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6.1.4Space Physics

Light Speed Calculations — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 8 parts Lesson 9 of 24 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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6.1.4 Light Speed Calculations — IGCSE Physics Notes — IGCSE PHYSICS notes: Use time = distance ÷ speed of light to find light travel times in space, and use the light-year as a unit of distance.Value — light travels through a vacuum at a constant speed of 3.0 × 108 m/s.Electroma…

  • Electromagnetic waves
  • Other media
  • Visibility
  • Sun to Earth
  • Moon to Earth
  • Sun to the outer Solar System

Section 1 of 8The speed of light

Value
  • Value light travels through a vacuum at a constant speed of 3.0 × 108 m/s.
Electromagnetic waves
  • Electromagnetic waves all electromagnetic waves, including radio waves, travel at this speed in a vacuum.
Fastest
  • Fastest nothing can travel faster than this speed.
Other media
  • Other media light is slower in glass or water.
Visibility
  • Visibility objects in the Solar System are seen because they reflect light from the Sun.

Section 2 of 8Equation

Basis
  • Basis average speed = total distance ÷ total time.
Light travel time
  • Light travel time time = distance ÷ speed of light, t = d / c.
Units
  • Units d in metres (m), c in metres per second (m/s) and t in seconds (s).
  • Convert km, minutes, hours or years before calculating.

Section 3 of 8Light travel times

Light travel times
Sun to Earth
  • Sun to Earth about 8 minutes (500 s).
  • We see the Sun as it was 8 minutes ago.
Moon to Earth
  • Moon to Earth about 1.3 s for a distance of 3.8 × 108 m.
Sun to the outer Solar System
  • Sun to the outer Solar System about 5 hours.
Nearest star after the Sun
  • Nearest star after the Sun about 4 years.
Radio signals between Earth and Mars
  • Radio signals between Earth and Mars between 3 and 22 minutes depending on the planets' positions.
Milky Way
  • Milky Way contains billions of stars, whose light takes even longer to reach us.

Section 4 of 8The light-year

1Definition
  • Definition a light-year is the distance travelled by light in one year.
  • It is a unit of distance, not of time.
2Value
  • 1 light-year is 9.5 × 1015 m, or about 9.5 × 1012 km.
3Seconds in a year
  • Seconds in a year about 3.15 × 107 s, since 365 × 24 × 3600 = 3.15 × 107.
4Looking back in time
  • Looking back in time light from a star 4.2 light-years away left it 4.2 years ago.
5Size
  • Size the Milky Way is about 100 000 light-years across.

Section 5 of 8Method for a calculation

1
Write down c = 3.0 × 108 m/s and the given quantity with its unit.
2
Convert distances to metres and times to seconds (1 km = 1000 m, 1 hour = 3600 s).
3
Choose t = d / c, or d = c × t.
4
For a reflected signal (echo or radar) divide the total distance by 2.
5
Give the answer with the correct unit, in standard form if it is very large or small.

Section 6 of 8More from the specification

1Light travel times to comets
  • Light travel times to comets calculating electromagnetic wave travel durations determines spatial distances to orbiting satellites, planets,
    • and distant comets.
2Speed of light in vacuum
  • Electromagnetic radiation propagates through empty space at the universal speed of light: c = 3.00 x 108 m/s.
3Astronomical distances and light-travel time
  • Vast astronomical distances are quantified using light-years, representing the distance traversed by light through vacuum in one Julian year.
4Satellite communications and signal delay
  • Microwave signals transmitted between terrestrial stations and orbital satellites experience measurable light-speed transit delays.
5Gravitational attraction in orbits
  • Gravitational attraction in orbits centripetal force maintaining planetary orbits arises from the strength of the Sun's central gravitational field.
6Orbital radius and period
  • Orbital radius and period planets with a larger orbital radius experience weaker gravitational field strength and travel longer orbital paths.
7Earth tilt and seasons
  • Earth tilt and seasons axial tilt alters daylight hours and solar energy concentration between the northern and southern hemisphere during an orbit.
8Field strength
  • G = Fg/m.
9Kinetic energy
  • K = ½mv2, energy of motion.
10Artificial satellite
  • A man-made object orbiting a body in space.
11Gravitational potential
  • Work done per unit mass in bringing a test mass from infinity to the point.
12Potential energy
  • Energy stored by the configuration of a system.
13Orbit
  • Path of one body around another under gravity.
14Hemisphere
  • Half of the Earth, northern or southern.
15Radius
  • Distance from the centre to the circular path.
16Period
  • Time for one revolution.
17Nebula
  • Cloud of gas and dust in which stars form.
18Comet
  • An icy body on a highly elliptical orbit around the Sun.
19Gravitational field
  • Region where mass experiences gravitational force.
20Orbital speed
  • Speed of an orbiting object, √(GM/r).

Section 7 of 8Key terms

TermMeaning
Speed of light3.0 × 10⁸ m/s in a vacuum.
Light-yearThe distance light travels in one year, 9.5 × 10¹⁵ m.
Electromagnetic waveA wave that can travel through a vacuum at the speed of light.
Standard formA number written as a × 10n with 1 ≤ a < 10.
EchoA reflected signal whose path is there and back.

Section 8 of 8Exam technique

1
Treating a light-year as a unit of time — it is a unit of distance.
2
Using distances in km with c in m/s — convert to metres first.
3
Forgetting that an echo travels there and back — halve the total distance.
4
Thinking light is slower in space — its speed in a vacuum is a fixed constant.

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