The electromagnetic spectrum.

Step 1 of 5

One family

Radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays are all electromagnetic waves: an electric field and a magnetic field shaking at right angles to each other and to the direction of travel. They need no medium, so they cross empty space. Together they make up the electromagnetic spectrum, arranged by wavelength.

The Wavelength (powers of ten) control moves on a logarithmic scale: the value $1$⁠ means $10^{1} = 10$⁠ metres, and the value $-6.3$⁠ means $10^{-6.3}$⁠ metres. Slide from Radio to X-ray.

Step 2 of 5

Frequency

Every wave obeys $v = f\lambda$⁠. For an electromagnetic wave in a vacuum the speed is always $c$⁠, the speed of light, about $3.00 \times 10^8\ \mathrm{m/s}$⁠, so:

$f = \frac{c}{\lambda}$$f =$$\frac{c}{\lambda}$⁠

A wave ten times shorter shakes ten times faster. Light’s frequencies are so high that they are handy to measure in terahertz: 1 THz = 10¹² Hz. The wave in the Scene is now fixed at a length of $10^{-6.3}$⁠ metres.

Step 3 of 5

Brightness and colour

Within the visible band, from about 400 nm to about 700 nm, the colour you see is set by the wavelength: red at the long end and violet at the short end. The Brightness control has nothing to do with the wavelength. Predict what it does change, then watch the Scene turn it up.

Step 4 of 5

The energy of a photon

Light comes in packets, photons, and each photon carries an energy that depends only on its frequency:

$E = hf = \frac{hc}{\lambda}$$E =$$hf =$$\frac{hc}{\lambda}$⁠

$h$⁠ is Planck’s constant, $6.63 \times 10^{-34}\ \mathrm{J \cdot s}$⁠. These energies are tiny, so they are also measured in electronvolts: 1 eV ≈ 1.60 × 10⁻¹⁹ J, the energy an electron gains crossing a potential difference of one volt.

Step 5 of 5

Energy per photon

Brightness is how many photons arrive each second; the frequency sets how much energy each one carries. When light strikes a molecule, it is absorbed one photon at a time. Slide from the X-rays back to radio, and notice how many powers of ten the wavelength changes by.