Optics Question 26
Question: Assertion : The focal length of lens does not change when red light is replaced by blue light. Reason : The focal length of lens does not depends on colour of light used.
Options:
A) If both assertion and reason are true and the reason is the correct explanation of the assertion.
B) If both assertion and reason are true but reason is not the correct explanation of the assertion.
C) If assertion is true but reason is false.
D) If the assertion and reason both are false.
E) If assertion is false but reason is true.
Show Answer
Answer:
Correct Answer: D
Solution:
Focal length of the lens depends upon it’s refractive index as $ \frac{1}{f}\propto (\mu -1) $ . Since $ {\mu _{b}}>{\mu _{r}} $ so $ f _{b}<f _{r} $
Therefore, the focal length of a lens decreases when red light is replaced by blue light.
