The speed of light in a vacuum is approximately 3.0 x 10^8 m/s (300,000,000 m/s). All electromagnetic waves travel at this speed in a vacuum.
Key points
- In a vacuum, all electromagnetic waves (radio, microwave, infrared, visible, UV, X-ray, gamma) travel at the same speed.
- Light slows down when it enters a denser medium (e.g., glass, water). This change in speed causes refraction.
- The speed of light in air is approximately equal to its speed in a vacuum for most IGCSE calculations.
In calculations
Use c = 3.0 x 10^8 m/s for the speed of light in a vacuum or air. Use the wave equation: c = f x lambda.
Example: A radio wave has a frequency of 100 MHz = 100 x 10^6 Hz. Wavelength = c / f = (3.0 x 10^8) / (100 x 10^6) = 3.0 m.
In exam questions
When a question gives the speed of light, it is usually 3.0 x 10^8 m/s. Enter it carefully using standard form on your calculator (3.0 EXP 8). A common error is entering 3 x 10 x 8 = 240, which is wrong.
Need help understanding this?
A 0625 specialist can explain this concept using the student's own questions and show how it appears in exam papers.