Question

What is the condition for which we consider two thin lenses to be in achromatic combination?

• A. The ratio of dispersive power of the thin lenses should be $-1$
• B. The ratio of dispersive power of the thin lenses should be $1$
• C. The ratio of dispersive power of the thin lenses should be $>1$
• D. The ratio of dispersive power of the thin lenses should be $<1$

Answer 1

The correct option is A The ratio of dispersive power of the thin lenses should be $-1$

For an achromatic combination of thin lenses we have the important condition as,
$\frac{\omega }{f}+\frac{{\omega }^{\prime }}{f}=0$
This gives us, $\omega =-{\omega }^{\prime }$or
$\frac{\omega }{{\omega }^{\prime }}=-1$
So, option (a) is the right answer.