The angles of incidence and refraction of a monochromatic ray of light of wavelength $\lambda$ at an air-glass interface are $i$ and $r$,respectively. $A$ parallel beam of light with a small spread $\delta \lambda$ in wavelength about a mean wavelength $\lambda$ is refracted at the same air-glass interface. The refractive index $\mu$ of glass depends on the wavelength $\lambda$ as $\mu(\lambda)=a+b / \lambda^2$,where $a$ and $b$ are constants. Then,the angular spread in the angle of refraction of the beam is

• A
  $\left|\frac{\sin i}{\lambda^3 \cos r} \delta \lambda\right|$
• B
  $\left|\frac{2 b}{\lambda^3} \delta \lambda\right|$
• C
  $\left|\frac{2 b \tan r}{a \lambda^3+b \lambda} \delta \lambda\right|$
• D
  $\left|\frac{2 b\left(a+b / \lambda^2\right) \sin i}{\lambda^3} \delta \lambda\right|$