In the visible region of the spectrum the rotation of the place of polarization is given by \[\theta =a+\frac{b}{{{\lambda }^{2}}}\]. The optical rotation produced by a particular material is found to be 30° per mm at \[\lambda =5000\]Å and 50° per mm at \[\lambda =4000\text{{ }\!\!\mathrm{\AA}\!\!\text{ }}\]. The value of constant a will be

• A. \[+\frac{50{}^\circ }{9}\]per mm
• B. \[-\frac{50{}^\circ }{9}\]per mm
• C. \[+\frac{9{}^\circ }{50}\]per mm
• D. \[-\frac{9{}^\circ }{50}\]per mm