The refractive index of a material $M_1$ changes by 0.014 and that of another material $M_2$ changes by 0.024 as the colour of the light is changed from red to violet. Two thin prisms one made of $M_1(A = 5.3^\circ)$ and other made of $M_2(A = 3.7^\circ)$ are combined with their refracting angles oppositely directed.
  1. Find the angular dispersion produced by the combination.
  2. The prisms are now combined with their refracting angles similarly directed. Find the angular dispersion produced by the combination.
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Given that, $\mu'_\text{v}-\mu'_\text{r}=0.014$ and $\mu_\text{v}-\mu_\text{r}=0.024$ $\text{A}' = 5.3^\circ\ \text{and A} = 3.7^\circ$
  1. When the prisms are oppositely directed,

angular dispersion $=(\mu_\text{v}-\mu_\text{r})\text{A}-(\mu'_\text{v}-\mu'_\text{r})\text{A}'$
$=0.024\times3.7^\circ-0.014\times5.3^\circ=0.0146^\circ$
  1. When they are similarly directed,

angular dispersion $=(\mu_\text{v}-\mu_\text{r})\text{A}+(\mu'_\text{v}-\mu'_\text{r})\text{A}'$
$=0.024\times3.7^\circ+0.014\times5.3^\circ=0.163^\circ$
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