A needle placed 45 cm from a lens forms an image on a screen placed 90 cm on other side of the lens. Identify the type of lens and determine its focal length, What is the size of the image, if the size of the needle is 5 cm?

Explanation

Here `u = 45 cm v= 90 cm f= ? H_(2) , ? H_(1) = 5`
`:. , (1)/( v) - (1)/(u ) = (1)/( f)`
`:' , (1)/(90) + (1)/( 45) = (1)/(f )`
`rArr (1+2)/( 90) = (1)/( f)` or ` f= 30 cm`