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. (Refraction, prisms and the lens-maker's formula have their own Refraction & prisms lab.)

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.

The physics behind it

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.

Formulas used in this lab

Spherical mirrors Class 10 & 12

Mirror formula: 1/v + 1/u = 1/f
Focal length and radius of curvature: f = R/2
Magnification: m = h′/h = −v/u
Sign convention (NCERT): distances from the pole, positive in the direction of the incident light; a concave mirror has f < 0, a convex mirror f > 0
Real object: u is always negative
Reading m: |m| > 1 enlarged, |m| < 1 diminished; m > 0 erect (virtual), m < 0 inverted (real)

Thin lenses Class 10 & 12

Lens formula: 1/v − 1/u = 1/f
Magnification: m = h′/h = v/u
Power of a lens: P = 1/f (f in metres, P in dioptres)
Sign convention (NCERT): a convex lens has f > 0, a concave lens f < 0; a real object has u < 0
Concave lens: always forms a virtual, erect, diminished image between the lens and F

Rays used to locate the image Class 10

Ray parallel to the axis: passes through the principal focus F after reflection or refraction (or appears to come from F)
Ray through the focus: emerges parallel to the principal axis
Mirror: ray through the centre of curvature C: returns along its own path
Lens: ray through the optical centre: passes straight through, undeviated