What is astigmatism?

When white light is incident on an object and it reflects all its constituent colors,what color does it appear to be?

$A$ student cannot read properly from the blackboard while sitting on the front desk in a large classroom. However,he can read clearly while sitting on the last desk of the classroom.
$(a)$ Draw ray diagrams to illustrate the formation of the image of the blackboard writing by his eye lens when he is seated at the $(i)$ front desk,$(ii)$ last desk.
$(b)$ Name the defect of the eye from which the student is suffering.
$(c)$ Name the type of lens that would enable him to see the blackboard writing clearly when seated at the front desk. Draw a ray diagram to illustrate how this lens helps him to see clearly.

What is the least distance of distinct vision of a normal human eye (in $cm$)?

Why does the clear sky appear blue? How would the sky appear in the absence of Earth's atmosphere?

$A$ person suffering from an eye defect uses lenses of power $-1\, D$. Name the defect he is suffering from and the nature of lens used.