What is the number of receptors contained in the retina?

The far point of a myopic person is $80 \, cm$ in front of the eyes. What is the nature and power of the lens required to enable him to see very distant objects distinctly?

Why is the blind spot so called?

Draw ray diagrams each showing $(i)$ myopic eye and $(ii)$ hypermetropic eye.

Name the part of the human eye that helps in changing the focal length of the eye lens.

$A$ person suffering from an eye defect uses lenses of power $-1 \,D$. Identify the eye defect. Draw a suitable diagram to illustrate how this type of lens corrects the defect. Give the nature of the corrective lens to be used. Also,calculate the focal length of the corrective lens.