Medium

The Laser

Simple Explanation

A laser produces light that is coherent (all waves in step), monochromatic (a single precise wavelength), and highly directional — very different from ordinary light sources like light bulbs, which emit a broad, disorganized mix of wavelengths in every direction.

Why Do We Need It?

A laser's precise, concentrated light makes possible everything from barcode scanners and fibre-optic communication to surgery and cutting tools — applications ordinary light simply cannot achieve.

Formula

Photon Energy

E = hf = hc/λ

Light (including laser light) is carried by photons, each with an energy directly proportional to its frequency (or inversely proportional to its wavelength).

E: — energy of a single photon, in joules (J)
h: — Planck's constant, 6.626×10⁻³⁴ J·s
f: — frequency of the light, in hertz (Hz)
c: — speed of light, 3×10⁸ m/s
λ: — wavelength of the light, in metres (m)

When to use it: Whenever the energy carried by a single photon of light (such as laser light of a known wavelength) needs to be found.

Worked Example

Find the photon energy of a helium-neon laser

A common helium-neon (He-Ne) laser emits light at a wavelength of 632.8 nm. Find the energy of each photon.

Why Does This Work?

A laser's light comes from stimulated emission — excited atoms releasing photons of one exact energy (and therefore one exact wavelength) in a coordinated cascade — so E=hc/λ applies directly and precisely, unlike ordinary light sources that mix many different photon energies together.

Real-Life Example

A laser pointer's tight, focused beam

A laser pointer's beam stays narrow and bright even across a large room, unlike a flashlight's beam, which spreads out and dims quickly.

The laser's coherent, directional light stays tightly focused over long distances — a direct consequence of every photon being emitted with the same wavelength and in step with the others.

Practice

A green laser pointer emits light at 532 nm. Find the energy of each photon.

Medium

Common mistake

Forgetting to convert the wavelength from nanometres to metres before substituting into the formula — using 632.8 directly (instead of 632.8×10⁻⁹) gives an answer wrong by a factor of a billion.

Quick Review

E = hf = hc/λ — photon energy.
Laser light: coherent, monochromatic, highly directional.
Comes from stimulated emission of one exact photon energy.