. Its real image…

As u shrinks toward f, v stretches toward infinity — the image runs away and balloons. At F it escapes to infinity; inside F it flips to virtual.

• moves away from the lens and keeps growing — correct
• moves closer and shrinks
• stays where it is

What you do

Drag the object through all five regions — beyond 2F, at 2F, between F and 2F, at F, inside F — and watch the image walk, flip and vanish.

Check yourself

A projector throws a big picture on a wall. The film must sit…

Between F and 2F gives a REAL (catchable on a screen), inverted, magnified image — that's why the film goes in upside-down.

A real image differs from a virtual one because it…

Real images: rays truly converge (and the image is inverted). Virtual: rays only appear to come from there.

With the object exactly at F, the image forms…

u = f makes 1/v = 0. This 'broken' case powers searchlights and lighthouse beams.

• just outside F (between F and 2F) — correct
• inside F
• beyond 2F
• can be caught on a screen — rays actually meet there — correct
• is always bigger
• is always upright
• at infinity — the rays leave parallel — correct
• at F on the other side
• on the object itself