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Physics · Grades 10–12

Light speed in a medium

The refractive index is dimensionless; use the given vacuum speed.

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Rule and assumptions

The refractive index is dimensionless; use the given vacuum speed.

v = c/n

Worked examples

A transparent medium has refractive index 1.3. Use c = 300 million m/s. Find light speed in MILLION m/s. Round to 3 decimal places if needed.

v = c/n 300 / 1.3 = 230.769

230.769

A transparent medium has refractive index 1.3. Use c = 300 million m/s. Find light speed in MILLION m/s. Round to 3 decimal places if needed.

v = c/n 300 / 1.3 = 230.769

230.769

Try two questions

  1. A transparent medium has refractive index 1.1. Use c = 300 million m/s. Find light speed in MILLION m/s. Round to 3 decimal places if needed.

  2. A transparent medium has refractive index 1.8. Use c = 300 million m/s. Find light speed in MILLION m/s. Round to 3 decimal places if needed.

Answers and reasoning

Answers and marking notes
  1. 272.727

    v = c/n 300 / 1.1 = 272.727

  2. 166.667

    v = c/n 300 / 1.8 = 166.667

Further reading and review references

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Learning preview · independent educational and translation review required. Suggested grades are not official curriculum alignment.