Physics and War: Night Vision

Physics of Night Vision

In war, it is always important to stay a step ahead of enemy. Scientific advancement is one of the means that helps in achieving it. Science and war are in a way interdependent for the same reason. As history tells us, progress in science has played a crucial role in winning wars; on the other hand the development process of science is expedited during war. Though most of the inventions during the war were made solely for the purpose of winning it, the same inventions have taken mankind a step ahead several times. Be it nuclear reaction based energy generation, RADAR, high speed aerial navigation etc, war has motivated either motivated their development or hastened the process. Let’s discuss a very simple aspect of night vision which is one such thing that helps in staying a step ahead of the enemy in all three kinds of warfare.

How do we see objects? Eyes absorb the light that falls on them and convert it into electrical signals enabling the brain to visualise it. The light that falls on the eye can be generated in two ways; one of which is when light from a source (like sun) falls on the object and is reflected off it (a tree under the sunlight) and the second is when the object itself generates light( ex: computer screen). Night vision technology largely depends on second form of light. As learnt during our Physics tuition classes, the light we see belongs to a wavelength range in the electromagnetic spectrum termed as visible range. Infrared radiation falls under electromagnetic spectrum with slightly higher wavelength compared with that of visible radiation. Infrared radiation is emitted by hot objects and is often termed as thermal radiation. The intensity of the radiation emitted by the object depends on its emissivity.

Emissivity is the efficacy of an object to emit heat in the form of radiation. Two objects of different temperatures may radiate different intensities of IR radiation due to the difference in their emissivity. Different objects show different intensities of IR radiation of the same frequency and also different objects emit IR radiation of different frequencies. Based on this difference, when one sees the IR profile of something like a war zone, one can differentiate trees and other background structures from target (like a tanker or an SUV). Since heat radiation is emitted from objects irrespective of the light falling on them one should be able to see this radiation irrespective of the presence of a light source (like Sun). This is exactly how night vision works.

A typical night vision goggles have a sensor to absorb all the IR radiation and the low intensity visible radiation which is then processed to form an image that is finally shown to the eye.