$A$ $14$ year old student is not able to see clearly the questions written on the blackboard placed at a distance of $5\, m$ from him.
$(a)$ Name the defect of vision he is suffering from.
$(b)$ With the help of labelled ray diagrams show how this defect can be corrected.
$(c)$ Name the type of lens used to correct this defect.

$A$ person needs a lens of power $-2.5 \ D$ for correcting his vision. Name the defect of vision he is suffering from. Which lens will he be using for the correction? Also,find the focal length of the lens.

$A$ student finds the writing on the blackboard blurred and unclear while sitting on the last desk in a classroom. He,however,sees it clearly while sitting on the front desk at about $2\, m$ from the blackboard.
$(a)$ Draw ray diagrams to illustrate the formation of image of the blackboard writing by his eye lens when he is seated at the $(i)$ last desk,$(ii)$ front desk.
$(b)$ Name the defect of the eye from which the student is suffering.
$(c)$ Name the kind of lens that would help him to see clearly even when he is seated at the last desk. Draw a ray diagram to illustrate how this lens helps him to see clearly.

Which two colors of light combine to form the color yellow?

When a light ray travels from an optically denser medium to an optically rarer medium, what is the angle of incidence called at which the angle of refraction becomes $90^{\circ}$?

Which of the following are the primary colours of light?