Can there be black light or is that just black hole?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

24 August 2008

Joe Mudaka says that he recently overheard someone talk about black light and he was left wondering “what the heck that was.

“I asked a friend and he told me that there is such thing as black light. Is it really true? I mean, isn't black light darkness and darkness the absence of light? Please help.”

Black light is a phrase used in reference to invisible electromagnetic radiation. Now, the “thing” we call light comprises of only the frequencies (or, alternatively, wavelengths) that ca be detected by our eyes.

The visible frequencies fall in a “narrow” range from about 400 trillion cycles per second (hertz or Hz) for violet colour to about 800 trillion Hz for red. In terms of wavelength, the range is from 750 nanometres (violet) to 380nm (red). And that’s not a mistake; the higher frequencies have shorter wavelengths and vice-versa.

The full range of frequencies of electromagnetic radiation that has been observed starts from a few cycles per second to a few thousand-trillion-trillion (a number with 27 zeros!). On that scale, the visible range is very narrow.

But why would the invisible frequencies be called “black light”? The answer lies in the fact that the radiation can be used to view otherwise invisible markings. You have probably seen the gadgets they use in banks to verify the authenticity of currency notes.

When the banknote is put in the gadget, some previously invisible markings are illuminated and this confirms that it is genuine. The fact that the “light” used in the illumination is invisible is what lends it the name “black” light.

The radiation used in these gadgets has frequencies slightly above those of violet colour – the so-called ultra-violet or UV. This is why some of the devices produce a fluorescent purple/violet glow.

In a related matter, Irungu Thiongo asks: “What’s the speed of light in a black hole?” Before answering that, we must first explain what a black hole is. This is a (heavenly) body that has accumulated an immense amount of mass to the point that nothing on its surface can escape from the resulting gravitational pull.

Now astronomers have observed that when a beam of light passes near very massive object it bends towards it due to the gravitational field – in the same way that a solid body (such as a planet) passing nearby would “feel” the gravity.

However, the pulling effect on a beam of light is much smaller than that on a solid body. A black hole is so massive that even a beam of light is bent back into the body. In addition, any solid particle in the vicinity is “sucked” in making the black hole even more massive.

The name black hole comes from the fact that objects seem to be pulled into an invisible place never to reappear again. That the raises another question: if no light can come from a black hole, how can we know that it exists? Well; that is a story for another day.