Question

what causes atmospheric refraction??mention its 2 effects..

Answer 1

atmospheric refraction is caused because different layers of the atmosphere are not equally denser. some are rarer and some are denser. so when the light rays strike s the atmosphere it gets refracted due to change in the layers.
effects are, star appears to be twinkle

we can see the sun rise and sunset about 2 mins earlier and 2mins after the actual sunset