Two thin lenses
"Two thin lenses" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the NCERT section "Power of a lens and combinations of thin lenses" in Ray Optics and Optical Instruments. Two thin lenses with a chosen gap: the first image becomes the second lens's object, traced by rays, with the in-contact equivalent power. NEET has asked about this idea 5 times in 10 years of papers. JEE Main asked 66 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson Power of a lens and combinations of thin lenses in Ray Optics and Optical Instruments. Change the values and watch what happens.
What it shows
NCERT § Power of a Lens
Two thin lenses with a chosen gap: the first image becomes the second lens's object, traced by rays, with the in-contact equivalent power.
Controls
4 to try
- Sliderf₁ −50 to +50 cm
- Sliderf₂ −50 to +50 cm
- Sliderseparation 0–60 cm
- Dragobject position
Try this
Put a +20 cm and a −40 cm lens in contact and read +2.5 D; then separate them so a distant object's final image lands 30 cm behind the second lens (≈2.9 cm gap)
Common mistake
Adding focal lengths instead of powers; forgetting the negative sign of a diverging lens.
In NEET
10 years of papers
Asked 5 times in 10 years of NEET.
See the past questions: Thin Lens Formula, Ray Optics and Optical Instruments (chapter)
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 § "Power of a Lens"
- The power of a lens is the reciprocal of its focal length in metres, P = 1/f. Its SI unit is the dioptre (D), 1 D = 1 m⁻¹.
- A converging lens has positive power; a diverging lens has negative power.
- Two thin lenses in contact behave as one lens with 1/f = 1/f₁ + 1/f₂, so powers add: P = P₁ + P₂ + ….
- The total magnification of a combination is the product of the individual magnifications: m = m₁m₂m₃….
- Lenses are combined in instruments to raise magnification and to reduce certain aberrations; a combination also lets designers make lenses of the required power.
- For lenses separated by a distance, find the image formed by the first lens and use it as the object (real or virtual) for the next lens, measuring each distance from that lens.
Questions
5 answers
- What does the "Two thin lenses" simulation show?
- Two thin lenses with a chosen gap: the first image becomes the second lens's object, traced by rays, with the in-contact equivalent power.
- What mistake does the "Two thin lenses" simulation fix?
- A common mistake it clears up: adding focal lengths instead of powers; forgetting the negative sign of a diverging lens.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 5 times in 10 years of papers.
- 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 "Power of a lens and combinations of thin lenses" in Ray Optics and Optical Instruments (Class 12 Physics).
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