A screen is placed 90 cm away from a luminous object. The image of the object on the screen is formed by a convex lens at two different positions separated by 30 cm. what is the focal length of the lens?

Options

A. 15 cm
B. 20 cm
C. 25 cm
D. 35 cm

Answer

B

Explanation

Distance between the object and the screen D=90 cm and distance between the two positions of the convex lens =30 cm
`therefore` for this displacement method,
`f=(D^(2)-d^(2))/(4D)=(90^(2)-30^(2))/(4xx90)`
`=((90+30)(90-30))/(4xx90)`
`therefore f=(120xx60)/(90xx4)=20cm`