Physics U28

Particle model: Explanation of properties of matter.

Considering matter is made up of particles. Here, the intensity of radiation is proportional to the number of photons present at each instant.

Wave model: Energy transfers in the form of waves. Here, the intensity of radiation is proportiona to square of the amplitude of the waves.

Photon = Packet of energy

Energy of 1 photon of a wave is related to its frequency.

E = hf

h = 6.63 x 10^-34 Js (Planck constant)

To measure energy of Photons, we use the unit eV (electronvolt). 1 eV = 1.6 x 10^-19 J.

Electronvolt measures energy. So if an electron is accelerated froom rest by supplying 1 ev then we can find velocity using: 1/2 m v^2 = 1 ev

In his special theory of relativity, Einstein showed that:

' A photon travelling in vacuum has momentum. It doesn't have mass but the momentum is related to its energy.'

p = E/c = hf/c

De Broglie proposed wave-like property of particles like electron. The diffraction of electrons confirmed their wave-like property.

All moving particles have De Broglie wavelength.

Slow moving electrons or neutrons are used to study arrangement of atoms as wavelength of neutrons ~ separation between atoms.

Steps: 1. Neutron are passes through the separation.

           2. Wavelength is calculated using lambda = 2dSin theta. Where d is spacing of atomic layers, and theta is angle of diffraction.

 Photoelectric effect

- Metals have free electrons.

- The energy of the electron in an atom is said to be quantised.

- So every metal has a threshold frequency.

When the frequency of incident light >= threshold frequency of the metal. A single photon interacts with a single electron to release it.

Energy of photon = Work function (energy used to escape the metal) + Maximum kinetic energy of electron (kinetic energy of the released electron).

Threshold frequency is the frequency of incident light.

Line Spectra: It is used to identify elements. 

For example:- In a specific atom, electrons can absorb or emit photons of certain fixed energies.

E = hf. So electrons can absorb energy from photons of fixed frequencies.

c = f x lambda. So electrons can absorb energy from photons of a unique collection of wavelengths.

Hence, each element has a spectrum with a unique collection of wavelengths. Thus line spectra ccan be used to identify elements.

Emission Line Spectra

- Light interacts with matter.

- Light of a unique wavelength and frequency is absorbed by the electron in the element.

This extra energy makes the electron unstable. It loses energy to attain stability.

When eectrons lose energy, light is emitted by matter in the form of photons. The distinctive energy levels of an atom mean that energy of the photons emitted and hence their wavelength will be unique to that atom.

Therefore, unique line spectra = unique spacing between the energy levels.

Absorption Line Spectra

Same as above but when after absorption of energy, the photon is emitted out. It is emitted in any direction (not necessarily the original direction of light.) This leads to dark absorption.

Q. Use the concept of discrete electron energy levels to explain the existence of these darker lines.

Incident photon gives energy to an electron in an inner shell. As difference in energy level is equal to energy of photon. Electron moves to a higher energy level, then the electron deexcites giving off photon of the same energy. But photons are emitted in all directions.

Q. Use band theory to explain the dependence on light intensity of the resistance of a light dependent resistor (LDR).

Incident photons give energy to valence band. Electron crosses Forbidden Band and jumps to Conduction Band. Consequently positive holes are created in Valence Band. As high intensity light is incident on LDR, there are more free electrons in Conduction Band. As a result, resistance of LDR decreases.

Q. With reference to the photoelectric efffect, state what is meant by work function energy.

Minimum energy of photon required to remove electron from metal surface.

Q. Explain why most of the emitted electrons will have a speed lower than Vmax.

The electrons below the surface spend some energy in doing work against the intermolecular forces.