What is the near point of a normal human eye (in $cm$)?

$(a)$ Explain with the help of a labelled diagram how a combination of two prisms can be used so that incident white light emerges out of the second prism as white light.
$(b)$ Name the scientist who first performed this activity.
$(c)$ Explain the term spectrum in brief.

Name the delicate membrane in the eye having an enormous number of light-sensitive cells.

$(a)$ 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.

Which color of light undergoes the least scattering due to the atmosphere?

What is long-sightedness (hypermetropia)? List two causes for the development of long-sightedness. Describe with a ray diagram how this defect may be corrected by using spectacles.