Why do I get different wavelength in a medium?

In a medium, light speed is v = c/n. Frequency stays the same, so wavelength decreases: λ = (c/n)/f.

What’s the fastest way to convert eV to nm?

Use the shortcut E(eV) ≈ 1240/λ(nm), so λ(nm) ≈ 1240/E(eV).

Use f = c/λ (or (c/n)/λ in a medium) and E = hf. You can also use E(eV) ≈ 1240/λ(nm).

Pick a mode: Wave (v & f), EM (f & c), Photon (E & hc), or Inverse (λ → f & E).
Enter values and choose units.
Click Calculate to get clean results + unit conversions.
Use Quick picks to sanity-check common cases.

It first converts your inputs into standard units (Hz, m/s, meters, joules) so calculations stay consistent.
Then it applies the correct relationship for your mode: Wave uses speed + frequency, EM uses the speed of light (and optional n), and Photon ties energy to wavelength.
In Inverse mode, you enter λ, and we compute frequency and photon energy (plus eV and kJ/mol conversions).
Finally, the calculator shows friendly unit conversions (nm/µm/m), adds a visible-color label when λ is 380–700 nm, and labels the EM spectrum region (IR/visible/UV/etc.) when relevant.

Wave relation:

$v = f λ$

Wavelength from frequency:

$λ = \frac{v}{f}$

Photon energy relation:

$E = \frac{h}{λ} c$

Wavelength from energy:

$λ = \frac{h}{c} / E$

Example 1 — Sound wave

A sound wave in air travels at 343 m/s with frequency 440 Hz.
λ = v/f = 343/440 = 0.7795 m.

Example 2 — Visible light from frequency

Light has frequency 5.00×10¹⁴ Hz.
λ = c/f = (3.00×10⁸)/(5.00×10¹⁴) = 6.00×10⁻⁷ m = 600 nm.

Example 3 — Photon wavelength from energy

A photon has energy 2.48 eV.
λ ≈ 1240 eV·nm / 2.48 eV = 500 nm.

Example 4 — Inverse (λ → f and E)

A green laser has wavelength 532 nm.
f = c/λ = (3.00×10⁸)/(5.32×10⁻⁷) ≈ 5.64×10¹⁴ Hz.
E = hf ≈ 3.74×10⁻¹⁹ J ≈ 2.33 eV.

Q: Why do I get different λ in a medium?

Because light slows down in a medium: v = c/n, so λ decreases while frequency stays the same.

Q: What’s the fastest way to convert eV ↔ nm?

Use E(eV) ≈ 1240 / λ(nm) (equivalently λ(nm) ≈ 1240 / E(eV)).

Q: Does frequency change when light enters water?

No — frequency stays the same at the boundary; speed and wavelength change.