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Photon energy from wavelength calculator

A-level Nuclear and particles

The energy of one photon found from its wavelength in a vacuum.

E = h c / λ

This free calculator is for A-level physics. Choose the quantity you want to find, enter the other values with their units, and it shows the working in the layout used in exams, with the answer to the number of significant figures you choose. The values shown are an example. Change them to your own.

Quantity to find

EPhoton energy to find 3.98 × 10⁻¹⁹ J

hPlanck constant constant J s

cSpeed of light constant m/s

λWavelength

Significant figures

Working

Write the equation
E = h c / λ
Constants used
h = 6.63 × 10⁻³⁴ J s; c = 3 × 10⁸ m/s
Substitute the values
E = (6.63 × 10⁻³⁴) × (3 × 10⁸) ÷ (5 × 10⁻⁷)
Calculate
E = 3.978 × 10⁻¹⁹ J
Give the answer to 3 significant figures
E = 3.98 × 10⁻¹⁹ J

Worked example

Find E, the photon energy. Given: wavelength λ = 5.00 × 10⁻⁷ m; Planck constant h = 6.63 × 10⁻³⁴ J s; speed of light c = 3 × 10⁸ m/s.

Write the equation
E = h c / λ
Constants used
h = 6.63 × 10⁻³⁴ J s; c = 3 × 10⁸ m/s
Substitute the values
E = (6.63 × 10⁻³⁴) × (3 × 10⁸) ÷ (5 × 10⁻⁷)
Calculate
E = 3.978 × 10⁻¹⁹ J
Give the answer to 3 significant figures
E = 3.98 × 10⁻¹⁹ J

Symbols and units

Symbol	Quantity	SI unit
E	Photon energy	J
h	Planck constant (constant)	J s
c	Speed of light (constant)	m/s
λ	Wavelength	m

Photon energy from wavelength: rearranged

To find photon energy: E = h c / λ
To find wavelength: λ = h c / E

Exam tip

Wavelength must be in metres. A wavelength in nm is multiplied by 10⁻⁹.

Common questions

What is the formula for photon energy from wavelength?

The equation is E = h c / λ, where E is the photon energy (J); λ is the wavelength (m); h is the Planck constant (J s); c is the speed of light (m/s).

What units should I use for photon energy from wavelength?

In SI units: photon energy in J, Planck constant in J s, speed of light in m/s, wavelength in m. The calculator also accepts common alternatives such as kilometres, grams or minutes, and converts them for you.

How do I rearrange E = h c / λ?

Rearranging gives these forms: to find photon energy: E = h c / λ. to find wavelength: λ = h c / E.

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Read the full explanation on the Photon energy formula page. More nuclear and particles equations: Photon energy, Photoelectric equation, de Broglie wavelength, Radioactive decay and half-life, Activity and decay constant, Half-life and decay constant, Mass–energy equivalence. See the full list on the a-level physics equation calculator page.