What light is

Electromagnetic radiation that we can see

TWO OPPOSING THEORIES. EACH TURNED OUT TO BE ACCURATE

People have tried to comprehend and define light since the beginning of human intelligence. Many great minds have applied themselves to the study. Two of the greatest minds to wonder about the nature of light were active at roughly the same time and came up with apparently diametrically-opposed theories as to what light might be.

The British mathematician and physicist, Sir Isaac Newton (1643 -1727), postulated that light is an emission of tiny particles. His chief rival in the matter, the Dutch astronomer, physicist and mathematician Christiaan Huygens (1629-95) developed a wave-theory of light. Each, of course, had data and scientific observations to back his theory. Each, with his own theory as a basis, went on to other discoveries and inventions that would not have held true if his theory had been incorrect. Huygens began grinding superior lenses, enabling him to discover a satellite of Saturn. Newton proposed that sunlight is composed of light of various colors and refined his theory along these lines.

It seemed that the two greatest minds ever to deal with the definition of light were pulling in opposite directions. If one was right, the other had to be wrong, didn't he? Could light be both energy and matter, wave and particle?

LIGHT DOES HAVE A DUAL PERSONALITY

The answer is yes. The argument has been resolved. Modern science allows that both theories are substantially accurate. Some experiments in quantum theory have light behaving as a wave. In others, light seems to take on all the characteristics of a particle. Light can be considered as both energy and matter.

Einstein’s well-known equation, E=mc ("E" being energy and "m" being the mass of matter), gave substance to the fact that all energy is a form of matter. Light is a prime example.

LIGHT AS A FORM OF MATTER - THE PHOTON

To photographers, most of the time it is more useful to think of light as a wave, but it is the solid or particle nature of light that interacts with the silver-based ingredients of film to produce your picture. When light reacts with photographic film, it acts like a particle, striking a molecule of silver bromide or silver iodide and giving it a jolt, moving it to make an exposure. It also acts as a particle when striking a digital camera's image sensor.

It is the smallest unit of light – the photon – that we are describing here: light as a particle, when it is most like a solid. A photon is a tiny bundle of energy, very much like a particle except that it has no mass. If you were able to collect a pile of photons, no matter what size the container, the contents would weigh nothing.

LIGHT AS A FORM OF ENERGY - THE LIGHT WAVE

In the majority of light’s relevances to photography, light can be described as acting like a wave, as energy - not matter - that can spread, bend and react with obstacles just like waves in water, and most of our study of light in this section will refer to light waves rather than to photons.

WHERE DOES A LIGHT WAVE COME FROM?

We have seen that light is a form of energy that travels as photons. The energy within the photon creates an electromagnetic field around the photon, resulting in electromagnetic radiation, part of a vast spectrum that includes radio waves, radar, heat, and X-rays. Visible light, which concerns us as photographers, is a minuscule part of the spectrum. (Note that objects illuminated by infrared light, which is invisible to humans, can also be photographed.)

An electromagnetic field is invisible. It can be detected, however, when it exerts its force on a material object in the field. This seems difficult to comprehend, but fortunately there is a common example to illustrate it, and that is the magnetic field surrounding an ordinary magnet. You can't see the magnet's field, but you know it is there when a magnet attracts a nail.

THE AMOUNT OF LIGHT'S ENERGY DETERMINES ITS COLOR

The strength of the electromagnetic field surrounding the photon fluctuates according to the amount of energy within - the more energy, the faster the fluctuation. The rate of fluctuation is called "frequency" and is measured in units called "hertz" - the number of wavelengths that pass a given point in one second.

It is the difference in the rate of fluctuation that the human eye perceives as color. Different color sensations are produced by light vibrating at different frequencies.

The visible spectrum of light can also be defined in terms of wavelength which ranges from violet light with a wavelength of 35 millionths of a centimetre to 75 millionths of a centimetre for red. Higher frequencies corresponding to shorter wavelengths are called ultraviolet and still higher frequencies are associated with X-rays (1,000,000 million megaHertz). Lower frequencies, at lower wavelengths, are called infrared while even lower frequencies are characteristic of radio waves (10 megaHertz, for instance). It seems weird to think of light waves as being almost the same as radio waves. Yet the only physical difference between the two is their length.

FILM AND DIGITAL SENSORS CAN "SEE" LIGHT THAT WE CAN'T

We needn't concern ourselves too much with the extremes of the electromagnetic spectrum, but the "colors" ultra violet and infrared that are just beyond the range of light available to our vision (i.e. that are invisible to us) can still be "seen" by the sensors of digital cameras and by certain types of film and equipment, and therefore do concern us. For example, all ordinary black-and-white films are sensitive to some light we cannot see, and they are more sensitive to blue light and less sensitive to red light than our eyes are. You may have seen the effects of this yourself in your photographs where, for example, the deep blue sky you thought you were capturing on black-and-white film turned out as blank white and the pale red flower you saw photographed as dark red. With knowledge of a few basic facts about light, you can compensate for effects such as these by employing filters.

ALL COLORS COMBINE TO MAKE WHITE

Although the sun radiates each individual color wavelength in the range of visible light, the mixture created by them all together registers in our brains as white.

Daylight consists of all the colors of the rainbow - which can be remembered by the familiar “ROY G BIV” that stands for Red, Orange, Yellow, Green, Blue, Indigo and Violet, the colors that we see in that order in a rainbow. We call it "white light," even though it appears colorless, because any object that reflects all of the colors in normal daylight is white. A colored object reflects only some of the colors found in daylight, and absorbs the rest. A lime is green because it reflects green light and absorbs blue and red light.

If something reflects none of the colors, but instead absorbs them all, it is black. See How Photographers Talk About Light for more information on light and color.

THE SOURCE OF LIGHT IS IMPORTANT TO PHOTOGRAPHERS

Light sources other than the sun, such as incandescent (common household) bulbs, emit a range of different wavelengths, containing more long ones (red) than short wavelengths (blue). This is because incandescent filaments cannot match the sun’s temperature, and wavelength balance is dependent on temperature. An incandescent bulb cannot produce as many short wavelengths as the sun.

When you know this as a photographer, you can therefore expect a reddish cast in your color pictures when taking pictures indoors in incandescent light with film that is balanced for daylight, or when you have not set your digital camera's white balance for this type of illumination. You can also take steps to counterbalance the reddish cast to create a more natural-looking photograph by using filters or by adjusting your white balance.

Fluorescent tubes give off light that is a mixture of relatively few colors. Fluorescent lighting can produce unnatural-looking results in photographs. You may, for example, have noticed a sickly-greenish tinge in pictures you have taken under fluorescent lighting. The good news is that some of today’s color films (like Fuji Reala) will produce natural-looking color when photographed in either daylight or fluorescent light conditions, and most digital cameras can be adjusted for white balance to help overcome the effect of fluorescent light sources.

Most electronic flash units are balanced so that their wavelengths closely simulate sunlight. Pictures taken using electronic flash appear to be more naturally-lit.