Humans cannot see in the dark however, some animals can do. The light-sensitive part of our eye is known as the retina which is made of some special types of cells called cones and rods. The cones see coloured light and the rods detect movement and dim light. Our eyes contain six (6) million cones and one hundred and twenty (120) millions rods in the ratio 1:20. On the other hand, nocturnal creatures such as tarsiers and cats etc. are created differently and are able to see in the dark because their retinas have many more rods than the human eye. Cat’s eyes contain a special kind of mirror, called tapetum, which reflects light back out of the eyes. The tapetum reflects the incoming light through the retina twice so cats have double chances of seeing things than humans. In spite of all these features, even then cats cannot see in complete darkness.
Human eye can only detect visible portion of light which comprises seven coulors i.e. Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR). However, there are some other types of radiation as well such as Gamma rays, X-rays, Ultraviolet rays, Infrared rays and Radio waves and our eye cannot detect them. Radioactive elements e.g. radium and cesium emit gamma rays. We use X-rays in hospitals to take images of different parts of our body. Ultraviolet rays are used in killing of germs in drinking water. With the help of infrared rays, electronic devices are turned on and off such as TV and AC remote controls. Radio programs are relayed through radio waves. Apart from all these features, there are numerous other uses and applications of the these radiation. Collectively, all the above mentioned radiation is called an electromagnetic spectrum. Our vision is confined only to a small fraction of the electromagnetic spectrum but some animals (e.g. mantis shrimp), unlike humans, can see infrared and ultraviolet portion of the electromagnetic spectrum.
Our eyes do not have tapetum and enough rods so what should we do to see in the dark. To do so, we have to make use of the technology based on night vision. Night vision is the ability to see in dim light conditions and it can be developed by a suitable combination of two approaches namely sufficient light intensity range and sufficient spectral range. The basic principle of night vision is to amplify the available low light to a desired level so that the thing around us could be visible. We use the same principle in eyeglasses, binoculars and telescopes by using lenses that focus more and more light rays coming from the illuminated object. Similarly electricity, along with some special electronic circuits, is used to amplify the available low light in night vision. Based on this scientific principle, scientists have invented night vision goggles that boost dim light to a sufficient level to see in the dark.
Night vision goggles consist of a front lens, a photocathode (like a solar panel), a photomultiplier (a type of photoelectric cell which converts light into very small particles called electrons) and a phosphor screen ( like that of an old-fashioned TV). Light (in the form of photons) from a night scene, after passing through the front lens, falls on the photocathode and is converted into electrons which are multiplied by the photomultiplier.
The electrons thus produced then strike the phosphor screen which generates sufficient light to make the original scene visible. The phosphors are deliberately chosen to produce a green image of the scene because human eye is more sensitive to green colour and it is comparatively easy to look at green screen for a longer period of time.
For proper working of night vision goggles, a tiny amount of light from the night scene should be available. In the absence of this tiny light, the night vision goggles do not work. Alternately, we use a technology which exploits heat (infrared radiation), instead of light, radiated by the objects including humans in the surroundings. The image obtained by heat is called a thermal image. The digital camera fitted in the night vision goggles is equipped with a charge-couple device (CCD) which only responds to infrared instead of visible light. Night vision goggles are usually used for search and rescue, professional security, camping, boating in foggy weather or at night, observing nocturnal wildlife and night hunting or fishing. Last but not least, the actual use of night vision goggles depend upon the users to fulfill a specific objective.
(The writer is Punjab University Physics Department Chairman and Member Syndicate on Associate Professor’s seat. He can be reached at: firstname.lastname@example.org)