$(a)$ How does the sky appear from the surface of the moon?
$(b)$ What is the cause of the blue colour of the ocean?

$A$ person cannot see objects distinctly when placed beyond $2\, m$.
$(a)$ Identify the eye defect.
$(b)$ Give two reasons for this defect.
$(c)$ Calculate the power and nature of the lens he should be using to see the distant objects clearly.
$(d)$ Draw the ray diagrams for the defective and the corrected eye.

Why do we observe differences in the colours of the sun during sunrise,sunset,and noon?

In the dispersion of white light by a prism,which color of light undergoes the minimum deviation?

What is the range of vision for a normal human eye?

Sushil went to an eye specialist for a check-up. The doctor prescribed him to use spectacle lenses of $+0.5 \text{ D}$ power.
$(a)$ Name the defect of vision he is suffering from.
$(b)$ Find the focal length of the spectacle lens.