From: Rob Burns 8/07/99 8:41:15 Subject: Blue Sky post id: 22397 Question - Why is the sky blue. Recently one of my colleagues asked me why the sky was blue. I gave him what I could remember of the answer my high school physics teacher had given me to the same question. I believe it has to do with the refraction of the light from the sun by the earth's atmosphere, much like a prism breaking white light into its full spectrum. The thickness and make up of the atmosphere results in blue being the most predominant colour that we see. The colour of the sky on another planet, say Mars, would therefore not be the same necessarily. I was also informed that sunrise and sunset is predominantly red or yellow because relative to you there is more atmosphere between you and the light source than there would be when the light is directly over head and the refractive properties are different. Can anyone add to this? From: Chris W (Plebeian) 8/07/99 9:10:29 Subject: re: Blue Sky post id: 22401 The sky's blueness is caused by an effect called Rayleigh Scattering. The molecules that make up the atmosphere scatter light at the blue end of the spectrum more efficiently than light at the red end of the spectrum. From: Martin B 8/07/99 9:45:18 Subject: re: Blue Sky post id: 22410 Further on this. We see the sky because air molecules scatter sunlight. Air molecules are much smaller than visible light waves and do not absorb them very well. But the visible light wavescan still make the air molecules vibrate. Vibrating molecules can briefly absorb energy then release it in a different direction. This is called scattering. Light waves that vibrate air molecules faster are scattered more. Blue light has a higher frequency than other colours, so it is scattered most. Usually we are looking away from the sun and see the scattered blue light. At sunset, looking towards the sun, we see the red light that is scattered least. From: Martin B 8/07/99 9:49:10 Subject: re: Blue Sky post id: 22412 And a bit more. The sky on Mars is pinkish. This is due not to scattering, but due to particles of (whatever it is that makes the surface of Mars red, iron oxide maybe?) suspended in the atmosphere. The sky in Venus is also quite red. This is due to the heavy atmosphere scattering all blue light well before it reaches the surface, similar to the red sunsets on Earth. In fact the atmosphere of Venus is so thick that you can't see the sun at all, and the light is very dull, but definitely red. From: Chris W (Plebeian) 8/07/99 9:52:41 Subject: re: Blue Sky post id: 22414 Thanks Martin B. I was looking for a succinct description of the process but didn't have one to hand. From: Martin B 8/07/99 10:04:37 Subject: re: Blue Sky post id: 22421 That's Ok. Producing succinct descriptions of these things is my stock-in-trade at the moment, so testing out these gems of prose on all of you is about the only way I can justify hanging out on this site as being work. So if any of you want to make any suggestions about my descriptions of 'blue sky', 'rainbows', 'lightning' or other natural phenomena, please feel free. Steve (who's never heard of primus)? Don't know how I'll get "petasmoots per femtotribulations" past the bosses if they ever look. From: Terry Frankcombe 8/07/99 10:22:35 Subject: re: Blue Sky post id: 22427 A different take on the blue sky problem: Light travelling through air gets scattered, with the scattering dependent on the wavelength. You can think of this as the scattering bending the light rays as they travel to your eye, with blue being bent more than red. So for red to get to your eye you pretty much need to look straight at the source of the light (the sun). The greater bend on the blue light means that when looking away from the sun, light beams that were originally travelling somewhere else are bent arount into your eye. From: Terry Frankcombe (Avatar) 26/08/99 19:10:12 Subject: re: Blue Sky post id: 33005 Re: my last post on this thread. I was under the impression that the scattering angle was frequency dependent. I have recently discovered that this is not so, light is scattered with spherical symmetry. However, the cross-section for the scattering probability decreases with increasing wavelength. So long wavelength photons are less likely to be scattered, rather than being scattered through a greater angle a la diffraction. This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.