Wavelength – Frequency – Energy (Light) + Chart – Tutorial
On this page, you can find the logic, usage, and important details of the Wavelength – Frequency – Energy (Light) + Chart calculator.
Light: Wave or Particle?
Light is one of the most debated concepts in physics: sometimes it behaves like a wave (diffraction, interference, polarisation) and sometimes like a particle (energy packets called photons). The modern view says "both": light is an electromagnetic wave, but exchanges energy in quantum packets called photons.
What Is an Electromagnetic Wave?
Light is a wave in which an electric field (E) and magnetic field (B) mutually generate each other and propagate through space. In a vacuum the propagation speed is constant: c = 299 792 458 m/s — an exact defined value since the 2019 SI revision.
Wavelength (λ) and Frequency (f)
- Wavelength (λ, lambda): The distance from one wave crest to the next. Unit: metres (m). In practice, nm (nanometres = 10⁻⁹ m) is used for light.
- Frequency (f): The number of oscillations per second. Unit: Hz (s⁻¹). Visible-light frequencies are in the 10¹⁴–10¹⁵ Hz range, i.e. hundreds of THz.
The Fundamental Relation: c = λ · f
In a vacuum, c is constant, so λ and f are inversely proportional.
Visible Light and Colour
The human eye perceives roughly 380–750 nm as visible light:
- ~380–450 nm: violet / purple (high frequency)
- ~450–495 nm: blue
- ~495–570 nm: green (eye most sensitive here)
- ~570–620 nm: yellow / orange
- ~620–750 nm: red (low frequency)
Photon Energy: E = h · f
Planck's 1900 discovery: light energy does not come in a continuous stream but in quantum packets proportional to frequency.
As frequency increases, so does photon energy: UV photons are more energetic, IR photons less so.
Why Use eV?
At the atomic scale, Joules are inconveniently small. The electronvolt (eV) is a more natural unit:
Visible-light photon energies fall in the ~1.65–3.26 eV range — the same order as semiconductor band gaps.
The Chart: A Hyperbolic Inverse Relationship
f = c/λ is a hyperbolic function: as λ grows from small values, f drops rapidly at first and then more slowly. This relationship is not linear — students sometimes expect a straight line, but it is a hyperbola.
Quick Sanity Check
- 500 nm → f ≈ 3×10⁸ / 5×10⁻⁷ = 6×10¹⁴ Hz (visible range ✓)
- 10¹⁰–10¹² Hz → microwave
- 10¹⁸ Hz → X-ray
Summary: c = λf describes the wave character of light; E = hf describes its quantum character. What appears to be "colour = wavelength" is ultimately rooted in the physical reality of "energy = frequency".