SENSATION OF LIGHT

Arguably our most important perceptual ability is vision. We know that vision depends on light: when there is no light, we cannot see. What are the important characteristics of light, and how do these affect the kind of information it conveys to us? Light is a form of electromagnetic radiation. ‘Visible’ light forms just a small part of the full spectrum of this radiation. The sun emits radiation over a much larger part of the spectrum than the chunk of it that we can see. Why might this be so? To answer this question, it may help to consider why we do not see the two parts of the spectrum that border on the visible part. Ultra-violet radiation There is plenty of ultra-violet (UV) radiation about, especially as you get nearer to the equator and at high altitude. You will have heard about your skin being at risk of sunburn when there is a lot of UV radiation around you. Sunburn is the first stage of the process of the skin dying as a result of damage. So we know that UV radiation is damaging o skin, and presumably other biological tissue too. This is the most likely explanation for our eyes having an in-built filter to remove UV radiation. To put it simply, if we were able to see UV rays, they would be likely to damage our eyes. Some animals do possess UV vision, especially insects and birds. It is thought that they are less vulnerable to this hazardous radiation because they live a shorter timespan than humans. Our eyes must function throughout a long lifetime. Other forms of short-wavelength information, such as X-rays and gamma rays, are even more damaging to tissue, but these are filtered out by the earth’s atmosphere. Infra-red radiation Why are we unable to see infra-red (IR) radiation? Would it be helpful if we could? The answer to the second question is certainly ‘yes’. IR radiation is given off in proportion to an object’s temperature. This is why it is used in night-vision devices, which can locate a warm object, such as a living body, even in the absence of light. This information coul be extremely useful to us. So why do we not see it? Precisely because we are warm creatures ourselves. Imagine trying to see while holding a strong light just below your eyes. The glare from the light prevents you from seeing other objects. In the same way, we would suffer from glare if we could see IR radiation. It would be like having light-bulbs inside your own eyes. Again, some animals do see IR radiation, but these are coldblooded creatures, such as pit vipers, which do not suffer from this glare problem. The IR information is very useful in helping them to locate warm objects, such as the small mammals they hunt for food.

Humans build devices that transform IR into visible light – useful for armies (and the psychopath in the movie Silence of the Lambs) needing to ‘see’ warm objects at night, such as vehicles with hot engines and living humans. More humane uses of this technology include looking for living earthquake victims. A Landover is clearly visible from its engine’s heat. A normal photo of this scene would simply look black. Visible light – speed and spatial precision Light travels extremely quickly, at a rate of about 300,000 km per second. In effect, this means that light transmission is instantaneous. So we cannot determine where light is coming from by perceiving differences in arrival time. No biological system exists that could respond quickly enough to signal such tiny time intervals. One of the fastest neural systems in humans is the auditory pathway, which can sense differences in the time of arrival of sound waves at each side of the head. Such differences are of the order 1 ms, or one-thousandth of a second. As light travels so much faster, the equivalent difference in time-of-arrival we would need to detect would be one millionth of a millisecond. This is impossible for neurons to resolve. Fortunately, the other major property of light means that we do not need time-of-arrival information to know where the light is coming from. In transparent media such as air, light rays travel in straight lines, enabling it to convey information with high spatial precision. This means that two rays of light coming to me from adjacent leaves on the tree outside the window, or adjacent letters on this page, fall on different parts of the retina – the part of the eye that translates optical image information into neural signals. As a result of this simple property (travelling in straight lines), we can resolve these separate details. In other words, we have a high degree of directional sensitivity, [directional sensitivity similar to acuity] or a hig acuity. [acuity the finest detail that the visual (or other) system can distinguish] Without this property, the light from adjacent letters on this page would become irretrievably jumbled and we would not be able to resolve the letters.