Optics lab

Drag the object or the focal point right on the diagram (or use the sliders), switch between mirrors and lenses, and watch the two-ray construction and the image update live.

Lessons

Pick your class, read the steps, then drag the object or the sliders in the sandbox below to see it happen.

Class 10 — Concave mirrors: where the image forms

Concept: a concave mirror's image changes character depending on where the object sits relative to the focal length f — this is exactly what the mirror formula 1/v + 1/u = 1/f captures.

Switch to Concave mirror mode.
Set Focal length |f| to 10 cm and Object distance |u| to 30 cm (beyond the centre of curvature). Read the nature line — it should say real, inverted, and diminished, like a shaving mirror held far away.
Now drag Object distance down to 5 cm — closer than the focal length. The image should flip to virtual, erect and magnified.

Takeaway: a concave mirror gives a real image when the object is beyond f, and a magnified virtual image (like a makeup mirror) when it's inside f.

Class 10 — Convex mirrors: always virtual, always smaller

Concept: a convex mirror always forms a virtual, erect, diminished image, no matter the object distance — which is exactly why it's used for vehicle side mirrors (a wider field of view, safely).

Switch to Convex mirror mode.
Drag Object distance |u| across its full range, from very close to very far. Watch the nature line — it never changes from virtual/erect/diminished.

Takeaway: unlike a concave mirror, a convex mirror's image type never switches — that predictability is exactly why it's used where a driver needs a reliable, wide, if slightly shrunk, view.

Class 12 — Convex lenses and the lens formula

Concept: the lens formula 1/v − 1/u = 1/f behaves like the mirror formula's mirror image — a convex lens also switches between real and virtual images depending on whether the object is beyond or within the focal length.

Switch to Convex lens mode.
Set Focal length |f| to 10 cm, Object distance |u| to 25 cm. The image should be real and inverted — this is how a camera lens forms an image on a sensor.
Drag the object distance to 5 cm, inside the focal length. The image flips to virtual, erect and magnified — this is how a convex lens works as a magnifying glass.

Takeaway: the same lens is a camera lens or a magnifying glass, purely depending on how far the object is from it.

Class 12 — Concave lenses: always virtual, always diminished

Concept: like a convex mirror, a concave (diverging) lens always forms a virtual, erect, diminished image — this is the lens used to correct short-sightedness (myopia).

Switch to Concave lens mode.
Drag Object distance |u| across its full range and confirm the nature line stays virtual/erect/diminished throughout.

Takeaway: converging (convex) elements can flip between real and virtual images; diverging (concave) elements never do.

Drag the orange object arrow to change object distance, or the small F marker on the axis to change focal length.

Reading the diagram

Two rays locate the image: one leaves the object tip parallel to the axis and refracts/reflects through the focal point; the other goes straight through the pole (mirror) or optical centre (lens) undeviated. Where they meet is the image tip — solid lines are real light paths, dashed lines are backward extensions used only to construct a virtual image (nothing physically travels along the dashed part).

Sign convention matches NCERT: distances are measured from the pole/optical centre, the object is always to the left. A concave mirror and a concave lens both have a negative focal length in this convention (even though one converges light and the other diverges it) — that asymmetry is a property of the convention, not a mistake.

Idealisations: thin-lens/thin-mirror, paraxial (small-angle) rays, and a fixed object height chosen for visibility — the vertical scale is exaggerated relative to the horizontal (distance) scale, which is accurate to the sliders. Same spirit as Circuit lab's ideal battery and Mechanics's no-air-resistance free fall.

Part of Forces — see Learn for the class 10 light and class 12 ray optics topics this maps to.

Test yourself

Pick Class 10 or Class 12 in the physics quiz to test yourself on mirrors, lenses and ray optics — 10 random questions, with an explanation for every answer.

Take the physics quiz →