what is astigmation and how is it caused?

also why the path of light becomes visible when milk is added to it 

Astigmatism:

Astigmatism  is the inability of the eye in focussing objects in horizontal or vertical lines clearly.

i.e. inability to focus all lines of an object in one plane in a single image plane.

This defect arises because the cornea does not have a spherical shape, it curves more in one direction than in another.

Astigmatism is an eye condition with blurred vision as its main symptom.

This defect can be corrected by using cylindrical lenses, so that the convergence produced by the eye lens and the cylindrical lens together will be the same.

 

If a beam of light is passed through a milk solution, the path of light becomes visible because in milk solution the particles are large enough to scatter the beam of light. This scattered light enters our eyes.