$A$ presbyopic patient has a near point of $30 \ cm$ and a far point of $40 \ cm$. The dioptric power of the corrective lens required for seeing distant objects is: (in $D$)

$A$ student studying the similarities and differences between a camera and the human eye makes the following observations:
$I$. Both the eye and the camera have convex lenses.
$II$. In order to focus,the eye lens expands or contracts while the camera lens moves forward or backward.
$III$. The camera lens produces upside down real images while the eye lens produces only upright real images.
$IV$. $A$ screen in a camera is equivalent to the retina in the eyes.
$V$. $A$ camera adjusts the amount of light entering in it by adjusting the aperture of the lens. In the eye,the cornea controls the amount of light. Which of the above statements are correct?

The circular part in the centre of the retina is called:

The sensation of vision in the retina is carried to the brain by

If the far point of a short-sighted person is $400 \ cm$, then the power of the lens required to enable him to see very distant objects clearly is

The far point of a myopic eye is $250 \, cm$. What is the focal length of the lens required to correct this defect?