Frequency: definition in chemistry
Frequency: The number of wave cycles that pass a point per second, measured in hertz (Hz).
For light and other electromagnetic radiation, frequency and wavelength are linked by c = λν, so a higher frequency means a shorter wavelength. Frequency is also linked to energy by E = hν, which is why ultraviolet light carries more energy per photon than red light. Atoms absorb and emit light only at specific frequencies, producing the line spectra that are used to identify elements.
Examples
- Red light — about 4.3 × 10¹⁴ Hz
- Violet light — about 7.5 × 10¹⁴ Hz
- FM radio — about 100 MHz (1 × 10⁸ Hz)
- Yellow light from sodium street lamps — about 5.1 × 10¹⁴ Hz
On the periodic table
Each element emits light at its own set of frequencies, which is how flame tests identify metals such as sodium and copper and how helium was first discovered in the Sun's spectrum.