Optics Question 18
Question: Assertion : The mirrors used in search lights are parabolic and not concave spherical. Reason : In a concave spherical mirror the image formed is always virtual.
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: C
Solution:
In search lights, we need an intense parallel beam of light.
If a source is placed at the focus of a concave spherical mirror, only paraxial rays are rendered parallel.
Due to large aperture of mirror, marginal rays give a divergent beam.
But in case of parabolic mirror, when source is at the focus, beam of light produced over the entire cross-section of the mirror is a parallel beam.
