Thin lens: the same drag, a different formula
"Thin lens: the same drag, a different formula" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the lesson "Thin lenses: lens maker's formula and thin lens formula" in Ray Optics and Optical Instruments. The same three rays, but now the light goes through. A real image forms on the far side of the lens. Move the object close to the lens, then try a concave lens. Lens: 1/v − 1/u = 1/f and m = v/u. Mirror: 1/v + 1/u = 1/f and m = −v/u. For a lens a positive v is a real image on the far side. A concave lens, like a convex mirror, only ever gives a virtual, erect, diminished image of a real object. JEE Main asked 66 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Thin lenses: lens maker's formula and thin lens formula in Ray Optics and Optical Instruments. Change the values and watch what happens.
In JEE Main
42 shifts analysed
66 questions from Ray Optics and Optical Instruments across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT § "Refraction by a Lens"
- Lens maker's formula: 1/f = (n₂₁ − 1)(1/R₁ − 1/R₂), where n₂₁ is the refractive index of the lens material relative to the surrounding medium.
- For a biconvex lens with light from the left, R₁ is positive and R₂ is negative; for a biconcave lens, R₁ is negative and R₂ is positive.
- A convex (converging) lens has positive f and a concave (diverging) lens has negative f in this convention.
- Thin lens formula: 1/v − 1/u = 1/f. Note the minus sign, unlike the mirror equation.
- Linear magnification of a lens is m = h′/h = v/u, again without the minus sign used for mirrors.
- When a lens is placed in a liquid, n₂₁ becomes n_lens/n_liquid, so the focal length changes. If the liquid's n equals the lens's n, the lens has no effect (f becomes infinite); if the liquid's n is larger, a convex lens behaves as a diverging lens.
- A convex lens gives a real inverted image for objects beyond F and a virtual erect enlarged image for objects between F and the lens; a concave lens always gives a virtual, erect, diminished image of a real object.
Questions
3 answers
- What does the "Thin lens: the same drag, a different formula" simulation show?
- The same three rays, but now the light goes through. A real image forms on the far side of the lens. Move the object close to the lens, then try a concave lens.
- Does JEE Main ask questions from Ray Optics and Optical Instruments?
- Yes. JEE Main asked 66 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Thin lenses: lens maker's formula and thin lens formula" in Ray Optics and Optical Instruments (Class 12 Physics).
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