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(B) The equivalent focal length $f$ of two thin lenses in contact is given by the formula: $\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}$.Given: Focal

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Convergent beam

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(B) The equivalent focal length $f$ of two thin lenses in contact is given by the formula: $\frac{1}{f} = \frac{1}{f_1} + \frac{1}{f_2}$.Given: Focal length of concave lens $f_1 = -50 \, cm$ and focal length of convex lens $f_2 = +25 \, cm$.Substituting the values: $\frac{1}{f} = \frac{1}{-50} + \frac{1}{25} = \frac{-1 + 2}{50} = \frac{1}{50}$.Therefore,$f = +50 \, cm$.Since the equivalent focal length is positive,the combination acts as a convex lens,which converges the incident parallel beam of light. Thus,the emergent beam will be a convergent beam.

As shown in the figure,a concave lens of focal length $50 \, cm$ and a convex lens of focal length $25 \, cm$ are placed in contact with each other. If a parallel beam of light is incident on the system,how will it emerge?

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