Subject: Does the earth's atmosphere act like a gas lens?

If you consider that the earth's atmosphere
forms an optical lens, a spherical obstructed(?) lens,
due to its spherical shape and varying density with
altitude, what would be its focal length?

If this is true, then sun light passing through the 'lens'
would be concentrated in an area behind the earth?

Does this area have a name? 
Has it been observed?

Thanks.

Re: Does the earth's atmosphere act like a gas lens?