$A$ prism $ABC$ (with $BC$ as base) is placed in different orientations. $A$ narrow beam of white light is incident on the prism as shown in the figure. In which of the following cases,after dispersion,the third colour from the top corresponds to the colour of the sky?

Name the component of white light that deviates the least and the most while passing through a prism.

What is the far point of a normal human eye?

Which part of the eye controls the amount of light entering the eye?

Dispersion of white light occurs because every colour travels with a different speed in a transparent medium.

$A$ 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.