Question

Speed of Light in vacuum is :

• A. $3 \times 10^6$ m/s
• B. $3 \times 10^8$ m/s
• C. $3 \times 10^5$ m/s
• D. $3 \times 10^7$ m/s

Hint:

A common mistake is choosing $3 \times 10^5 \text{ km/s}$. While that value is correct in kilometers, always check the units. In meters per second (m/s), which is the SI standard, the exponent must be $10^8$.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question asks for the standard value of the speed of light in a vacuum, a universal constant in physics denoted by the symbol \( c \).
Step 2: Detailed Explanation:

• Exact Value: The speed of light in a vacuum is defined with a precise value of $299,792,458$ meters per second. For most scientific and engineering calculations, this is conveniently approximated to $3 \times 10^8 \text{ m/s}$.
  
• Universal Constant: A fundamental postulate of Einstein's Theory of Special Relativity is that the speed of light in a vacuum is the same for all observers, irrespective of the motion of the light source. It represents the maximum speed at which energy, matter, and information can travel.
  
• Significance in Calculations: This constant is a key component in numerous famous physics equations, including the mass-energy equivalence formula, \( E = mc^2 \). It is also used to define astronomical distances, such as the light-year.
  
• Speed in Media: Light slows down when it passes through a transparent medium like air, water, or glass. The factor by which it slows is known as the medium's "refractive index" (\( n = c/v \)).
  
• Magnitude Comparison: To put its speed into perspective, a beam of light could travel around the Earth's equator about 7.5 times in just one second. This immense speed is why light travel appears instantaneous in our everyday experience.
  

Step 3: Final Answer:
The widely accepted scientific value for the speed of light in a vacuum is $3 \times 10^8 \text{ m/s}$.