▁the ▁indirect ▁sun light ▁t int ing ▁the ▁sky ▁yellow , ▁orange , ▁red , ▁and ▁blue . ▁This ▁effect ▁is ▁caused ▁by ▁the ▁relative ▁diffus ibility ▁of ▁shorter ▁w avel ength s ▁( bl uer ▁ray s ) ▁of ▁visible ▁light ▁versus ▁the ▁longer ▁w avel ength s ▁( red der ▁ray s ). ▁During ▁the ▁blue ▁" hour ", ▁red ▁light ▁passes ▁through ▁space ▁while ▁blue ▁light ▁is ▁scattered ▁in ▁the ▁atmosphere , ▁and ▁thus ▁reaches ▁Earth ' s ▁surface . ▁Blue ▁hour ▁usually ▁last s ▁about ▁ 2 0 – 3 0 ▁minutes ▁right ▁after ▁sun set ▁and ▁right ▁before ▁sun r ise . ▁For ▁instance , ▁if ▁the ▁sun ▁sets ▁at ▁ 6 : 3 0   p . m ., ▁blue ▁hour ▁would ▁occur ▁from ▁ 6 : 4 0   p . m . ▁to ▁ 7 ▁p . m .. ▁If ▁the ▁sun ▁were ▁to ▁rise ▁at ▁ 7 : 3 0   a . m ., ▁blue ▁hour ▁would ▁occur ▁from ▁ 7 ▁a . m . ▁to ▁ 7 : 2 0   a . m .. ▁Time ▁of ▁year , ▁location , ▁and ▁air ▁quality ▁all ▁have ▁an ▁impact ▁on ▁the ▁exact ▁timing ▁of ▁blue ▁hour . ▁ ▁Blue ▁hour ▁phot ography ▁Many ▁artists ▁value ▁this ▁period ▁for ▁the ▁quality ▁of ▁the ▁soft ▁light . ▁Although ▁the ▁blue ▁hour ▁does ▁not ▁have ▁an ▁official ▁definition , ▁the ▁blue ▁color ▁spectrum ▁is ▁most ▁prominent ▁when ▁the ▁Sun ▁is ▁between ▁ 4 ° ▁and ▁