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Exam level4 past questions

Optical instruments

Understand the working principles, magnification, resolving power, and design of devices like microscopes and telescopes, including their adjustments and measurement techniques.

Why this shows up in the exam

NEET often asks about the construction and functioning of microscopes and telescopes, and how they enhance vision.

How NEET tests this

Statement analysis · 2 QsNumericalAssertion–reason

Learn the idea

A telescope works because a long‑focal‑length objective makes a larger real image and a large aperture lets that image be bright and sharp. The three advantages – bigger image scale, more light and finer resolution – all come together in one objective lens.

🧠 Memory hook: Big lens = big view: Long focal length stretches the picture, big aperture brightens and sharpens it.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Angular magnification M = f_o / f_e
  • Resolving power improves with larger aperture (θ_min = 1.22 λ / D)
  • Light‑gathering power ∝ aperture area ∝ D²
  • For an astronomical telescope the object is at infinity, so the objective forms its image at its focal plane
  • A larger focal length gives a larger linear size of the image, increasing the effective magnification

How to approach it

  1. 1Identify whether the question asks for magnification, brightness or resolution
  2. 2Choose an objective with the longest practical focal length to enlarge the image
  3. 3Ensure the aperture is as large as possible to collect more light and to lower θ_min
  4. 4If numerical values are required, use M = f_o/f_e and θ_min = 1.22 λ/D

Worked example — watch it click

A lens of large focal length and large aperture is best suited as an objective of an astronomical telescope since:

  • A)A large aperture contributes to the quality and visibility of the images.
  • B)A large area of the objective ensures better light gathering power.
  • C)A large aperture provides a better resolution.
  • ✅All of the above.

The concept behind this problem

The example asks why a lens with both large focal length and large aperture is ideal for the objective; it checks that you know the three linked benefits – larger image scale, more light, and finer resolution.

Step by step

  1. 1For astronomical telescope objective: (a) Large aperture improves image quality and visibility by reducing diffraction effects.
  2. 2TRUE.
  3. 3(b) Large aperture (area) ensures better light-gathering power, crucial for observing faint celestial objects.
  4. 4TRUE.
  5. 5(c) Resolving power ∝ aperture diameter; larger aperture provides better resolution.
  6. 6TRUE.
  7. 7(d) Large focal length provides higher magnification (M = f_o/f_e).
  8. 8Combined with large aperture, this is ideal.
  9. 9All statements are correct.

Watch out

Students often pick only one benefit, forgetting that all three statements are simultaneously true.

Common slip-ups that cost marks

  • •Confusing “resolving power” with “angular resolution” – remember larger D means better (smaller) minimum angle
  • •Assuming a long focal length alone gives high resolution; aperture size is also essential
  • •Using the eyepiece focal length in the magnification formula incorrectly (it belongs in the denominator)

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 4NEET 2021

A lens of large focal length and large aperture is best suited as an objective of an astronomical telescope since:

Push further

More challenging

13 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 13

Assertion (A): Radio telescopes often have much larger dish diameters compared to optical telescopes, even though radio waves have much longer wavelengths. Reason (R): The resolving power of a telescope is inversely proportional to the wavelength of the radiation being observed.

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