Light Year, Astronomical Unit, Parsec Units-Class 11 Physics | Question-Answer, FAQS, Worksheet, MCQS, Test Paper

Complete Study Material for JEE, NEET, CBSE Board Class 11 Exams

Understanding vast distances in space requires specific units beyond conventional meters and kilometers. The three primary astronomical units used for measuring such distances are:

• Light Year (ly)
• Astronomical Unit (AU)
• Parsec (pc)

These units help astronomers express the enormous distances between celestial objects conveniently.

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Light Year (ly)

Definition:

A light year is the distance that light travels in a vacuum in one year.

Formula:

\begin{array}{l} 1 \text{ light year} = \text{Speed of light} \times \text{1 year} \end{array}

Since the speed of light is 3×10⁸ m/s, and 1 year is 365.25×24×60×60 seconds,

\begin{array}{l} 1 \text{ ly} = 3 \times 10^8 \times (365.25 \times 24 \times 60 \times 60) \end{array}

\begin{array}{l} 1 ly = 9.46 \times 10^{15} \text{ m} \end{array}

Significance:

• Used to measure distances to stars and galaxies.
• Helps in understanding the concept of how long light takes to reach Earth from distant celestial bodies.

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Astronomical Unit (AU)

Definition:

An astronomical unit (AU) is the average distance between the Earth and the Sun (center to center).

Value:

\begin{array}{l} 1 \text{ AU} = 1.496 \times 10^{11} \text{ m} \end{array}

Significance:

• Used to measure distances within our solar system.
• Helps in defining planetary orbits and distances from the Sun.

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Parsec (pc) – Parallactic Second

Definition:

A parsec is the distance at which an arc of 1 AU subtends an angle of 1 second of arc.

Formula:

\begin{array}{l} 1 \text{ parsec} = \frac{1 \text{ AU}}{1”} \end{array}

Since 1 arcsecond = π/180×60×60 radians,

\begin{array}{l} 1 \text{ pc} = \frac{1.496 \times 10^{11}}{\frac{\pi}{180 \times 60 \times 60}} \end{array}

\begin{array}{l} 1 pc = 3.08 \times 10^{16} \text{ m} \end{array}

Also,

\begin{array}{l} 1 \text{ parsec} = 3.26 \text{ light years} \end{array}

Significance:

• Used to measure distances to stars and galaxies beyond the solar system.
• A fundamental unit in stellar parallax method of distance measurement.

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Question-Answer Format for Exams (JEE, NEET, CBSE Class 11)

Q: Convert 5 parsecs into light years.

Ans:

\begin{array}{l} 1 \text{ parsec} = 3.26 \text{ light years} \end{array}

\begin{array}{l} 5 \text{ parsecs} = 5 \times 3.26 \end{array}

\begin{array}{l} = 16.3 \text{ light years} \end{array}

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Conceptual Questions with Answers

Q1: Why is the parsec a preferred unit in astronomy?

Ans: Parsec is derived from the method of stellar parallax, making it a natural unit for measuring large cosmic distances accurately.

Q2: Can the distance to a planet be measured in light years?

Ans: No, light years are too large for planetary distances; AU is more appropriate.

Q3: How does the astronomical unit help in measuring planetary distances?

Ans: The AU provides a standard reference for measuring distances within our solar system, allowing astronomers to compare planetary orbits.

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Test Your Knowledge(Quiz)

body { margin: 0; padding: 0; font-family: Arial, sans-serif; } .quiz-container { width: 60%; margin: 20px auto; padding: 20px; border: 1px solid #ccc; border-radius: 10px; } .question { font-size: 18px; font-weight: bold; } .options { margin-top: 10px; } .option { display: block; margin: 5px 0; } .result { margin-top: 20px; font-size: 20px; font-weight: bold; } .explanation { font-size: 14px; color: green; margin-top: 5px; } .btn { padding: 10px 15px; background: #007BFF; color: white; border: none; cursor: pointer; margin-top: 10px; } const quizData = [ { question: “What is 1 light year in meters?”, options: [“9.46 × 10¹⁵ m”, “1.496 × 10¹¹ m”, “3.08 × 10¹⁶ m”, “3.26 AU”], correct: 0, explanation: “A light year is defined as the distance light travels in one year, which is 9.46 × 10¹⁵ meters.” }, { question: “How many light years are in 1 parsec?”, options: [“1 AU”, “3.26 ly”, “206,265 AU”, “9.46 × 10¹⁵ m”], correct: 1, explanation: “By definition, 1 parsec is equal to 3.26 light years.” }, ]; function loadQuiz() { let quizHTML = “”; quizData.forEach((q, index) => { quizHTML += ``; q.options.forEach((opt, i) => { quizHTML += ` ${opt}`; }); }); document.getElementById(“quiz”).innerHTML = quizHTML; } function submitQuiz() { let score = 0; let output = “”; quizData.forEach((q, index) => { let selected = document.querySelector(`input[name=’q${index}’]:checked`); if (selected) { let answer = parseInt(selected.value); if (answer === q.correct) { score++; output += `

${index + 1}. ✅ Correct! ${q.explanation}

`; } else { output += `

${index + 1}. ❌ Incorrect. ${q.explanation}

`; } } else { output += `

${index + 1}. ❌ No answer selected.

`; } }); output += `

Your Score: ${score}/${quizData.length}

`; document.getElementById(“result”).innerHTML = output; } loadQuiz();

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Do You Know?

• The nearest star, Proxima Centauri, is 4.24 light years away from Earth.
• The Milky Way galaxy is 100,000 light years in diameter.
• 1 parsec is equivalent to 206,265 AU.

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Worksheet

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Test Paper -Total Marks: 20 (Marks Distribution Included)

Section A: Objective Questions (1 Mark Each)

Q.1 : 1 parsec is equal to how many astronomical units?

a) 3.26 AU
b) 206,265 AU
c) 1 AU
d) 9.46 x 10¹⁵ AU

Q.2 : What is the distance of 1 light year in meters?

a) 9.46 x 10¹⁵ m
b) 9.46 x 10¹¹ m
c) 9.46 x 10¹⁶ m
d) 9.46 x 10¹⁰ m

Section B: Short Answer Questions (2 Marks Each)

3. Explain the relationship between a parsec and a light year.
4. Define an astronomical unit and its significance.

Section C: Long Answer Questions (4 Marks Each)

5. Derive the expression for 1 parsec in terms of AU and meters.
6. Why is a light year not a unit of time?

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Important Points for Quick Revision

• 1 light year = 9.46 × 10¹⁵ m
• 1 AU = 1.496 × 10¹¹ m
• 1 parsec = 3.08 × 10¹⁶ m = 3.26 light years
• Used in measuring astronomical distances beyond our solar system.

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