Astronomical telescope
"Astronomical telescope" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the NCERT section "Telescope" in Ray Optics and Optical Instruments. Parallel light from two distant stars through objective and eyepiece in normal adjustment, with the angle subtended before and after and the tube length. NEET has asked about this idea 3 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 Telescope in Ray Optics and Optical Instruments. Change the values and watch what happens.
What it shows
NCERT § Telescope
Parallel light from two distant stars through objective and eyepiece in normal adjustment, with the angle subtended before and after and the tube length.
Controls
4 to try
- Sliderobjective focal length 20–200 cm
- Slidereyepiece focal length 1–10 cm
- Sliderobjective aperture 2–20 cm
- Togglerefracting / Cassegrain reflecting
Try this
Build a telescope with magnifying power 25 and a tube no longer than 105 cm (e.g. 100 cm + 4 cm)
Common mistake
Thinking magnifying power is f_e/f_o or that a long-focus eyepiece helps.
In NEET
10 years of papers
Asked 3 times in 10 years of NEET.
See the past questions: Prisms, Dispersion and Optical Instruments, 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 § "Telescope"
- A refracting (astronomical) telescope has an objective of large focal length and large aperture and an eyepiece of short focal length; it gives angular magnification of distant objects.
- In normal adjustment the final image is at infinity, the magnifying power is m = f_o/f_e, and the tube length (separation of the lenses) is f_o + f_e.
- The final image of an astronomical telescope is inverted.
- A large objective aperture collects more light and improves resolving power, but large lenses are heavy, hard to make free of defects, and can be supported only at their edges.
- Reflecting telescopes use a concave mirror as the objective; this avoids chromatic aberration, and a mirror weighs less and can be supported over its whole back surface.
- In the Cassegrain design the light from the concave primary mirror is reflected by a small convex secondary mirror through a hole in the primary to the eyepiece or detector.
- For a telescope, magnification is about angles, not sizes: the image subtends a larger angle at the eye than the distant object does.
Questions
5 answers
- What does the "Astronomical telescope" simulation show?
- Parallel light from two distant stars through objective and eyepiece in normal adjustment, with the angle subtended before and after and the tube length.
- What mistake does the "Astronomical telescope" simulation fix?
- A common mistake it clears up: thinking magnifying power is f_e/f_o or that a long-focus eyepiece helps.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 3 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 "Telescope" in Ray Optics and Optical Instruments (Class 12 Physics).
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