Which is `//` are incorrect observations related to photoelectric effect ?

Options

A. When intensity of light is increased then kinetic energy of photo electron increases
B. If frequency of incident light isless than threshold frequency photoelectron is ejected from the metal surface, if we increase the intensity light
C. Photoelectric current is proportionalto the frequency of incident light
D. When frequency of light is increased , velocity of photoelectron increased

Answer

To determine which observations related to the photoelectric effect are incorrect, we can analyze each statement provided in the question.

### Step-by-Step Solution:

1. **Observation A**: "When intensity of light is increased, kinetic energy of photoelectrons increases."
- **Analysis**: This statement is incorrect. Increasing the intensity of light increases the number of photoelectrons emitted, but it does not increase their kinetic energy. The kinetic energy of photoelectrons is determined by the frequency of the incident light, not its intensity.

2. **Observation B**: "If frequency of incident light is less than threshold frequency, photoelectrons are ejected from the metal surface."
- **Analysis**: This statement is also incorrect. If the frequency of the incident light is below the threshold frequency, no photoelectrons will be ejected from the metal surface, regardless of the intensity of the light.

3. **Observation C**: "Photoelectric current is proportional to the frequency of incident light."
- **Analysis**: This statement is incorrect as well. The photoelectric current is proportional to the number of photoelectrons emitted, which is related to the intensity of the light, not the frequency. The frequency must be above the threshold for any current to be generated, but once that is achieved, increasing frequency does not increase the current.

4. **Observation D**: "When frequency of light is increased, velocity of photoelectrons increases."
- **Analysis**: This statement is correct. As the frequency of the light increases, the energy of the emitted photoelectrons increases, which leads to an increase in their velocity.

### Conclusion:
The incorrect observations related to the photoelectric effect are:
- A: When intensity of light is increased, kinetic energy of photoelectrons increases.
- B: If frequency of incident light is less than threshold frequency, photoelectrons are ejected from the metal surface.
- C: Photoelectric current is proportional to the frequency of incident light.

Thus, the answer is A, B, and C.

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