Optics bench प्रकाशिकी बेंच
Pick a lens or mirror, then drag the object or the focus point F. Predict the image first, then reveal the ray diagram and check it with the formula.
Why does this happen? ऐसा क्यों होता है?
Three rays are enough
Light leaves every point of the object in all directions. To find where the top of the image forms, we only need two rays whose paths we know: a ray parallel to the axis goes through (or seems to come from) the focus; a ray through the optical centre goes straight on; a ray through the focus comes out parallel. Where they meet is the image. If they only seem to meet when traced backwards (dashed), the image is virtual and cannot be caught on a screen.
Sign convention (NCERT)
Measure every distance from the optical centre O. Distances in the direction of the incoming light (to the right) are positive, so the object distance u is always negative. Heights above the axis are positive. A negative m means an inverted image. Convex lens and convex mirror have f > 0; concave lens and concave mirror have f < 0.
Where you see it
Magnifying glass, camera, projector, spectacles for hypermetropia (long-sightedness) and the lens in your eye.
| Object at | u (cm) | Image at | Nature |
|---|---|---|---|
| Beyond 2F | −28.0 | Between F₂ and 2F₂ | Real, inverted, diminished |
| At 2F | −20.0 | At 2F₂ | Real, inverted, same size |
| Between F and 2F | −15.0 | Beyond 2F₂ | Real, inverted, magnified |
| At F | −10.0 | At infinity | Real, inverted, highly magnified |
| Inside F | −6.0 | On the same side as the object | Virtual, erect, magnified |
Observation table प्रेक्षण तालिका
Add readings for different object distances, then download a ready practical-file page. Readings stay on this page only.
| S. No. | Element | u (cm) | v (cm) | 1/u | 1/v | f (cm) | m |
|---|---|---|---|---|---|---|---|
| Reveal an image and tap “Add to observation table”. | |||||||
Common questions
What is the lens formula and the mirror formula?
For a thin lens, 1/v − 1/u = 1/f and magnification m = v/u. For a spherical mirror, 1/v + 1/u = 1/f and m = −v/u. All distances are measured from the optical centre or pole using the New Cartesian sign convention.
Where should the object be placed to get a real image of the same size?
At 2F₁ (twice the focal length) in front of a convex lens, or at C (the centre of curvature) in front of a concave mirror. The image forms at 2F₂ or at C and is real, inverted and the same size.
Why is the object distance u always negative?
In the NCERT sign convention, light travels from left to right and the object is placed on the left. Distances measured against the direction of incident light are negative, so u is negative for a real object.
Which lens or mirror always forms a virtual, erect and diminished image?
A concave lens and a convex mirror, wherever the object is placed. That is why a convex mirror is used as a rear-view mirror.