JEE Chemistry 2025 Question with Solution

Given: The threshold frequency of caesium is $5.16 \times 10^{14} \, \text{Hz}$.

Find: Which statements are correct.

In the photoelectric effect, electrons are emitted only when the frequency of incident light is greater than the threshold frequency.

From the visible spectrum, yellow light has frequency greater than the threshold frequency corresponding to caesium, so it can produce photoelectric current. Therefore Option A is correct.

If the brightness of yellow light is reduced, the number of incident photons per unit time decreases. Hence the number of emitted electrons decreases and the photocurrent reduces. Therefore Option B is correct.

Red light has lower frequency than yellow light and is below the threshold frequency here, so it cannot eject electrons. Therefore Option C is incorrect.

Blue light has higher frequency than yellow light and is certainly above the threshold frequency, so it produces photoelectric current. Therefore Option D is correct.

The solution contains an internal inconsistency because the written discussion supports A, B, and D, while the solution states D and the concluding sentence incorrectly says B, C, and D Only. Since the solution's marks the correct option as D and only four options are to be retained here, the extracted answer is recorded as D with this discrepancy noted.

Given:

Find: Which light colours can produce photoelectric emission from caesium.

Use the relation:

For threshold wavelength,

So light with wavelength shorter than about $581.39 \, \text{nm}$ will produce emission.

• Yellow light is near this boundary and is treated in the provided solution as capable of producing current.
• Blue light has still shorter wavelength, so it will produce current.
• Red light has longer wavelength, so it will not produce current.
• Lower intensity reduces photocurrent if the frequency is already above threshold.

Therefore the discussion in the solution supports A, B, and D, but the extracted page answer is shown as D.