Waves and photons

Electro-magnetic radiation (EM) can be modelled in two ways: by waves, or by radiant particles called photons. The first publications on the wave theory date back to the 17th century. According to the wave theory, light travels in a straight line (unless there are external influences) with its physical properties changing in a wave-like fashion. Light waves have two oscillating components: an electric field and a magnetic field. We refer, therefore, in this context to electromagnetic waves.

Electric fields and magnetic field interact—an instance of a positive electric field coincides with a moment of negative magnetic field (Figure 1). The wave behaviour of light is common to all forms of EM radiation. All EM waves travels at the speed of light, which is approximately equal to 2.998×10⁸ m s^-1. This is fast, but the distances in space are literally astronomical: it takes eight minutes for the sun light to reach the Earth, thus when we see, a sunrise, for example, the light particles actually left the Sun that much earlier. Because they travel in a straight line, we use the notion of light rays in optics. 

A sine wave can be described as:

where α is the amplitude of the wave, φ is the phase (it depends on time) and λ is the Wavelength.

Although wave theory provides a good explanation for many EM radiation phenomena, for some purposes we can better rely on particle theory, which explains EM radiation in terms of photons. We take this approach when quantifying the radiation detected by a multispectral sensor. The amount of energy carried by a photon of a specific wavelength is:

where Q is the energy of a photon measured in joules (J) and h is Planck’s constant (h ≈ 6.626 × 10^-34 J s).

The energy carried by a single photon of light is just sufficient to excite a single molecule of a photosensitive cell of the human eye, thus contributing to vision. It follows from Equation 2 that long-wavelength radiation has a low level of energy while short-wavelength radiation has a high level. Blue light has more energy than red light. EM radiation beyond violet light is progressively more dangerous to our body as its frequency increases. UV radiation can already be harmful to our eyes, so we wear sunglasses to protect them. An important consequence of Equation 2 for RS is that it is more difficult to detect radiation of longer wavelengths than radiation of shorter wavelengths.