Kinetic Energy of Ejected Electrons:
When light of a certain frequency falls on a metal surface, electrons are ejected from the metal. The energy required to eject an electron is called the work function of the metal and is denoted by Φ.

Threshold Frequency:
The threshold frequency is the minimum frequency of light required to eject electrons from the surface of a metal. If the frequency of the light is less than the threshold frequency, no electrons are ejected.

Relation between Frequency and Kinetic Energy:
The kinetic energy of ejected electrons can be calculated using the following formula:
K.E. = hf - Φ

where K.E. is the kinetic energy of the ejected electron, h is the Planck constant, f is the frequency of the incident light, and Φ is the work function of the metal.

If the frequency of the incident light is equal to the threshold frequency, then the kinetic energy of the ejected electron will be zero. This is because the energy of the incident light is just enough to overcome the work function and eject the electron, but there is no excess energy left for the electron to have any kinetic energy.

If the frequency of the incident light is greater than the threshold frequency, then the kinetic energy of the ejected electron will be positive. The excess energy of the incident light is converted into the kinetic energy of the ejected electron.

Conclusion:
In conclusion, the kinetic energy of electrons ejected by using light having frequency equals to threshold frequency is zero because the energy of the incident light is just enough to overcome the work function and eject the electron, but there is no excess energy left for the electron to have any kinetic energy.