IGCSEPhysicsCIERevision Notes6. Space PhysicsEarth & The Solar SystemLight Speed Calculations

Light Speed Calculations (CIE IGCSE Physics)

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Katie M

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Light speed calculations

  • Light is a type of electromagnetic wave which travels at a constant speed of 3 × 108 m/s
  • Objects in the Solar System are visible from Earth as they reflect light from the Sun
  • It takes time for light to travel such large distances, for example:
    • it takes 8 minutes for light from the Sun to reach Earth
    • it takes around 5 hours for light from the Sun to reach the outer regions of the Solar System
    • it takes 4 years for light from our nearest star (after the Sun) to reach Earth
  • The Milky Way galaxy contains billions of stars, huge distances away, with the light taking even longer to be seen from Earth
  • To carry out light speed calculations, we can rearrange the equation:

average space speed space equals space fraction numerator total space distance over denominator total space time end fraction

  • So, the time taken for light to travel a distance can be calculated using:

time space equals space fraction numerator distance over denominator speed space of space light end fraction

Worked example

The radius of Mercury's orbit around the Sun is 5.8 × 1010 m.

The speed of light is 3.0 × 108 m/s.

Calculate the time taken for light from the Sun to reach Mercury.

Answer:

Step 1: List the known quantities

  • Distance travelled = 5.8 × 1010 m
  • Speed of light = 3.0 × 108 m/s

Step 2: State the equation for the time taken for light to travel a distance

time space equals space fraction numerator distance over denominator speed space of space light end fraction

Step 3: Substitute the values into the equation

time space equals space fraction numerator 5.8 space cross times space 10 to the power of 10 over denominator 3.0 space cross times space 10 to the power of 8 end fraction space equals space 193 space straight s

  • It takes about 193 s, or 3 min, for light from the Sun to reach Mercury

Exam Tip

The speed of light is very fast. This is why in our everyday life things like switching on a light seem to be instant. However, this is only because the light travels very fast and the distances are very small. In large, astronomical distances which can be millions or even billions of kilometres, the limit of the speed of light starts to have an effect.

For example, it takes light 8 minutes to travel from the Sun to the Earth. This means we are seeing the Sun as it was eight minutes ago. If the Sun was to disappear, we would not notice till eight minutes later. Although, by that time, time delay would be the least of our worries...

p.s.: The Sun is not going to vanish!

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    Katie M

    Author: Katie M

    Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.