Question

Direction Read the following question choose if: (a) Both Assertion and Reason are true and Reason is correct explanation of Assertion. (b) Both Assertion and Reason are true, but Reason is not correct explanation of Assertion. (c) Assertion is true but the Reason is false. (d) Both Assertion and Reason is false. Assertion: Metals like Na or \(\mathrm{K}\), emit electrons even when visible lights fall on them. Reason: This is because their work function is low. (A) a (B) \(\mathrm{b}\) (C) \(\mathrm{c}\) (D) d

Short answer

The correct answer is (C) c.

Step-by-step solution

Examine the Assertion

The Assertion states that metals like sodium (Na) and potassium (K) emit electrons when visible light falls on them. This phenomenon, known as the photoelectric effect, occurs when light (or photons of light) collide with electrons in the metal, transferring energy and allowing the electrons to escape from the surface. The Assertion claims that this process happens in sodium and potassium when exposed to visible light specifically. We'll come back to this point after looking at the Reason.

Examine the Reason

The Reason attributes the Assertion to the metals' low work function. The work function is the minimum energy required to remove an electron from the surface of a metal. Metals with a low work function require less energy to remove the electrons, making it easier for them to emit electrons when exposed to light. Sodium and potassium indeed have low work functions, so the Reason is true as well.

Check the Relationship between the Assertion and the Reason

The actual critical part is whether low work function metals can emit electrons under visible light accordingly to the Assertion. The energy of the incoming light needs to be greater or equal to the work function for the photoelectric effect to occur. However, for metals like sodium and potassium, the work function is higher than the energy provided by visible light. Therefore, the metals will not emit electrons when exposed to visible light, and the Assertion is false.

Determine the Correct Option

Now that we have concluded that the Assertion is false and the Reason is true, we can identify the correct option. In this case, it is (c) Assertion is true, but the Reason is false. The correct answer is (C) c.