How far into the sky does the light from Yoko Ono's Peace Tower travel?
Guðjón Rúnar Sigurðsson
AnswerThe short answer is that there is no particular limit to the distance it travels. If we were out in space and inside the ray of light, and there were no clouds between us and the light source, we could see it, either with the naked eye or with the appropriate equipment. With sufficiently good equipment, we would be able to "see" or detect the light quite far out in space. The distance at which it is detectable depends on the quality of the equipment used, and increases as it gets better. Seen from the outside, the light column fades in proportion to altitude while it penetrates the atmosphere. This is because the light itself is attenuated, while the column widens and the air gets thinner. The light mostly fades due to scattering, but also due to absorption by particles in the atmosphere. The column also spreads over a greater surface area the further up it goes, so the proportion of light to surface area decreases with altitude. This is just as much the case in the vacuum of space as it is within the Earth's atmosphere. If we stand outside the light column, we do not see the white light emitted directly by the light source. We only see signs of it, and we interpret these signs as a column. They reach us as photons (light particles) that scatter from the column and subsequently hit our eyes. Particles in the air scatter the light and thus become visible. The particle density falls with increased altitude in the atmosphere, and signs of the light in the column are reduced. The effects are still visible at 100 km altitude, where the density of the atmosphere is less than a millionth of that at the surface of the Earth. We might say the light column ceases to be visible from the outside at approximately this height, if it hasn't already been obscured by clouds.
- Rayleigh scattering is named after the English physicist Lord Rayleigh (1842-1919). It is caused by particles or uneven density in particles (pressure variations) smaller than the wavelength of light. The wavelength of visible light is between 400 and 700 nm (nm, nanometer is 10-9 m or one billionth of a meter). Rayleigh scattering is highly dependent on wavelength, and is 10 times stronger for blue light than for red. It gives the sky its blue colour despite the fact that sunlight is white. In the same way, it gives the Peace Tower its blue texture where the air is clear, although the original light source is white, as can be seen in the picture.
- Mie scattering is named after the German physicist Gustav Mie (1869-1957). It is caused by particles that are much larger than a wavelength in diameter. Its strength is independent of wavelength and it therefore has a white texture. Tiny drops of water, small ice crystals (clouds and fog) and dust particles create Mie scattering, as can be seen from the white cloud and its surroundings in the picture.
- The photograph of the Peace Tower was taken by Jón Reykdal at the request of the Icelandic Web of Science and we express our gratitude to him. More photos of the Peace Tower can be viewed here © Jón Reykdal.
Um þessa spurningu
Ari Ólafsson. „How far into the sky does the light from Yoko Ono's Peace Tower travel?“. The Icelandic Web of Science 30.4.2008. http://why.is/svar.php?id=47440. (Skoðað 23.3.2017).
Ari Ólafssondósent í eðlisfræði við HÍ