Example 1
E = 2 eV -> λ ≈ 620 nm (red).
Wavelength from photon energy
Enter photon energy in electronvolts. The calculator inverts E = hc/λ and you get wavelength. 2 eV is about 620 nm, red. 3 eV drops to 413 nm. When you know nanometers and want eV, open E = hc/λ.
E from λ: E = hc/λ. E from f: E = h·f. λ = v/f: wavelength.
Result
Energy E (eV). The result shows up here.
How it works
You invert E = hc/λ. You type energy in eV, because that is how photons are quoted in school and in spectroscopy. First eV become joules: 1 eV = 1.602176634e-19 J, then λ = hc/E. Larger E, shorter λ. The table shortcut is λ(nm) ≈ 1240/E(eV). The calculator uses full h and c, not the 1240 rounding; the result is close, not identical.
Take 2 eV from a red-LED problem. λ = hc/E ≈ 6.199e-7 m, or 620 nm. 3 eV gives 413 nm, already violet. 2.25 eV lands on 551 nm, green. 1.8 eV returns to 689 nm, deep red. 4 eV drops to 310 nm, UV, outside the eye’s window.
The field wants eV, not joules. 3.2e-19 J should first be divided by 1.602e-19 to get about 2 eV, or go to E = h·f when you know frequency. Type 1 keV as 1000: λ ≈ 1.24 nm, soft X-ray. 10 eV is 124 nm, vacuum UV. 0.001 eV is already 1.24 mm, microwaves.
E has to be positive. Zero is division by zero. Beside λ sits f = c/λ in vacuum: the same wave, a second label. At 2 eV, f ≈ 4.84e14 Hz. λ shows m and, in US, ft; in visible light it also writes nm. The E field stays in eV no matter the header. The chart of λ versus E falls as 1/E.
The form needs one field. The calculator example 2 → 620 nm. 3 → 413 nm. A comma and a period both parse, so 2.25 can be 2,25. If you have E in joules from another formula, convert to eV before you type.
Sound in air does not belong here: this is an electromagnetic photon. Sound wavelength is λ = v/f on the wavelength page. The inverse of this sum, from meters to eV, lives on E = hc/λ.
How to use
- Type energy in electronvolts. Visible light is usually 1.6 to 3.3 eV. Red in problems is often 2 eV.
- Compute. 2 eV gives about 620 nm. 3 eV drops to 413 nm. Beside λ sits f = c/λ, at 2 eV about 4.84e14 Hz.
- Convert larger units. 1 keV is 1000 eV and λ ≈ 1.24 nm. 10 eV is 124 nm. Do not type joules in this field.
- The 1240/E shortcut is close, but the calculator uses full h and c. 2 eV is 619.9 nm, not exactly 620; the rounding is school-level.
- When you know nanometers and want eV, open E = hc/λ. When you know hertz, open E = h·f.
Formula
λ = h · c / E
You enter E in eV. E > 0. 1 eV = 1.602176634×10⁻¹⁹ J.
Letters in λ = hc/E
You invert E = hc/λ: type eV, get meters. 2 eV is about 620 nm (red); 3 eV drops to 413 nm.
- λ
- Wavelength in meters. At 2 eV the calculator returns ≈ 6.199×10⁻⁷ m, or 620 nm, not the 1240/E shortcut.
- h
- Planck constant in the hc numerator. Full h and c, not the 1240 nm·eV table rounding.
- c
- Speed of light in that same numerator. Larger E shortens λ: 3 eV is already 413 nm.
- E
- Energy in electronvolts. 1 eV = 1.602176634×10⁻¹⁹ J; the field wants eV, not joules from the E = h·f card.
Real-life examples
Example 2
E = 2.25 eV -> λ ≈ 551 nm.
Example 3
E = 3 eV -> λ ≈ 413 nm.
Example 4
E = 1.8 eV -> λ ≈ 689 nm.
Example 5
E = 4 eV -> λ ≈ 310 nm.
Example 6
E = 10 eV -> λ ≈ 124 nm (VUV).
Example 7
E = 1000 eV -> λ ≈ 1.24 nm (soft X-ray).
Example 8
E = 0.001 eV -> λ ≈ 1.24 mm.
Ways to use this calculator
- From a semiconductor gap in eV you read the edge color: 2 eV ≈ 620 nm.
- From 1 keV you estimate a soft X-ray λ, about 1.24 nm.
Frequently asked questions
What λ at E = 2 eV?
λ ≈ 620 nm, red. 3 eV is ≈ 413 nm, violet. Shortcut 1240/2 = 620; full constants give 619.9 nm.
How do I type keV or MeV?
1 keV = 1000 eV, λ ≈ 1.24 nm. 1 MeV = 1e6 eV. Type the number in eV, not in keV.
How much at E = 2.25 eV?
λ ≈ 551 nm, green. 1.8 eV gives 689 nm, 4 eV gives 310 nm (UV).
Why is f beside the λ result?
In vacuum f = c/λ. At 2 eV and 620 nm that is about 4.84e14 Hz. Same wave, second label.
Does this work for sound in air?
No. This is an electromagnetic photon. Sound is λ = v/f on the wavelength page.
How do I get energy back from λ?
The E = hc/λ page. Type meters (620 nm is 6.2e-7) and you get about 2 eV back.
Is 1240 hard-coded in the calculator?
No. It uses full h and c. The result is close to 1240/E, not identical: 2 eV is 619.9 nm, not exactly 620.
What if I have E in joules?
Divide E by 1.602e-19 to get eV, or use E = h·f when you know frequency.
What if E is zero?
You would divide by zero. Photon energy must be positive.
Why is 2 eV red, not 2 nm?
Visible light is 380-750 nm. 2 eV lands at 620 nm. 2 nm is X-ray, keV energies, not a bulb.
Knowledge sources
The formula is the school one. Units follow SI; NIST SP 330 and BIPM define the measures, not your result.
Page updated in 2026.