Question

In a photoelectric effect experiment a light of frequency 1.5 times the threshold frequency is made to fall on the surface of photosensitive material. Now if the frequency is halved and intensity is doubled, the number of photo electrons emitted will be:

• A. Doubled
• B. Quadrupled
• C. Zero
• D. Halved

Answer 1

Answer: (C)

Zero

Answer 2

In the photoelectric effect, electrons are emitted only when the frequency of the incident light $f$ is greater than or equal to the threshold frequency $f_0$ of the material. This condition can be written as:

$f \geq f_0.$

Initially, the frequency of the light is $1.5 f_0$, which is above the threshold frequency, so photoelectrons are emitted. However, if the frequency is halved, the new frequency $f'$ becomes:

$f' = \frac{1.5 f_0}{2} = 0.75 f_0.$

Since $f' < f_0$, the incident frequency is now less than the threshold frequency. Therefore, no photoelectrons will be emitted, regardless of the light’s intensity. Emission of photoelectrons depends on frequency, not intensity, when the frequency is below the threshold.

Thus, the answer is: Zero.