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$A$ beam of light has a small wavelength spread $\delta \,\lambda$ about a central wavelength $\lambda$ . The beam travels in vaccum until it enters a glass plate at an angle $\theta$ relative to the normal to the plate, as shown in figure. The index of refraction of the glass is given by $n(\lambda )$. The angular spread $\delta \theta '$ of the refracted beam is given by

• A
  $\delta \theta ' = \left| {\frac{1}{n}\delta \lambda } \right|$
• B
  $\delta \theta ' = \left| {\frac{{dn\left( \lambda  \right)}}{{d\lambda }}\delta \lambda } \right|$
• ✓
  $\delta \theta ' = \left| {\frac{{\tan \theta '}}{n}\frac{{dn\left( \lambda  \right)}}{{d\lambda }}\delta \lambda } \right|$
• D
  $\delta \theta ' = \left| {\frac{{\sin \theta }}{{\sin \theta '}}\frac{{\delta \lambda }}{\lambda }} \right|$