A candle placed 25 cm from a lens forms an image on a screen placed 75 cm on the other side of the lens. The focal length and type of the lens should be

Options

A. `+18.75` cm and convex lens
B. `-18.75` cm and convex lens
C. `+20.25` cm and convex lens
D. `-20.25` cm and convex lens

Answer

A

Explanation

Here , as per the Cartesian sign convention, Object distance , u = - 25 cm
Image distance , `v=+75 cm` (as image is real)
According to thin lens formula
`1/f+1/v-1/4=1/(+75)-1/(-25)=1/75+1/25`
`=(1+3)/75=4/(75)" or " f = (75)/4=+18.75cm `
Since f is positive , the lens should be convex lens .