Describe Hallwach's and Lenard's observations regarding the photoelectric effect in short.

(N/A) Between $1886$ and $1902$,Hallwach and Lenard studied the phenomenon of photoelectric emission.
Lenard's experiment:
As shown in the figure,two metal electrodes are kept at opposite ends inside an evacuated glass tube. One electrode is called the cathode $(C)$ (emitter),which is a photosensitive surface,and the second electrode is called the collector $(A)$. $A$ potential difference is applied between $C$ and $A$.
$A$ quartz window is provided in this tube through which ultraviolet rays are incident on the cathode $(C)$.
When ultraviolet radiation is incident on the cathode $(C)$,photoelectrons are emitted from the surface.
The anode $(A)$ is kept at a positive voltage with respect to the cathode $(C)$,hence it attracts the emitted electrons,resulting in a flow of electrons. Consequently,a current flows in the circuit.