Black.

Hello everyone!

Today I shall be writing about the colour: Black. Yes, black the colour. Though the question of whether black itself is a colour has been debated over the years, I feel black is definitely a unique colour by itself because of its distinct appeal. The colour black has a lot of symbolism and is also ubiquitous in nature: crows, hair, the most common eye iris colour, the fabric of our existence:carbon and the amazing black colour of outer space and the beautiful night sky.

blk
The magnificent black night sky.

Thermodynamically speaking, a black body is any body which absorbs all the incident radiation on its surface, irrespective of the angle of incidence of the incoming radiation. This means it can absorb any amount of energy that is incident on its surface without reflecting it in any direction. An ideal black body has a reflectivity (ability to reflect radiation without absorbing it) and transmissivity (ability to allow radiation to pass through, like glass) of zero and an absorbtivity of one.

Idealized black body, no way for radiation to escape!

Black bodies can be conceptualized as a container from which the incident radiation is unable to escape after entering. However, as a part of a thermodynamic system, a black body has to obey the second law of thermodynamics and accordingly, release this absorbed energy. The measure of its ability to release this energy is known as emissivity and its value for an ideal black body is one, which means all black bodies release this energy completely to their surroundings in order to achieve thermal equillibrium.

Now, by virtue of this phenomenon, there exist times when black bodies do not appear black to an observer. This is because, any object with a surface temperature above absolute zero emits some form of electromagnetic radiation and if this radiation is within the range of the visible light spectrum, we can actually observe a change in its colour with temperature. Consequently, we can model the temperature of a hot body by matching the colour of its surface using an Optical Pyrometer to a known temperature range.

Colour of Black body with change in temperature in Kelvin (Wikipedia).

Practically, an ideal black body is almost impossible to manufacture. However, a lot of artificially created materials exhibit an emissivity greater than 0.95.  One of these materials is Vantablack which is probably the blackest black material created. Vantablack is so frighteningly black that people have described looking at it akin to staring in the face of a never-ending black abyss.

vantablack
Vantablack.(Mentalfloss.com)

This material is manufactured by arranging carbon nanotubes in vertical arrays with microscopically small gaps between individual molecules that prevents the photons of light from escaping the grasp of the object made of this material. Such near-black materials have a very special application in space telescopes, where they are used to absorb the stray radiation and prevent their interference with the field of view. Near black materials could also be used for military purposes as it facilitates stealth and reduces the possibility of detection by radar or thermal probing.

lockheed_sr_71_blackbird-1920x1200
Lockheed-Martin SR-71 “Blackbird” High Altitude Stealth Reconnaissance Aircraft 

I would like to end this post with a short nudge to the curiously magnificent world of Black holes. Black holes are pretty much singularities or abnormalities that kind of tweak the laws of physics. Such an amazing topic deserves an entirely different blog post, so stay tuned for the next one!

I hope this post was interesting and informative.

Until Next time! 🙂

-Chaitanya

References and further reading:

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