(a) The ray incident on BC passes undeflected through it
If `n_1=n_2` for wavelength `lambda_0`
`1.20+(10.8xx10^4)/(lambda_0^2)=1.45+(1.8xx10^4)/(lambda_0^2)`
`((10.8-1.8)xx10^4)/(lambda_0^2)=1.45-1.20`
`lambda_0^2=(9xx10^4)/(0.25)implieslambda_0=(3xx10^2)/(0.5)=600nm`
(b) For `lambda_0=600nm`, `n_1=n_2=1.2+(10.8xx10^4)/((600)^2)=1.5`
Now complete the prism, angle of prism, `A=60^@`
When angle of deviation is minimum, `r=A//2=30^@`
Refraction on AC: `1xxsini=muxxsinr`
`sini=1.5xxsin30^@=0.75`
`i=sin^-1(0.75)`