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How we see light

Most living creatures have the ability to see, albeit in very different ways. Generally speaking, what we see is defined by three factors:

  1. Light Source

  2. Receiving Object

  3. Observer

Although this sounds fairly straightforward, the interaction between these three factors is hugely complex and I won’t even attempt to provide a full explanation. However, in terms of understanding printed artwork, for instance, it is worth having a basic appreciation of how and what we see, if only to understand that what we think we see depends on the conditions of these three factors and these conditions can change and, hence, give us a different result. Confused? Let’s try to give it some substance:

Light Source

It all starts when the atoms of a heat source give off thermal energy, resulting in producing photons. These photons are literally ‘packets’ of energy, travelling through space, whilst pulsating in a regular pattern. All photons travel at the same speed (the speed of light), but they may pulsate at different intervals. Those that pulsate faster, contain more energy than the slower pulsating photons. These pulsating actions are better known as waves. The distance between each wave (or pulse) is measured in nanometers (one nm equals a billionth of a metre) and the faster pulsating photons have a shorter wavelength and contain more energy. It’s these differences in wavelength that produce the different colours we see. As humans, we can only see colours that are produced by wavelengths ranging from 380nm to 700nm. Shorter wavelengths produce colours towards the blue end of the spectrum and longer ones tend to be more towards the red. This range is called the visible spectrum. Anything outside these boundaries, cannot be detected by the human eye. Roughly speaking light could be broken down in the following bands (from longer to shorter waves):

Red, Yellow, Green, Blue, Indigo, Violet

or as many will remember from their school days:

Richard of York Gained Battles In Vain

Receiving Object

The second requirement in order to see light is for wavelengths to interact with an object. The result is that some wavelengths will be absorbed by the atoms that make up the object, whilst other wavelengths will bounce back. This phenomenon is also known as spectral reflectance. As surfaces absorb and re-emit lightwaves differently, due to their chemical structures, their light will also be perceived as different.

So, for example, if an object absorbs the longer wavelengths and bounces back more shorter wavelengths then the colour will appear towards the blue side of the spectrum. Vice versa, if the longer wavelengths are bounced back, the object may appear more red. Black is produced when all light is absorbed and an object appears white when all wavelengths bounce back from its surface.

The Observer

We now have an appreciation of the origin of a light source (packets of energy travelling through space at the speed of light) and the creation of light by interacting with an object. Finally, it’s down to us (the observer) to translate this phenomenon and tell our brain what we see. This is by far the most complex part of the story as it is dependent on many variables.

We all know that the eye observes light and colour and then sends it to the brain, where it’s explained and starts to make sense.

Very briefly, these are the stages that we go through in order to see colour.

First, light passes through the cornea (the clear front layer of the eye). The cornea is shaped like a dome and bends light to help the eye focus.

Some of this light enters the eye through an opening called the pupil. The iris (the colored part of the eye) controls how much light the pupil lets in, a bit like opening or closing the aperture inside a camera.

Next, light passes through the lens (a clear inner part of the eye). The lens works together with the cornea to focus light correctly on the retina.

When light hits the retina (a light-sensitive layer at the back of the eye), special cells called photoreceptors turn the light into electrical signals.

The photoreceptors are divided into “rods” and “cones”, where rods are more sensitive to low light and cones function better in bright light conditions. The cones are split into three types, dealing with short, medium and long wavelengths respectively.

Once the eye has done its work, receiving the colour signals, the optic nerves in both eyes pick up the results and send it to the brain. Then the brain turns the signals into the images you see. Of course, it gets pretty complex, between the signals travel through the brain and eventually being translated into something we recognise but perhaps that part would be best left to the medical profession to deal with.

Finally, it’s fairly obvious now that the light we see can be very subjective and depends on a series of interactions between the light source itself, the object that is being seen and the observer, who makes sense of the spectacle displayed in front of them.

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