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CBSE Class 11 Physics Numericals: Speed of Light = 1 Unit Explained

 Distance Between Sun and Earth When c = 1 | Class 11 Physics Solution

 - Dr.Sanjaykumar Pawar 

Sun and Earth connected by light path showing time 500 seconds and speed of light equal to 1 unit per second in physics concept.
When speed of light is taken as 1 unit, distance becomes numerically equal to time in seconds.


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CBSE Class 11 Physics - Speed of Light Unit Question

CBSE Class 11 Physics - Topper Notes

Question

A new unit of length is chosen such that the speed of light in vacuum is unity. What is the distance between the Sun and the Earth in terms of the new unit if light takes 8 min 20 s to cover this distance?

Solution

Given:

  • Speed of light, c = 1 (in new unit system)
  • Time = 8 min 20 s

Step 1: Convert time into seconds

8 min 20 s = (8 × 60) + 20 = 480 + 20 = 500 s

Step 2: Use formula

Distance = Speed × Time

Distance = 1 × 500 = 500 units

Final Answer: Distance = 500 units

Short Notes (Exam Revision)

  • When speed of light c = 1, units become simplified natural units.
  • 1 unit of length = distance travelled by light in 1 second.
  • Distance = Speed × Time → here Speed = 1, so Distance = Time.
  • Always convert time into seconds before solving.
  • This concept is useful in relativity and theoretical physics.

Topper Trick

If c = 1, then simply convert time into seconds → that is the answer!

Practice Questions - Speed of Light (c = 1)

Practice Questions & Answers (c = 1 System)

Q1.

If speed of light is 1 unit/sec, how much distance does light cover in 15 seconds?

Answer:

Distance = Speed × Time = 1 × 15 = 15 units

Q2.

Light takes 2 minutes to travel a distance. Find the distance in this system.

Answer:

2 minutes = 120 seconds

Distance = 1 × 120 = 120 units

Q3.

A signal takes 45 seconds to reach Earth. Find the distance.

Answer:

Distance = 1 × 45 = 45 units

Q4.

Light takes 5 minutes 30 seconds to travel from a star to Earth. Find distance.

Answer:

5 min 30 sec = (5 × 60) + 30 = 330 seconds

Distance = 1 × 330 = 330 units

Q5.

If distance is 1000 units in this system, how much time does light take?

Answer:

Time = Distance / Speed = 1000 / 1 = 1000 seconds

Q6.

Why does distance become equal to time when c = 1?

Answer:

Because speed of light c = distance / time = 1

So, distance = time (numerically in this unit system)

Q7.

Light takes 8 min 20 sec to travel from Sun to Earth. Find distance.

Answer:

8 min 20 sec = 500 seconds

Distance = 1 × 500 = 500 units

Q8.

A spaceship signal takes 1.5 hours to reach Earth. Find distance.

Answer:

1.5 hours = 1.5 × 3600 = 5400 seconds

Distance = 1 × 5400 = 5400 units

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