Photoelectric Effect Flashcards
What is photoelectric effect?
The phenomenon of emission of electrons from the surface of metals when irradiated with light ( electromagnetic radiation ) of suitable frequency is known as photoelectric effect.
Who discovered photoelectric effect? How?
Heinrich hertz discovered photoelectric effect in 1888. In his experiment on electromagnetic waves he found that the electric discharge ( high voltage spark) of the cathode ray was facilitated when the cathode ray was illuminated by UV light.
Who followed up on hertz’s discovery? What did he observe and and conclude?
Hallways followed up hertz’s discovery in 1888. He observed ising a leaf electroscope that when exposed to UV radiation
1. Negatively chafed zincs ate lost its charge.
2. Positively charged zinc plate became more positively charged
3. Neutral zinc plate acquired positive charge.
From this he concluded that negatively charged particles (electrons) are emitted by zinc in the process .
Which substances reomd photo electrically to the following radiations
- UV light
- Visible light
- X rays
- Infrared rays
- Zinc cadmium magnesium
- Alakali metals - sodium potassium caesium
- Heavy metals
- Semi conductor devices - transistors
What are photo electrons ?what is photo current ?
The electrons emitted during the phenomenon of photoelectric effect are known as photo electrons.
The current due to the movement of photo electrons is known as photoelectric current or photo current
Experimental study of photoelectric effect is conducted to study…
The dependence of photoelectric current on
Intensity of incident radiation
Frequency of radiation
Potential difference between the electrodes A and C
Nature of material of plate C
Describe apparatus of experimental study of photoelectric effect
The apparatus consists of a highly evacuated glass tube(G). A photosensitive metal plate(C) and another metal plate A are enclosed in the tube. Glass tube has a quartz window W through which UV rays from source S is incident on C . C and A are connected to H.T battery. Potential difference between A and C can be varied. A high quality volt meter V is used to measure the applied p.d. The polarities of applied voltage between A and C can be altered using commutator Cm . Resulting photo current is measured using the micro ammeter in the circuit
Why is C also called photo cathode and photo emitter
And why Is A considered the anode
It is held at negative potential with respect to A. It emits electrons under the influence of light.
A collects electrons and is held at positive potential with respect to C.
What is saturation current?
V is the anode voltage current attains maximum value at V = V’ for V > V’ current does not increase I.e. it saturates . This maximum value for photo current for given intensity and frequency is known as saturation current.
What is stopping potential?
The minimum negative potential or retarding potential Vs given to the anode for which photoelectric current becomes zero is called stopping potential.it is a measure of max kinetic energy of photo electrons .
What is threshold frequency?
The minimum frequency of incident light below which photoelectric emission completely ceases is known as the threshold frequency of the material.
What is threshold wavelength?
The threshold wavelength of a photomaterial is the maximum wavelength above which photoelectric emission completely ceases.
State the experimental observations or laws of photoelectric emission.
Photoelectric emission is an instantaneous process. Time interval between incidence of radiation and emission of electrons is of the order of nanoseconds.
For every photomaterial there is a certain minimum value of frequency of incident radiation called threshold frequency below which photoelectric emission doesn’t take place
Far given photo material and frequency of incident radiation above threshold frequency current is directly proportional to intensity of radiation. In other words number of photo electrons emitted per second is directly proportional to intensity of radiation.
For a given photo material and frequency stopping potential and hence the maximum kinetic energy of emitted electrons is directly proportional to frequency of radiation but is independent of intensity of radiation.
What is work function if a photoelectric metal?
Work function of a photometal is the minimum energy required to just liberate an electron from the metal surface.
Who coined the term photon?
Who experimentally verified einstein’s equation
Gilbert Lewis
Millikan
