$A$ person suffering from an eye defect uses lenses of power $-1\, D$. Name the defect he is suffering from and the nature of lens used.

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.

Draw a neat labelled diagram of the structure of the human eye and mention the functions of any four parts.

$A$ person is advised to wear spectacles with concave lenses. What type of defect of vision is he suffering from?

$(a)$ State two main causes of a person developing near-sightedness (myopia). With the help of a ray diagram, suggest how this disability can be corrected.
$(b)$ The far point of a myopic person is $150 \ cm$ in front of the eye. Calculate the focal length and power of the lens required to enable him to see distant objects clearly.

$(a)$ 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.