IIT JEE , AIEEE Forum - Physics , Optics - Help meeeeeeeeeeeeeeeeeeeeee...........................................

ManuAgrawal

Please solve the question no 40 or the last question of the ray optics of ncert text book part II

iitkgp_bipin

Please post the complete question here itself.

ManuAgrawal

An equiconvex lens with radii of curvature of magnitude r each is put over a liquid layer poured on top of a plane mirror. A small needle with its tip on the principle axis of the lens is moved along the axis until its inverted real image concides with the needle itself. The distance of the needle from the lens is measured to be 'a'. On removing the liquid layer and repeating the experiment the distance is found to be 'b'.
Given that two values of distances measured represent the focal length values in the two cases, obtain a formula for the refractive index of the liquid.

ManuAgrawal

Please replyyyyyyyyyyyyyyyyy.................. someone my boards are on the head.

nandithac

when there is liquid the lens acts as a planoconcex lens
with R1=R(RADIUS OF CURVATURE OF CONCEx LENS)
R2=INFINITY(PLANE SURFACE)
THE FOCAL LENGHT OF THE COMBINATION IS GIVEN BY;
1/F=1/f1+1/f2
where f1=f of convex lens
f2=focal lenght of liq lens
1/f2=(n-1){1/R1-1/R2)
=(n-1)/R1
in the problem a=F and b=f1 so solve for f2 and substitute it in the eqn and then u'll get an expression for n

nandithac

i hope it is clear

ManuAgrawal

Please explain it more clearly and in detail.

nandithac

when there is water below the lens it acts as a planoconvex lens and when the object coincides with the image it impies it is the focal length of the combination this can be understood frm the ray diagram...
and when there is no liq it wud b the focal length of the convex lens because of the presence of plane mirror......
u'll understand it by drawing the ray diagram.......
and from this the above follows

gaurav3sharma

HERE IT IS... I HAVE DONE IT BY TWO METHODS.....NONE OF THEM ARE IN THE NCERT BOOK....PLS DOWNLOAD THE IMAGE FOR MORE CLARITY...

gaurav3sharma

here goes the second method..

Ask & Discuss Questions with Community & Experts