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NEET 2021 Physics Question 5 Solution

Topic: Ray Optics and Optical Instruments · Official NTA Booklet Code M1

Question

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

(1)a large aperture contributes to the quality and visibility of the images.
(2)a large area of the objective ensures better light gathering power.
(3)a large aperture provides a better resolution.
(4)all of the above.

✓

Verified Against NTA Final Answer Key

Option (4) : all of the above.

📚 Ray Optics and Optical Instruments Easy

📋 Given Parameters & Problem Setup

Optical Instrument Astronomical Refracting Telescope

Objective Lens Properties Large focal length and large aperture (diameter D)

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The 30-Second Exam Hall Shortcut: ⚡ The "All of the Above" Telescope Triad

⏱️ Save ~20 seconds

A large objective aperture $D$ provides: (1) Higher light-gathering power ( $A \propto D^2$ ), giving bright visible images of faint stars; (2) Higher resolving power ( $\text{RP} = \frac{D}{1.22\lambda}$ ), resolving close stars; and (3) Superior image quality and contrast. All three options are valid! Match Option (4)!

1 Check light gathering power: Area

A = \pi D^2 / 4 \propto D^2

(Option 2 is correct).

2 Check resolution: Limit of resolution

\Delta\theta = 1.22\lambda / D \implies

Resolving power

\propto D

(Option 3 is correct).

3 Check image quality/visibility: Brighter and better-resolved images provide superior clarity (Option 1 is correct).

4 Therefore, "all of the above" is the correct choice.

5 Match Option (4).

🛡️

Why You Can 100% Trust This Shortcut: Canonical wave optics diffraction limit (Rayleigh criterion) and geometric light collection physics.

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Step-by-Step Rigorous Workflow & Mathematical Proof

Step 1

Light Gathering Power of Large Aperture

Astronomical telescopes observe extremely distant, faint celestial bodies. The amount of light gathered from a distant star is directly proportional to the surface area of the objective lens:

\text{Light Gathering Power} \propto \text{Area} = \frac{\pi D^2}{4} \propto D^2

Step 2

Resolving Power and Rayleigh Criterion

Due to diffraction at the circular aperture of diameter $D$ , the angular limit of resolution is $\Delta\theta = \frac{1.22\lambda}{D}$ . The resolving power ( $RP$ ) is the reciprocal of $\Delta\theta$ :

RP = \frac{1}{\Delta\theta} = \frac{D}{1.22\lambda} \implies \text{Larger } D \implies \text{Better Resolution}

Step 3

Image Quality and Visibility

Because a larger aperture collects more light photons and reduces diffraction blurring, the resultant image has superior brightness, sharpness, and visibility. Hence, all statements (1), (2), and (3) are scientifically true.

⭐

360° Option Elimination Engine Key Feature

Diagnose every choice: Know exactly why the right option works and why distractors fail.

4-Option Deep Scan

1 1

✗ Incorrect

Verdict & Scientific Reason:

True statement, but incomplete because statements (2) and (3) are also true.

⚠️

Student Fallacy (Why candidates pick this trap): Selecting the first true statement without reading the remaining choices.

2 2

✗ Incorrect

Verdict & Scientific Reason:

True statement, but incomplete on its own.

⚠️

Student Fallacy (Why candidates pick this trap): Overlooking the resolution benefits of large apertures.

3 3

✗ Incorrect

Verdict & Scientific Reason:

True statement, but incomplete on its own.

⚠️

Student Fallacy (Why candidates pick this trap): Overlooking the light gathering power benefit.

4 4

✓ 100% Correct

Verdict & Scientific Reason:

All three statements (1), (2), and (3) are genuine, verified optical advantages of large aperture telescope objectives.

💡

Why this is airtight: None; comprehensive optical instrument analysis.

⚠️

NTA Trap Alert: Premature Option Selection Trap

Students read statement (1) or (2), realize it is correct, and immediately bubble it without checking whether "all of the above" is present as Option (4).

🛡️ How to avoid the -1 mark: Whenever "all of the above" is an option, verify whether two or more statements are true before bubbling.

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Core NEET Syllabus Standard

NCERT Physics · Ray Optics and Optical Instruments Standard Benchmark

Core Scientific Principle Tested:

"The objective of an astronomical telescope has a large focal length and large aperture to collect more light from faint stars and provide high resolving power ( $D/1.22\lambda$ )."

Syllabus Focus: Telescope

✓ Core NEET UG Syllabus Anchor Syllabus & Academic Standards ↗

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NEET 2021 Physics Question 5 Solution

Question

The 30-Second Exam Hall Shortcut: ⚡ The "All of the Above" Telescope Triad

Step-by-Step Rigorous Workflow & Mathematical Proof

Light Gathering Power of Large Aperture

Resolving Power and Rayleigh Criterion

Image Quality and Visibility

360° Option Elimination Engine Key Feature

NTA Trap Alert: Premature Option Selection Trap

Core NEET Syllabus Standard

Ready to attempt under real exam pressure?

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