Consider the following statements about Light year: 1. Light year is a unit for measurement of very large distances. 2. Light year is a unit for measurement of very large time intervals. 3. Light year is a unit for measurement of intensity of light. Which of the statements given above is/are correct?
1 only
Understanding the Light Year Unit
The question asks us to consider several statements about the Light year and determine which ones are correct. Let's examine each statement carefully based on the definition of a light year.
What is a Light Year?
A Light year is a unit used in astronomy to measure very large distances. It is defined as the distance that light travels in vacuum in one Julian year (365.25 days).
The speed of light in a vacuum is approximately \(299,792,458\) meters per second (\(m/s\)).
The distance covered by light in one year can be calculated as:
Distance = Speed \(\times\) Time
First, let's convert one Julian year into seconds:
- \(1\) Julian year = \(365.25\) days
- \(1\) day = \(24\) hours
- \(1\) hour = \(60\) minutes
- \(1\) minute = \(60\) seconds
- Time in seconds = \(365.25 \times 24 \times 60 \times 60\) seconds \(\approx 31,557,600\) seconds
Now, calculate the distance:
Distance = \(299,792,458 \, m/s \times 31,557,600 \, s\)
This calculation gives a distance of approximately \(9.461 \times 10^{15}\) meters, or about \(9.461\) trillion kilometers. This enormous distance is what constitutes one Light year.
This clearly shows that a Light year is a measure of distance.
Analyzing the Statements about Light year
Let's evaluate each statement provided in the question:
- Light year is a unit for measurement of very large distances.
- Light year is a unit for measurement of very large time intervals.
- Light year is a unit for measurement of intensity of light.
Statement 1: Light year is a unit for measurement of very large distances.
Based on our definition and calculation, a Light year represents the distance light travels in a year. This distance is extremely large (\(\approx 9.461 \times 10^{15}\) meters). Therefore, this statement is correct. The Light year is indeed used to measure vast distances between stars, galaxies, and other celestial objects in astronomy.
Statement 2: Light year is a unit for measurement of very large time intervals.
While the definition of a Light year involves the time period of one year (the time it takes light to travel that distance), the Light year itself is a unit of distance, not time. Units of time include seconds, minutes, hours, days, years, etc. Therefore, this statement is incorrect.
Statement 3: Light year is a unit for measurement of intensity of light.
The intensity of light refers to the power emitted by a light source per unit solid angle or per unit area. It is measured in units like candela (cd) for luminous intensity or watts per square meter (\(W/m^2\)) for radiant intensity. A Light year has no relation to the brightness or intensity of light. Therefore, this statement is incorrect.
Conclusion on Light year Statements
Based on the analysis of each statement:
- Statement 1 is correct.
- Statement 2 is incorrect.
- Statement 3 is incorrect.
Therefore, only statement 1 is correct.
| Statement | Analysis | Correctness |
|---|---|---|
| 1. Light year is a unit for measurement of very large distances. | Defines light year as distance light travels in one year; used for astronomical distances. | Correct |
| 2. Light year is a unit for measurement of very large time intervals. | Light year is distance, not time. Time units are like years, days, etc. | Incorrect |
| 3. Light year is a unit for measurement of intensity of light. | Light intensity units are like candela or W/m<sup>2</sup>, not light year. | Incorrect |
The correct option is the one that states "1 only".
Revision Table: Key Concepts
| Term | Definition/Meaning | Unit Type | Common Use |
|---|---|---|---|
| Light year | Distance light travels in one year in a vacuum | Distance | Measuring interstellar and intergalactic distances |
| Year | Time it takes Earth to orbit the Sun | Time | Measuring time intervals |
| Light Intensity | Power of light per unit area or solid angle | Intensity (e.g., cd, W/m<sup>2</sup>) | Measuring brightness or power of light sources |
Additional Information on Astronomical Units
Besides the Light year, other units are used to measure distances in space:
- Astronomical Unit (AU): The average distance between the Earth and the Sun. It is approximately \(150\) million kilometers or \(93\) million miles. AU is typically used for distances within our solar system.
- Parsec (pc): Another unit of distance used in astronomy. One parsec is defined as the distance at which one astronomical unit subtends an angle of one arcsecond. One parsec is approximately equal to \(3.26\) Light years or about \(3.086 \times 10^{16}\) meters. Parsecs are often used for distances to stars and galaxies.
These units, including the Light year, were developed to make it easier to work with the immense scales involved in studying the universe, avoiding the need to use extremely large numbers of kilometers or miles.
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