The change in focal length of an eye lens to focus the image of objects at varying distances is done by the action of the

How is the eye lens held in its position?

Which of the following statements is correct?

$A$ narrow beam $PQ$ of white light is passing through a glass prism $ABC$ as shown in the diagram.
Trace it on your answer sheet and show the path of the emergent beam as observed on the screen $DE$.
$(i)$ Write the name and cause of the phenomenon observed.
$(ii)$ Where else in nature is this phenomenon observed?
$(iii)$ Based on this observation,state the conclusion which can be drawn about the constituents of white light.

$A$ prism causes dispersion of white light while a rectangular glass block does not. Explain.

$A$ 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.