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

Which scientist established that white light is composed of seven constituent colors?

Which of the following statements is correct regarding the propagation of light of different colours of white light in air?

How is astigmatism corrected?

What is meant by the far point?

$A$ person needs a lens of power $-4.5\, D$ for the correction of her vision.
$(a)$ What kind of defect in vision is she suffering from?
$(b)$ What is the focal length of the corrective lens?
$(c)$ What is the nature of the corrective lens?