Calculate one-way light travel across the Solar System
Use t = d/c with c = 3.0 × 10⁸ m/s and convert the result into useful time units.
Section 6.1.2 · The Solar System
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Use supplied distances, periods and stated orbital models. Drawings are not to scale. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Use distance = speed × time.
- Convert kilometres and seconds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is travel time at constant speed?
What you will learn
- Use t = d/c with c = 3.0 × 10⁸ m/s and convert the result into useful time units.
- Calculate a light delay using the actual supplied path and consistent units.
Light travel is not instantaneous
For a stated path in vacuum, light travels at approximately c = 3.0 × 10⁸ m/s. Travel time is distance divided by speed: t = d/c. Solar System distances can be large enough for delays of seconds, minutes or hours even at this high speed.
Use the distance between the emitting and receiving objects at the relevant time. An orbital radius from the Sun is not automatically the separation of two planets, because their positions differ. Most school problems supply a separation or a simple geometric arrangement to remove that uncertainty.
Convert distance before dividing
A kilometre is 1000 metres. Convert kilometres into metres before using the quoted c in metres per second. The quotient then gives seconds. To express the answer as minutes and seconds, divide into complete minutes and a remaining number of seconds.
A supplied distance and constant-speed model are approximations. Keep their precision in the final answer and do not infer an exact instantaneous separation from an illustrative diagram. One-way reception also differs from the time for a request and reply, developed in the following lesson.
Pause and explain
A supplied path is 9.0 × 10¹⁰ m. With c = 3.0 × 10⁸ m/s, what is one-way time?
Worked example
Find the one-way light time for a supplied Sun–Earth separation of 1.50 × 10¹¹ m.
Show the worked solution
- Use t = d/c.
- Calculate 1.50 × 10¹¹ ÷ 3.0 × 10⁸ = 500 s.
- Convert 500 s to 8 complete minutes and 20 s.
Answer 500 s, or 8 min 20 s.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find the one-way light time for a supplied separation of 6.0 × 10¹⁰ m.
Give me a hint
Divide by the vacuum light speed expressed in the same distance unit.
Compare my reasoning
- Use t = d/c.
- Calculate 6.0 × 10¹⁰ ÷ 3.0 × 10⁸.
- Convert 200 s if useful.
200 s, or 3 min 20 s.
Look for these in your work
- Use the supplied separation.
- Give consistent time units.
A supplied spacecraft separation is 4.5 × 10⁷ km. Find one-way signal time.
Give me a hint
Convert kilometres before using c in metres per second.
Compare my reasoning
- d = 4.5 × 10¹⁰ m.
- t = 4.5 × 10¹⁰ ÷ 3.0 × 10⁸ = 150 s.
- 150 s is 2 min 30 s.
150 s, or 2 min 30 s.
Look for these in your work
- Convert the distance correctly.
- Keep the time one-way.
Two planetary orbital radii are supplied but their relative orbital positions are omitted. Can you give one unique signal delay between them?
Give me a hint
A path requires their separation, not just both Sun-relative distances.
Compare my reasoning
- Their positions affect the separation.
- They can be on similar or opposite sides of the Sun.
- A geometry or direct separation must be supplied.
No unique delay follows without their relative positions or separation.
Look for these in your work
- Identify the missing path distance.
- Avoid substituting one orbital radius as the separation.
Common mistakes
- Convert kilometres to metres.
- Planet orbital radius is not automatically interplanetary separation.
- One-way time is not round-trip time.
What can you explain now?
A supplied path is 9.0 × 10¹⁰ m. With c = 3.0 × 10⁸ m/s, what is one-way time?
Compare with the explanation
300 s
Dividing the distance by c gives 3.0 × 10² seconds, equivalent to five minutes.
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Make it stick
Calculate a light delay using the actual supplied path and consistent units.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.