The images are superimposed and final image re-inverted in the brain. The effective optional functioning of the eye is hampered if any of these components is affected. Transparency of the media can be affected by infection, dryness, injury, systemic diseases like diabetes, old age etc. The light sensitivity of the retina depends upon a pigment called visual purple which is derived from vitamin 'A', Retina can also be affected by strong light, systemic diseases like diabetes, hypertension, etc. Infections like tuberculosis also affect it.
Systemic diseases can affect the third component or the optic nerve.
Infections, alcohol,tobacco, certain drugs especially anti tubercular drugs. The lids protect it from foreign bodies and injuries. Tears act as lubricant, and cleansing as well as antiseptic agents. They also provide nourishment to the cornea. The she oft pupil becomes small in strong light thus cutting off excess light reaching the retina.