## Photons

### Photons

1) These are the packets of energy (or energy particles) which are emitted by a source of radiation. They are travel in straight line.
2) The energy of a photon, E= hv = hc/λ .
3) The photons emitted from a source, travel through space with the same speed (equal to the speed of light).
4) The frequency of a photon does not change when it travels through different medium but its wavelength changes in different medium as the velocity of a photon in different media is different.
5) The rest mass of photon is zero.

6) Its momentum is hv /c or h/λ.

### Photoelectric effect

1) It is the phenomenon of emission of electrons from the surface of metals, when light radiations of suitable frequency fall on them. The emitted electrons are called photoelectrons and the current so produced is called photoelectric current.

2) Alkali metals: like lithium, sodium, potassium, cesium etc. show photoelectric effect with visible light, whereas metals like zinc, cadmium, magnesium etc. are sensitive only to ultra-violet light for photoelectric effect.

### Law of photoelectric emission

1) For a given metal and frequency of radiation, the number of photoelectrons ejected is directly proportional to the intensity of the incident light.
2) For a given metal, there exists a certain minimum frequency of the incident radiation below which no emission of photoelectrons takes place. This frequency is called threshold frequency.
3) Above the threshold frequency, the maximum K.E. of the emitted photoelectrons is independent of the incident light but depends upon the frequency (or wavelength) of the inside light.
4) The photoelectric emission is an instantaneous process.

### Einstein’s photoelectric equation

1/2 mv2 = hv – w0 = hv – hv0 = h(v-v0)
Where v is the frequency of the incident photons, v0 is the threshold frequency of metal, u is the velocity of ejected photo- electron from the metal surface and m is the mass of photo-electron.

### Cut off potential or stopping potential

It is the minimum negative potential given to anode in photo-cell for which the photo-electronic current becomes zero. It is denoted by V0 .
eV0 = 1/2 mv2 = hv – w0 . The value of stopping potential is different for different metals but is dependent of the intensity of the incident light.

### Photoelectric cell

It is a device which converts the light energy into electrical energy. Photoelectric cells are of three types.

1) Photo emissive cell
2) Photo voltaic cell
3) Photo conductive cell

### In Campton effect

When high energy photon collides with a free electron of light metal, it gets scattered. Two wavelength of the scattered photon is more than that of incident photon. Then the Compton shift.
∆λ = h/m0c(1-cosφ)
Where φ Is the angle at which the photon is scattered.