If tube length of astronomical telescope is 96 cm and magnification is 15, then focal length of objective is ...... cm.

Options

A. 100
B. 90
C. 105
D. 92

Answer

B

Explanation

`m=(f_0)/(f_e)`
`therefore f_e=(f_0)/(m)` and `L=f_0+f_e`
`96=f_0+(f_0)/(m)=f_0[1+(1)/(15)]=f_0xx(16)/(15)`
`therefore f_0 =(96xx1.5)/(16)=90` cm