IIT JEE , AIEEE Forum - Physics , Optics

varun.tinkle

An object and a screen are placed 80 cm apart . There r 2 positions at which a convex lens forms a real image on the screen, the magnification in the 2 cases being 3/2 and 2/3 .Calculate the focal length of the screen.

(p.s plzzzzz give me the way hw to solv these types of sum)

supralara

take the dist of object from lens =x
screen =80-x

then apply mag formulae

then solve for x

eistien

hey for that magnification to be 3/2 then the convex lens must be placed greater than 2f from object let the distance from 2f be x now this screen will be f+x from screen now put this in magnification formula and then 2f+x+f+x =80 thats all 2 equations 2 unknowns the shortcut here is that u never considered the second case of magnification think abt it!!!!

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