c is the speed of light and every electromagnetic wave in vacuum. In a medium it drops to c/n, where n is the refractive index.
Value of c in different units
| Value | Use it when |
|---|---|
| 2.998 × 10⁸ m s⁻¹ | Exact: 299 792 458 m/s. |
| 3 × 10⁸ m s⁻¹ | The value used in almost every numerical. |
| 3 × 10¹⁰ cm s⁻¹ | CGS units, older chemistry books. |
| 1.08 × 10⁹ km h⁻¹ | For a sense of scale. |
Formulas that use c
- c = νλ
- E = mc²
- c = 1/√(μ₀ε₀)
- n = c/v
Its dimensional formula is worked out on the dimensional formula of velocity page.
Other constants
- Gas constant (R): 8.314 J mol⁻¹ K⁻¹
- Planck's constant (h): 6.626 × 10⁻³⁴ J s
- Avogadro's number (Nₐ): 6.022 × 10²³ mol⁻¹
- Speed of light (c): 3 × 10⁸ m/s
- Charge of an electron (e): 1.602 × 10⁻¹⁹ C
- Mass of an electron (mₑ): 9.109 × 10⁻³¹ kg
- Mass of a proton (mₚ): 1.673 × 10⁻²⁷ kg
- Mass of a neutron (mₙ): 1.675 × 10⁻²⁷ kg
- Universal gravitational constant (G): 6.674 × 10⁻¹¹ N m² kg⁻²
- Acceleration due to gravity (g): 9.8 m/s²
- Boltzmann constant (k): 1.381 × 10⁻²³ J/K
- Faraday constant (F): 96 485 C/mol
- Permittivity of free space (ε₀): 8.854 × 10⁻¹² C² N⁻¹ m⁻²
- Permeability of free space (μ₀): 4π × 10⁻⁷ T m/A
- Rydberg constant (R): 1.097 × 10⁷ m⁻¹
- Stefan–Boltzmann constant (σ): 5.67 × 10⁻⁸ W m⁻² K⁻⁴
- Wien's constant (b): 2.898 × 10⁻³ m K
- Bohr radius (a₀): 0.529 Å
- Molar volume of a gas at STP (Vₘ): 22.4 L/mol
The full list, searchable, is on the physical constants page.
Questions students ask
What is the value of speed of light?
c = 3 × 10⁸ m/s. In other units: 2.998 × 10⁸ m s⁻¹; 3 × 10⁸ m s⁻¹; 3 × 10¹⁰ cm s⁻¹; 1.08 × 10⁹ km h⁻¹.
What is c in physics and chemistry?
c is the speed of light and every electromagnetic wave in vacuum. In a medium it drops to c/n, where n is the refractive index.
Which value of c should I use?
2.998 × 10⁸ m s⁻¹: Exact: 299 792 458 m/s. 3 × 10⁸ m s⁻¹: The value used in almost every numerical. 3 × 10¹⁰ cm s⁻¹: CGS units, older chemistry books. 1.08 × 10⁹ km h⁻¹: For a sense of scale.
What is the dimensional formula of speed of light?
See the dimensional formula of speed of light, worked out step by step on its own page.