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.

The focal length of the eye lens can be changed due to the action of $......................$.

What is the other name for the accommodation of the eye?

$(a)$ 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.

$A$ person needs a lens of power $-5.5 \, D$ for correcting his distant vision. For correcting his near vision,he needs a lens of power $+1.5 \, D$. What is the focal length (in $cm$) of the lens required for correcting near vision?

Study the diagram given below and answer the questions that follow it.
$(i)$ Identify the defect of vision represented in this case. Give a reason for your answer.
$(ii)$ Mention the two causes of this defect.