Reason
$$K$$ and $$Cs$$ emit electrons on exposure to light.
Assertion
$$K$$ and $$Cs$$ are used in photo electric cells
$$K$$ and $$Cs$$ are used in photo electric cells
A
Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B
Both Assertion and Reason are incorrect
C
Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
D
Assertion is correct but Reason is incorrect
Correct option is A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
Correct option $$\text{(A) Both A and R are true and R is the correct }$$
The valence shell electron in the alkali metals is so loosely held that even the low energy photons (light) can eject this electron from the metal surface. This phenomenon is called photoelectric effect. K and Cs are used in photoelectric cells which arc sensitive to blue light.