In article om,
Dave Liquorice writes:
On Sun, 22 May 2005 09:52:22 +0100, John Hall wrote:

That's fairly small for visible light, isn't it? I know that when
working with radar, refraction is commonly allowed for by assuming
an Earth radius of 4/3 times the true value.

Light is affected by temperature/density variations in the atmosphere,
think desert mirages or, closer to home, those patches of "water" on
summer roads that disappear as you approach. The latter is a
reflection of the sky in the the hot layer of air close to the road
surface.

Yep. But mirage conditions are the exception rather than the rule (at
least in the UK). How much is visible light refracted in the "standard
atmosphere"?
--
John Hall

"I don't even butter my bread; I consider that cooking."
Katherine Cebrian