Hey Friends,

            I have a doubt in the topic refraction. Please help me in clearing this doubt. The que. is :

           When a ray of light passes from rarer medium to denser medium, it bend towards the normal and when it passes from dense medium to rarer medium it bends away from the normal on the ending surface of both the medium.

           But when a ray is passed through a lens, it should bend towards the normal on the ending surface of the air (means on the surface of the lens.) and where it goes out from the lens, there also it should bend away from the normal, since a lens is also a peice glass.

            But in books, it is written that when a ray of light parallel to the principal axis is passed through the lens it bends from the pole and passes from the principal focus.

            Please explain with reasons.

Actually the refraction takes place as soon as there is change in medium,i.e immediately the ray shall bend towards the normal as it enters the glass from air......

But because in our terms the deflection between the incident ray and refracted ray(within the glass) isnt much significant (as in the diagram) so we take that bending takes place from the pole.But nonetheless,we must remember that the ray has already deviated and is already moving in a deviated path right since the time it entered the glass,which bends towards the normal....

try drawing ray diagrams yourself to understand more.....