Frequency
Definition and meaning of Frequency in chemistry.
Frequency is the number of complete wave cycles that pass a fixed point per unit time, expressed in hertz (Hz, or s⁻¹).
In more detail
In chemistry, frequency (symbol ν, "nu") is most important when describing electromagnetic radiation, since it links directly to a photon's energy through Planck's equation, E = hν, where h is Planck's constant. Frequency is inversely proportional to wavelength (ν = c/λ, with c the speed of light), so short-wavelength radiation like ultraviolet or X-rays has high frequency and high energy, while long-wavelength radiation like radio waves has low frequency and low energy. This relationship underlies spectroscopy, where absorbed or emitted frequencies reveal a molecule's electronic, vibrational, or rotational energy levels, and explains why only certain frequencies of light can ionize atoms or break chemical bonds.
Key facts
| Symbol | ν (nu) |
|---|---|
| SI Unit | hertz (Hz) = s⁻¹ |
| Key Relation | ν = c/λ; E = hν |
| Field | Physical Chemistry |
Visible light with a wavelength of 500 nm has a frequency of ν = c/λ = (3.00 × 10^8 m/s) / (5.00 × 10^-7 m) ≈ 6.00 × 10^14 Hz.
Frequently asked questions
How does frequency relate to a photon's energy?
Photon energy is given by E = hν, where h is Planck's constant (6.626 × 10^-34 J·s). Frequency and photon energy are directly proportional, so higher-frequency radiation delivers more energy per photon.
How does frequency differ from wavelength?
Frequency counts wave cycles passing a point per second, while wavelength measures the distance between successive crests. For electromagnetic waves they are inversely related by ν = c/λ, where c is the speed of light.