🔬 Modern Physics – Photoelectric Effect
(NEET)
📘 1. Definition of Photoelectric Effect
The Photoelectric Effect is the emission of electrons from a metal surface when
electromagnetic radiation (light) of sufficiently high frequency is incident on it.
Electrons emitted = photoelectrons
Current produced = photoelectric current
Discovered by Hertz (1887) and explained by Einstein (1905)
Key Concepts:
Only light above a threshold frequency (ν₀) causes emission.
Emission occurs without delay, indicating an instantaneous interaction.
Proves quantized nature of light (photon model).
2. Experimental Setup
Apparatus:
Evacuated glass tube
Metal plate as cathode
Collector plate as anode
, Variable voltage power supply
Monochromatic light source
Galvanometer to measure current
Process:
1. Light hits cathode → electrons are ejected.
2. Ejected electrons move to anode → photoelectric current.
3. Varying the voltage allows measurement of stopping potential
and saturation current.
3. Key Experimental Observations
✅ From Lenard's and Millikan's experiments:
1. Threshold Frequency (ν₀) exists:
o Below this frequency, no photoelectrons are emitted,
regardless of intensity.
2. Instantaneous Emission:
o Electrons are ejected as soon as light hits — no time lag,
even at low intensity.
3. Kinetic Energy ∝ Frequency:
o Kinetic energy of photoelectrons increases with increasing
frequency (not intensity).
4. Current ∝ Intensity:
o Brighter light → more photons → more electrons ejected →
higher current.
5. Stopping Potential (V₀):
o A negative potential is required to stop the fastest
photoelectrons.
o Related to maximum kinetic energy:
o
o eV0=K.Emax
(NEET)
📘 1. Definition of Photoelectric Effect
The Photoelectric Effect is the emission of electrons from a metal surface when
electromagnetic radiation (light) of sufficiently high frequency is incident on it.
Electrons emitted = photoelectrons
Current produced = photoelectric current
Discovered by Hertz (1887) and explained by Einstein (1905)
Key Concepts:
Only light above a threshold frequency (ν₀) causes emission.
Emission occurs without delay, indicating an instantaneous interaction.
Proves quantized nature of light (photon model).
2. Experimental Setup
Apparatus:
Evacuated glass tube
Metal plate as cathode
Collector plate as anode
, Variable voltage power supply
Monochromatic light source
Galvanometer to measure current
Process:
1. Light hits cathode → electrons are ejected.
2. Ejected electrons move to anode → photoelectric current.
3. Varying the voltage allows measurement of stopping potential
and saturation current.
3. Key Experimental Observations
✅ From Lenard's and Millikan's experiments:
1. Threshold Frequency (ν₀) exists:
o Below this frequency, no photoelectrons are emitted,
regardless of intensity.
2. Instantaneous Emission:
o Electrons are ejected as soon as light hits — no time lag,
even at low intensity.
3. Kinetic Energy ∝ Frequency:
o Kinetic energy of photoelectrons increases with increasing
frequency (not intensity).
4. Current ∝ Intensity:
o Brighter light → more photons → more electrons ejected →
higher current.
5. Stopping Potential (V₀):
o A negative potential is required to stop the fastest
photoelectrons.
o Related to maximum kinetic energy:
o
o eV0=K.Emax
