Derive `D_m = A(n_(21) - 1)` for thin prism.

Explanation

Thin prism : "The prism which has small prisn angle is called thin prism". Refractive index of prism,
`n=(sin((A+delta_m)/(2)))/(sin(A/2))` …........ (1)
If A and `delta_m` is kept small then,
`(sin((A+delta_m)/(2)))/(sin(A/2))` =`(A+delta_m)/(2)` radian
`sin(A/2)=A/2` radian
`therefore` Then equation (1) will be
`n=((A+delta_m)/(2))/(A/2)`
`n=(A+sin_m)/(A)`
`therefore` nA=A+`delta_m`
`therefore delta_m=A(n-1)`
`delta_m` depends on refractive index and prism angle of prism.