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 $90^{\circ}$?

Is the position of a star as seen by us its true position? Justify your answer.

$A$ person needs a lens of power $-0.5 \text{ D}$ for correcting his distant vision. For correcting his near vision,he needs a lens of power $+1.5 \text{ D}$. What is the focal length of the lens required for correcting his $(i)$ distant vision,$(ii)$ near vision?

How is the sense of vision carried from the eye to the brain?

$(i)$ What is meant by dispersion of white light? Draw a diagram to show the dispersion of white light by a glass prism.
$(ii)$ Light of two colours $A$ and $B$ pass through a prism. $A$ deviates more than $B$ from its path of incidence. Which colour has a higher speed in the prism?

Draw a neat diagram to show the refraction of a light ray through a glass prism,and label on it the angle of incidence,and angle of deviation.