Name the colours of the visible spectrum.

$(a)$ How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?
$(b)$ State any two effects of atmospheric refraction on the sun as observed from the Earth.

How is long-sightedness corrected?

Give one cause of long-sightedness (hypermetropia).

What is the other name for the accommodation of the eye?

$A$ person cannot read a book at distances less than $50\, cm$. Name the defect of vision he is suffering from. How can it be corrected? Draw ray diagrams to show the image formation
$(i)$ by defective eye and
$(ii)$ after using corrective lens.