<P> where R is the refraction in seconds of arc, b is the barometric pressure in millimeters of mercury, and t is the Celsius temperature . Comstock considered that this formula gave results within one arcsecond of Bessel's values for refraction from 15 ° above the horizon to the zenith . </P> <P> A further expansion in terms of the third power of the cotangent of the apparent altitude incorporates H, the height of the homogenous atmosphere, in addition to the usual conditions at the observer: </P> <Dl> <Dd> R = (n 0 − 1) (1 − H 0) cot ⁡ h a − (n 0 − 1) (H 0 − 1 2 (n 0 − 1)) cot 3 ⁡ h a . (\ displaystyle R = (n_ (0) - 1) (1 - H_ (0)) \ cot h_ (\ mathrm (a)) - (n_ (0) - 1) (H_ (0) - (\ frac (1) (2)) (n_ (0) - 1)) \ cot ^ (3) h_ (\ mathrm (a)).) </Dd> </Dl> <Dd> R = (n 0 − 1) (1 − H 0) cot ⁡ h a − (n 0 − 1) (H 0 − 1 2 (n 0 − 1)) cot 3 ⁡ h a . (\ displaystyle R = (n_ (0) - 1) (1 - H_ (0)) \ cot h_ (\ mathrm (a)) - (n_ (0) - 1) (H_ (0) - (\ frac (1) (2)) (n_ (0) - 1)) \ cot ^ (3) h_ (\ mathrm (a)).) </Dd>

What happens to visible light that enters the atmosphere