Question

Which one of the following statements is INCORRECT regarding the photoelectric effect?

• A. Threshold frequency is different for different metals
• B. The same metal gives same response to light of different wavelengths
• C. The emission of photoelectrons is an instantaneous process
• D. Above threshold frequency, the number of photoelectrons is proportional to intensity
• E. Maximum K.E. is independent of the intensity of incident radiation

Hint:

Intensity affects the number of electrons; Frequency affects the energy of electrons.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
We need to identify the incorrect statement among five statements describing the photoelectric effect. This requires knowledge of the fundamental laws and observations of this phenomenon.
Step 2: Detailed Explanation:
Let's analyze each statement based on the principles of the photoelectric effect: - (A) Threshold frequency is different for different metals: The threshold frequency (\( \nu_0 \)) is the minimum frequency of incident light required to eject an electron. It is related to the metal's work function (\( \phi = h\nu_0 \)). Since different metals have different work functions, their threshold frequencies are also different. This statement is correct. - (B) The same metal gives same response to light of different wavelengths: A metal's response depends critically on the wavelength (or frequency) of the incident light. If the wavelength is too long (frequency too low, i.e., below the threshold frequency), no photoelectrons are emitted, regardless of the intensity. If the wavelength is short enough (frequency above threshold), electrons are emitted with a kinetic energy that depends on the wavelength (\( K_{max} = hf - \phi = hc/\lambda - \phi \)). Therefore, the response is very different for different wavelengths. This statement is incorrect. - (C) The emission of photoelectrons is an instantaneous process: Experiments show that if the light frequency is above the threshold, electron emission starts almost immediately (within nanoseconds) after the light strikes the metal. This supports the particle (photon) model of light. This statement is correct. - (D) Above the threshold frequency the number of photoelectrons emitted per sec is directly proportional to the intensity of incident radiation: The intensity of light is related to the number of photons arriving per second. Each photon (if its energy is sufficient) ejects one electron. Therefore, a higher intensity (more photons) leads to more photoelectrons being emitted per second. This statement is correct. - (E) The maximum K.E. of the photoelectrons is independent of the intensity of incident radiation: The maximum kinetic energy of an ejected electron depends only on the frequency of the incident photon and the work function of the metal (\( K_{max} = hf - \phi \)), not on the intensity (number of photons). This statement is correct. Step 3: Final Answer:
The incorrect statement is (B).