What would have been the colour of the earth's sky if there were no atmosphere?

What is the range of clear vision for a person with normal vision?

$(a)$ With the help of a suitable diagram,explain why the Sun is visible to us two minutes before the actual sunrise and two minutes after the actual sunset.
$(b)$ Name the phenomenon responsible for the apparent flattening of the Sun's disc at sunrise and sunset.

$(a)$ What will be the colour of the sky in the absence of an atmosphere?
$(b)$ Why are traffic light signals (or danger signals) of red colour?

Which of the following is not a secondary color?

Which liquid fills the space behind the cornea?