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Wavelength from photon energy

Turn photon energy into wavelength straight away: 2.25 eV gives 551.040881925 nm, green light. Enter electronvolts, millielectronvolts, kiloelectronvolts, megaelectronvolts or joules and read the answer in metres, nanometres and angstroms. Free in your browser.

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Worked example

If you enter

Energy: 2.25
Energy unit: eV

You get

Energy unit label: electronvolt
Energy J: 0
Energy eV: 2.25
Wavelength m: 0.000000551

Every photon carries energy tied to its wavelength by Planck’s relation: wavelength equals the product of Planck’s constant and the speed of light divided by the photon energy. Optical engineers, spectroscopists, laser designers, and physics students need that conversion constantly—from electronvolts on a detector spec sheet to nanometres on a diffraction grating or filter catalogue. This wavelength from photon energy calculator accepts a positive energy value, optional unit (electronvolts by default, or millielectronvolts, kiloelectronvolts, megaelectronvolts, or joules), and returns wavelength in metres, nanometres, and angstroms using exact SI and CODATA 2019 constants. There is no network call, no model token, and no wall-clock dependency—only arithmetic you can pin in tests.

How λ = h·c / E links photon energy to wavelength

A single photon of electromagnetic radiation has energy E equal to Planck’s constant h times its frequency f, and because frequency and wavelength in vacuum are related by the speed of light c through c equals f times lambda, the two statements combine into the compact form lambda equals h times c over E. That is the formula this capability evaluates.

Larger energy means shorter wavelength: a soft X-ray photon at one kiloelectronvolt is far bluer than a green-light photon near two and a quarter electronvolts, which sits near five hundred fifty nanometres. The implementation uses the exact fixed values h equals 6.62607015 times ten to the minus thirty-four joule-seconds and c equals two hundred ninety-nine million seven hundred ninety-two thousand four hundred fifty-eight metres per second, together with the exact elementary charge so that one electronvolt converts to joules without floating-point guesswork.

The page does not convert wavelength back into energy, does not treat frequency or spectroscopic wavenumber as primary inputs, and does not model medium refractive index—those are different search intents. It is the directional calculator people mean when they type wavelength from photon energy and expect λ equals hc over E.

Where wavelength-from-energy calculators earn their keep

Laser and LED product pages quote photon energy in electronvolts while optical filters, gratings, and safety eyewear are specified in nanometres; converting with λ equals hc over E is the first step before comparing a source to a bandpass. Spectroscopy labs map detector channel energies to wavelength axes when calibrating monochromators or checking emission lines against tabulated values.

University quantum and modern-physics courses assign endless homework that asks for the wavelength of a photon of given energy in electronvolts or joules; a single trusted oracle with fixed CODATA constants speeds grading and removes calculator-mode drift between students. Process engineers in photolithography and thin-film coating translate dose-related photon energies into design wavelengths when discussing deep ultraviolet or soft X-ray tools with suppliers.

If you later need bidirectional multi-unit conversion among electronvolts, nanometres, terahertz, and wavenumber, or classical wave speed from frequency and wavelength, treat those as separate SEO landings rather than overloading this directional wavelength-from-photon-energy page.

What you can do with it

Laser and LED energy-to-wavelength specs

Convert a source photon energy in eV to nanometres before matching filters, gratings, or eyewear labelled only in wavelength.

Homework and quantum-physics answer keys

Return deterministic λ = h·c / E from textbook electronvolt energies so teachers and auto-graders share one ground truth.

FAQ

What happens if energy is zero or negative?

Photon energy must be strictly positive for λ = h·c / E to yield a physical wavelength.

What wavelength does a 2.25 eV photon have?

About 551.040881925 nanometres, which is green light. The same photon has a wavelength of 5510.40881925 angstroms, and the calculator shows the value in metres too.

Which energy units can I use?

