The Physics of Light
Light is a form of electromagnetic radiation that carries energy.
It has a dual nature, behaving both as a wave and as a particle. As a wave, light exhibits phenomena such as diffraction, interference, and polarization. As a particle, in the form of photons, it participates in processes like the photoelectric effect, photon absorption, and photon emission.
Light is also defined by two key properties: its frequency (f), which represents the number of oscillations per second and is measured in Hertz (Hz), and its wavelength (λ), which is the distance between successive wave crests. These two properties are interconnected through the fundamental equation c = λ · f, where c is the speed of light.
The main cue for our clocks is light. Special light-sensitive proteins in our eyes and skin, called opsins, and molecules within cells that absorb photons to trigger cellular processes (such as Chromophores) detect different wavelengths of light and relay this information to the brain.
The length of the wavelength affects how light penetrates:
- Short wavelengths (blue and violet) are high-energy and scatter more in the atmosphere. They penetrate less deeply into surfaces but strongly influence the SCN and circadian signaling.
- Long wavelengths (red, orange) are lower-energy, penetrate deeper, and dominate during sunrise and sunset when the sun is at a low angle.
Because the Earth rotates and tilts on its axis, the angle of sunlight changes throughout the day:
- Morning (sunrise, low angle): UV missing, blue light is still present but mixed with red/orange, giving a gentle wake-up signal.
- Midday (sun overhead, high angle): UV rises, Blue light present and the without missing the red component (almost 50% of the spectrum)
- Evening (sunset, low angle again): Blue light fades, red and orange dominate.
This shifting spectral composition from short, high-energy UV and blue in the middle of the day to long, red wavelengths at sunrise and sunset is what keeps our master clock and peripheral clocks in sync with the environment. Our body evolved to anticipate these changes, adjusting hormone release, metabolism, and energy use according to the natural light spectrum.
Because the Earth rotates and tilts on its axis, the angle of sunlight changes throughout the day:
- Morning (sunrise, low angle): UV missing, blue light is still present but mixed with red/orange, giving a gentle wake-up signal.
- Midday (sun overhead, high angle): UV rises, Blue light present and the without missing the red component (almost 50% of the spectrum)
- Evening (sunset, low angle again): Blue light fades, red and orange dominate.
This shifting spectral composition from short, high-energy UV and blue in the middle of the day to long, red wavelengths at sunrise and sunset is what keeps our master clock and peripheral clocks in sync with the environment. Our body evolved to anticipate these changes, adjusting hormone release, metabolism, and energy use according to the natural light spectrum.
