Generating light – more than just flicking a switch

You are here: Nature Science Photography – Natural light – The light

So much for the basics, but true to the motto „only what we can see counts„, the rest of the material deals largely with the narrow range of the visible spectrum of electromagnetic radiation, for which we want to retain the designation „light“.

Starting at 400 nm wavelength, light first appears to us as violet-blue, then green (500 nm), yellow (570 nm), orange (600 nm) and red at 650 nm.

Among the most important light-generating groups are the temperature radiators, such as the sun, a candle or even an incandescent lamp. Heating a body means supplying it with energy and thus stimulating its atoms to emit radiation. The higher the temperature, the greater the excitation, and the higher the frequency of the emitted radiation. Everyday life teaches us that a heated body will eventually glow and emit light. The higher the temperature, the greater the portion visible to us. At approximately 6,000 °C, which is the approximate temperature of the sun’s surface, the maximum of the emitted radiation precisely falls in the middle of the visible spectrum. Values below this calibration mark lead to maxima in the more long-wave red range; values above it lead to maxima at the short-wave blue end of the spectrum. The heater at home, which is around 50 °C warm, therefore emits only infrared radiation that is invisible to us, and an incandescent bulb whose filament reaches a good 2,000 °C still emits a light that is slightly reddish compared to the sun. The annoying color cast of this type of illumination on an image carrier tuned to daylight bears eloquent witness to this.

The sun emits most of its radiation between 225 nm and 3,200 nm wavelengths, but not all of it reaches us on earth. For instance, ozone reduces the various components of the atmosphere by the short blue region below 320 nm, while water vapor, or water droplets and carbon dioxide, reduces it by the portion above 1,100 and 2,300 nm. Of the remaining part, only half then falls within the section visible to us.

The discharge of an electrically charged gas also produces light. Typical representatives of this type of lamp are neon, xenon or sodium vapor lamps. They work according to the following principle: An alternating current flows through the electrodes at both ends of the gas-charged tube, continuously reversing the polarity of the charge. The resulting electric field releases some electrons from the electrodes, which collide with the gas atoms and excite them to emit a photon, i.e., discharge. The quality of the light depends on the density and especially the type of gas. Mercury vapor, for example, produces a large amount of UV radiation, while sodium or neon emit more radiation in the visible range.

The process of luminescence is another, more rarely encountered, process of light production. Unlike heat emitters, luminescence operates without heating the material due to the ability of the phosphor compounds to absorb the energy of the incident UV component and subsequently release it in an altered or unaltered form. We refer to the release as fluorescence if it occurs simultaneously with the absorption, and phosphorescence if it persists longer than the absorption.

But irrespective of the various ways in which it is generated, we encounter light in nature and under controlled conditions in a wide variety of qualities, forms and nuances, the effects of which the following chapters study with photographic interest.

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