Visible Light
Visible light is the narrow slice of the electromagnetic spectrum that human eyes can detect — spanning wavelengths from roughly 380 nm (violet) to 700 nm (red). To put that in perspective, the entire visible range is less than one octave wide on the EM spectrum, yet it contains every colour you have ever seen. The Sun emits most of its energy in this window, which is why evolution shaped our eyes to be sensitive to precisely these wavelengths. Like all electromagnetic waves, visible light consists of oscillating electric and magnetic fields perpendicular to each other and to the direction of travel. It travels at c ≈ 3 × 10⁸ m s⁻¹ in a vacuum. The frequency and wavelength of light are inversely related through this speed: higher-frequency violet light carries more energy per photon than lower-frequency red light, which is why UV causes sunburn while radio waves do not. Understanding visible light is central to optics, photography, astronomy, chemistry (atomic emission spectra), and even quantum mechanics. When white light passes through a prism, it disperses into a continuous rainbow because different wavelengths refract by slightly different angles — a phenomenon Isaac Newton first studied systematically in 1666. Every colour you perceive is your brain's interpretation of a specific range of frequencies stimulating the cone cells in your retina.
- The Electromagnetic Nature of Light
- Wavelength, Frequency, and Colour
- Energy of a Photon
- Dispersion and the Prism
- Intensity and the Inverse-Square Law
The Electromagnetic Nature of Light
Visible light is a transverse electromagnetic wave. The electric field E and magnetic field B oscillate at 90° to each other and to the propagation direction. No medium is needed — light travels through the vacuum of space at exactly c = 299 792 458 m s⁻¹ (defined). In a medium with refractive index n, speed drops to v = c/n.
Wavelength, Frequency, and Colour
The colour we perceive corresponds directly to the wavelength (or equivalently, the frequency) of the light. Approximate ranges: violet 380–450 nm, blue 450–495 nm, green 495–570 nm, yellow 570–590 nm, orange 590–625 nm, red 625–700 nm. Your eye has three types of cone cells with peak sensitivities near blue (420 nm), green (530 nm), and red (560 nm); all perceived colours arise from the ratio of stimulation among these three types.
Energy of a Photon
Light is also quantised into packets called photons. The energy of a single photon is E = hf = hc/λ, where h = 6.626 × 10⁻³⁴ J s is Planck's constant. Violet photons (λ ≈ 400 nm) carry about 3.1 eV of energy; red photons (λ ≈ 700 nm) carry about 1.8 eV. This ~70% energy difference explains why violet light can cause photochemical reactions (e.g., fading of dyes) that red light cannot.
Dispersion and the Prism
When white light enters glass, different frequencies travel at slightly different speeds because the refractive index n depends on wavelength — a phenomenon called dispersion. Snell's law n₁ sin θ₁ = n₂ sin θ₂ bends each wavelength by a different angle, spreading the beam into a spectrum. Violet light bends most (highest n), red bends least. Rainbows form by the same mechanism inside spherical water droplets.
Intensity and the Inverse-Square Law
The intensity of light (power per unit area, W m⁻²) decreases with the square of distance from a point source: I = P/(4πr²). This is purely geometric — the same energy spreads over an ever-larger spherical surface. Doubling your distance from a lamp makes it appear four times dimmer. This law governs everything from star brightness measurements to camera exposure settings.
The Key Equation
- E — Photon energy (Joule (J)): Energy carried by a single quantum of light. Larger for shorter wavelengths.
- h — Planck's constant (J s): h = 6.626 × 10⁻³⁴ J s; the fundamental quantum of action relating energy to frequency.
- c — Speed of light in vacuum (m s⁻¹): c = 2.998 × 10⁸ m s⁻¹; the universal speed limit for electromagnetic waves.
- \lambda — Wavelength (metre (m)): Distance between successive wave crests. For visible light, λ ranges from ~380 nm to ~700 nm.
Worked Example
Problem. A green laser pointer emits light with a wavelength of 532 nm. (a) Calculate the frequency of this light. (b) Calculate the energy of a single photon in eV. (c) If the laser has a power output of 5.0 mW, how many photons does it emit per second?
Answer. Frequency: 5.64 × 10¹⁴ Hz; photon energy: 2.34 eV; photon emission rate: 1.34 × 10¹⁶ photons per second.
Origins · 1 minIsaac Newton and how this was discovered
Newton performed the first rigorous scientific investigation of visible light and colour. In 1666, using a glass prism, he showed that white sunlight is not pure but a mixture of all colours — each wavelength refracting by a different angle. He correctly concluded that colour is an intrinsic property of light, not something added by the prism. His book Opticks (1704) systematised these findings and shaped optics for a century. Newton used a second prism to recombine the dispersed spectrum back into white light, decisively proving that the prism was separating existing colours rather than creating new ones — an elegant double-experiment design.
Related Concepts
- Infrared Radiation — Infrared lies immediately below the visible range (λ > 700 nm) in the EM spectrum; understanding where visible light ends helps define where infrared begins.
- Ultraviolet Radiation — UV light sits just above the visible range (λ < 380 nm) with higher photon energies, making it important for photochemistry and health effects.
- Snell's Law of Refraction — Dispersion of visible light into a spectrum by a prism is a direct consequence of Snell's law applied wavelength-by-wavelength.
- Photon Energy and E = hf — The wave-particle duality of visible light is captured by the photon energy equation, essential for understanding the photoelectric effect and atomic spectra.