CBSE Class 12 Physics Dual Nature Of Matter and Radiation chapter notes and important questions

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 DUAL NATURE OF MATTER & RADIATION

GIST

ELECTRON EMISSION

1. There are three types of electron emission, namely, Thermionic Emission, Photoelectric Emission and Field Emission.

2. The minimum energy required by an electron to escape from the metal surface is called work function.

3. Work function is conveniently expressed in electron volts ( e V )

4. One electron volt is the energy gained or lost by an electron while passing through a potential difference of one volt.

PHOTOELECTRIC EFFECT

1. The minimum energy required by an electron to come out from metal surface is called the work function of a metal.

2. Photo electric effect is the phenomenon of electrons by metals when illuminated by light of suitable frequency

3. Photo electric current depends on

i) The intensity of incident light

ii) he potential difference applied between two electrodes

iii) he nature of the emitter material

EXPERIMENTAL STUDY OF PHOTOELECTRIC EFFECT

1. The minimum negative potential given to the anode plate for which the photo electric current becomes zero is called stopping potential.

2. The stopping potential Vo depends on i) The frequency of incident light and ii) the nature of emitter material. For a given frequency of incident light, the stopping potential is independent of its intensity. 

3. Below a certain frequency (threshold frequency) n , characteristics of the metal , no 0 photo electric emission takes place, no matter how large the intensity may be.

EINSTEINS PHOTO ELECTRIC EQUATION: ENERGY QUANTUM OF RADIATION

1. Light is composed of discrete packets of energy called quanta or photons.

2. The energy carried by each photon is E = hν, where ν is the frequency and momentum. p= h/λ. The energy of the photon depends on the frequency γ of the incident light and not

on its intensity.

3. Photo electric emission from the metal surface occurs due to absorption of a photon by an electron

4. Einstein’s photo electric equation: 

PARTICLE NATURE OF LIGHT: THE PHOTON

1. Radiation has dual nature: wave and particle. The wave nature is revealed in phenomenon like interference, diffraction and polarization. The particle nature is revealed by the phenomenon photo electric effect.

2. By symmetry, matter also should have dual nature: wave and particle. The waves associated with the moving material particle are called matter waves or De Broglie waves.

3. The De Broglie wave length (λ) associated with the moving particle is related to its moment p as: λ =h/p = h/mv

4. An equation for the De Broglie wavelength of an electron accelerated through a potential V. Consider an electron with mass ‘m’ and charge ‘e’ accelerated from rest through a potential V.

K = eV

K =1/2 mv²= 2mK

P = √2mK = √2meV

λ = h/√2meV

ONE MARK QUESTIONS

1. A proton and an electron have same kinetic energy. Which one has greater de-Broglie wavelength and why?

2. Define the term 'stopping potential' in relation to photoelectric effect.

3. The stopping potential in an experiment on photoelectric effect is 1 .5 V. What is the maximum kinetic energy of the photoelectrons itted?

4. The maximum kinetic energy of a photoelectron is 3eV.What is its stopping potential?

5. With what purpose was famous Davisson-Germer experiment with electrons performed.

6. If the intensity of incident radiation in a photoelectric experiment is doubled what, happens to kinetic energy of emitted photo electrons?

7. Calculate the frequency associated with photon of energy 3.3 x 10^-10 J?

Ans: ν = 5 x 10 ²³ Hz.

8. What is the momentum of a photon of energy 1 MeV? Energy E = 1 MeV = 1.6 x 10 ^-13J, p = E/c= 5.33x 10^-22 Kgm/s

9. What happens to the velocity of emitted electrons when the wave length of incident light is decreased?

10 . Which of the two photons is more energetic: red light or violet light?

11. What will be the stopping potential when a photon of 25eV is incident of metal surface of work function 6eV? Ans : 19 volt

12. Why is alkali metal surface better suited as photosensitive surface?

13. Blue light can eject electrons from a photo-sensitive surface while orange light cannot. Will violet and red light eject electrons from the same surface?

14. Matter waves associated with electrons could be verified by crystal diffraction experiments. Why?

 Ans: The wave length of the matter waves associated with electrons has wavelength comparable to the spacing between the atomic planes of their crystals.

15. How do matter waves differ from light waves as regards to the velocity of the particle and the wave?

Ans: In case of matter waves, the wave velocity is different from the particle velocity. But in case of light, particle velocity and wave velocity are same.

16. An electron and an alpha particle have same kinetic energy. Which of these particles has the shortest de- Broglie wavelength Ans: Alpha particle

17. An electron, an alpha-particle and a proton have the same kinetic energy. Which one of these particles has the largest de-Broglie wave length?

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