IIT JEE , AIEEE Forum - Physics , Optics - A SIMPLE & VERY IMP. QUE. ABOUT REFRACTION...........

Ankur_1

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.

Asked by Ankur Shrivastava.

armaan

we donot take into consideration ...the thickness of the lens...

so don think this way that lens has mre ref index so the ray shud bend towards the normal....

the ray doesnt stay in the lens ,it emerges out of the lens into air....

sandeepramesh

yes its a thin lens approximation that we take

kapilBansal

This is not as they are saying to u,

We can suppose that lenses r made of large no. of prisms
In convex lens the base of prism is towards the principal axis and in concave it is away from the principal axis.

weknow that when light is incident on a prism then after refraction

thro both surfaces it is deviated towards the base.

Hence the rays incident on a convex lens after refraction at both surfaces of the prism, it bends towards the principal axis of the lens.greater is the distance of the prism from the principal axis,greater is the angle and hence greater is the deviation produced in the light ray.therefore the convex lens is convergent one and vice versa in the case of concave lens.

Rate me if u r satisfied.

pardesi

@kapil how do u justify ur prism approximations??

actually Ankur if u don't wanna have thin lens approximation leave it!!!
what u shud do is find the image of th eobject by refraction formule for a curved medium seperating two media's then find the image of the object and then use this image as the object for the second curvature and agin use the fomulae.

here the importance of thin lens comes into play see if u don't consider thin lens then the object distance as to be considred from two different points.
for the first refraction from the pole of first curvature and for the second refraction from tghe pole of the second .insted if we tqke the lens to be thing we can consider object distance froma point inebetween them at the centre it's kind of approximation
1/(x-a) + 1/(x+a) nearly equal to 2/x if a<<<<x
if u do this urself u will see the imp and convinience of such an aprroximation

kapilBansal

huy pardesi,
if u cut lens in no. of parts then they appear as no. of prisms inclined at some angle.

u can do it at home,just cut the lens horizontally in no. of parts but
if u deny i readily admit that b'coz there r more than one way of solving a problem,this was my way of thinking but u think once more then u probably admit my stand.

spideyunlimited

kapil's logic is correct dude :)

arpan1

both kapil and pardesi r right

see kapil described the actual reasoning (the theoretical part and backed it up with right arguments , yeah he is absolutely right) while pardesi defined the approximation of thin lens formula using lensmaker formula ( a more mathematical approach)

both the methods r right and inter-related

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