Question

a) How much time does it take light to travel from the moon to the earth, a distance of 384,000km ?
b) Light from the star Sirius takes 8.61 years to reach the earth. What is the distance to Sirius in kilometers?

Answer 1

A. The time taken for light to travel from the moon to the earth, a distance of 384000 Km is 1.28 s

B. The distance to Sirius in kilometers is 8.15×10¹³ Km

What is speed?

Speed is the distance travelled per unit. Mathematically, it can be expressed as:

Speed = distance / time

A. How to determine the time taken

• Speed of light = 3×10⁸ m/s
• Distance = 384000 Km = 384000000 m
• Time =?

Speed = distance / time

3×10⁸ = 384000000 / time

Cross multiply

3×10⁸ × time = 384000000

Divide both sides by 3×10⁸

Time = 384000000 / 3×10⁸

Time = 1.28 s

B. How to determine the distance

• Speed of light = 3×10⁸ m/s = 3×10⁸ × 3.6 = 1080000000 Km/h
• Time = 8.61 years = 8.61 × 8760 = 75423.6 hours
• Distance =?

Speed = distance / time

1080000000 = distance / 75423.6

Cross multiply

Distance = 1080000000 × 75423.6

Distance = 8.15×10¹³ Km

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