You are given three lenses of power `0.5 D`, `4D` and `10D` to design a telescope.
`(i)` Which lenses should be used as objective and eyepiece? Justify your answer.
`(ii)` Why is the aperture of the objective preferred to be large ?

Explanation

`(a)` N/A
`(b) (i) 0.5D` for objective lens and `10D` for eys piece because `M=(f_(0))/(f_(c ))`
`(ii)` To collect sufficient light to form a bright image.
Using `(n_(2))/(v)-(n_(1))/(u)=(n_(2)-n_(1))/(R )`
`n_(1)=1`, `n_(2)=1.5`, `u=-100cm`, `v=?`, `R=20cm`
`(1.5)/(v)-(1)/(-100)=(1.5-1)/(20)implies(1.5)/(v)+(1)/(100)=(0.5)/(20)`
`(1.5)/(v)=(1)/(40)-(1)/(100)=(3)/(200)impliesv=100cm`