Light, Dark, and the Electromagnetic Spectrum
Light, Dark, and the Electromagnetic Spectrum
by Scott Benjamin Gracie
GENRE: Non-Fiction (SCIENCE / Physics)
Since the dawn of time, electromagnetic energy has permeated through the universe, surrounding and interacting with everything it touches, illuminating, destroying and giving life.
The colourful section of the electromagnetic spectrum we see in the form of visible light, rainbows and other phenomena is tiny when compared to its vast entirety. Using many parts of the electromagnetic spectrum have become common everyday occurrence for the majority of people on Earth, as it's been integrated into our lives in ways that we don't even think twice about it.
This book illuminates many parts of the electromagnetic spectrum and its effects, the endless ways we have harnessed its energy, and how we interact and live with its influence.
How We See Light
The Human Eye
Our visual perception is enabled due to light entering the eye, either directly from light sources or reflected off objects that we are looking at. This provides colour vision, light perception and depth perception. Light enters the eye first through the cornea then travels through the pupil where the amount of light entering the eye is regulated. It then travels through the lens, where it is focused as required, and is concentrated and projected onto the retina, the photosensitive cells at the rear of the eyeball. The difference in luminance colour (wavelength) and contrast is then translated into a biological electric signal and transmitted along the optic nerve to the visual processing areas in the brain. How light goes through this process and is interpreted by the brain can be slightly different between individuals. This can result in slight variations in colour perception compared to other people, of which the individual may never become aware.
The rear of the eye contains light-sensitive cells where projected light from the lens is directed. These light-sensitive cells are called cones and rods. There is a higher concentration of these cells at the fovea, the 0° position from where light enters the eye. Just below the fovea is what’s known as the ‘blind spot’, where the optic nerve attaches to the eye.
Cone Cells of the Eye & Photopic Vision
Cones cells are more active at higher light levels (photopic vision). They are highly capable of colour vision and are responsible for high spatial acuity. During photopic vision, human eyes are most sensitive to light that is yellow-green. The area on the retina called the fovea is heavily populated by three types of cones cells of short (s), medium (m) and long (l) wavelength receptors.
The properties of cones are: less photopigment than rods, fast response and short integration time, less light amplification, directionally selective, less convergent retinal pathways, lower absolute sensitivity, chromatic with three types of pigment and high acuity.
Rod Cells of the Eye & Scotopic Vision
Rods are responsible for vision at low light levels (scotopic vision). They do not mediate colour vision and have a low spatial acuity. During scotopic vision, people are more sensitive to light, which appears blueish- green. Properties of rods are: more photopigment than cones, slow response and long integration time, high light amplification, achromatic with one type of pigment, high sensitivity, low acuity, highly convergent retinal pathways and not directionally selective.
This type of vision typically occurs when both scotopic and photopic vision are operating at the same time. The kind of situation where this vision is working is in low light but not quite dark, such as being outside in night-time street lighting.
The switching from cones to rods when the eye is processing light is called ‘Purkinje shift’.
About the Author:
Scott's passion for light and the electromagnetic spectrum began in his early teens while tinkering with electronics as a hobby. He was formally introduced to the lighting industry while studying Electrical and Electronics in Sydney, Australia, and in 1998 he started his career making custom light fittings and lamps in his dad's garage. Formal lighting design qualifications expanded his opportunity to help project-manage and design on many industry levels.
Twenty-two years on, and Scott has worked on many major infrastructure, commercial and architectural lighting projects. He is a member of the Illuminating Engineering Society of Australia & New Zealand, the Colour Society of Australia, the International Dark Sky Association and the Astronomical Society of the Pacific.
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