Electronvolts are the default, and you can also choose millielectronvolts, kiloelectronvolts, megaelectronvolts or joules. The wavelength always comes back in metres, nanometres and angstroms.

Why does a higher photon energy mean a shorter wavelength?

Wavelength equals Planck's constant times the speed of light divided by the energy, so the two are inversely proportional. Doubling the energy halves the wavelength.

Which constants are used for the conversion?

Planck's constant, the speed of light and the electron charge as exact SI values, following CODATA 2019. That keeps the result identical every time you repeat the same calculation.

Related tools

For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/optics/wavelength-from-photon-energy

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/optics/wavelength-from-photon-energy \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"energy":2.25}'

const res = await fetch("https://api.kit.forhosting.com/optics/wavelength-from-photon-energy", {
  method: "POST",
  headers: {
    "Authorization": `Bearer ${process.env.KIT_KEY}`,
    "Content-Type": "application/json"
  },
  body: JSON.stringify({
    "energy": 2.25
  })
});
const { task_id } = await res.json();

import os, requests

res = requests.post(
    "https://api.kit.forhosting.com/optics/wavelength-from-photon-energy",
    headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
    json={
    "energy": 2.25
},
)
task_id = res.json()["task_id"]

<?php
$res = file_get_contents("https://api.kit.forhosting.com/optics/wavelength-from-photon-energy", false, stream_context_create([
  "http" => [
    "method"  => "POST",
    "header"  => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
    "content" => '{"energy":2.25}',
  ],
]));
$task = json_decode($res, true);

body := bytes.NewBufferString(`{"energy":2.25}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/optics/wavelength-from-photon-energy", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)

Example request

{
  "energy": 2.25
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "optics.wavelength_from_photon_energy",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Errors

HTTP	Code	Meaning
`401`	`unauthorized`	Missing or invalid API key.
`402`	`insufficient_balance`	Your balance doesn't cover the task price.
`404`	`unknown_type`	That task type doesn't exist.
`429`	`rate_limited`	Too many requests. Use the webhook instead of polling.

Technical notes

Input contract, units, response fields, and errors

Unknown unit strings fail the same way with a message listing supported units. Classic checks include about two point two five electronvolts for green light near five hundred fifty nanometres, one electronvolt near twelve hundred forty nanometres in the near infrared, and a zero or negative energy path as the mandatory error example. Wire the free browser form for classroom demos, then call the identical contract from automation when you need bulk tables. POST a JSON body with the required field energy: a finite positive number. Optionally send energy_unit as eV (the default when omitted), meV, keV, MeV, or J. Aliases energy, E, photon_energy, and energy_value are accepted for the value; unit, energy_u, and eu are accepted for the unit string so scripts written against textbook notation do not need a rename layer. Missing energy, empty strings, NaN, Infinity, zero, and negative energy all fail with invalid_input and are not billed. A short formula string λ equals h·c / E and a constants object with h, c, elementary charge, and hc factors complete the payload so notebooks can print the physical basis next to the number. Successful API calls use the published base of $0.002 per item; validation failures do not consume prepaid balance.

Integration use cases

Spectroscopy and monochromator checks

Map detector channel energies to wavelength_nm and wavelength_angstrom for line tables and calibration scripts.

Technical questions

How do I compute wavelength from photon energy?

Send a positive energy (default unit eV) and optionally energy_unit (eV, meV, keV, MeV, or J). The API returns λ = h·c / E in metres, nanometres, and angstroms using exact SI/CODATA constants.

What units of energy are supported?

Electronvolts (default), millielectronvolts, kiloelectronvolts, megaelectronvolts, and joules. Unknown units raise invalid_input and are not billed.

Which constants does the calculator use?

Exact CODATA 2019 / SI values: h = 6.62607015×10⁻³⁴ J·s, c = 299792458 m/s, and elementary charge for eV↔J conversion. They appear in the response constants object.

What does the wavelength from photon energy calculator cost?

The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.

Read the full KIT documentation →

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