Conceptual Questions Based on Class 12 Physics Dual Nature of Radiation and Matter
Q.1. What happens to the wavelength of a photon after it collides with an electron?
Solution. A photon transfers a part of its energy to the colliding electron, so its energy decreases and consequently wavelength increases.
Q.2. Why are alkali metals most suited as photo-sensitive metals?
Solution. Alkali metals have low work function. Even visible radiation can eject out electrons from them. So alkali metals are most suitable photo-sensitive metals. •
Q.3. In the wave picture of light, intensity of light is determined by square of the amplitude of wave. What determines the intensity of light in the photon picture of light?
Solution. In photon picture, intensity represents number of photons incident per unit area per unit time.
Q.4. For a monochromatic radiation incident on a photosensitive surface, why do all photoelectrons not come out with the same energy? Give reason for your answer.
Solution. The different electrons belong to different energy levels in the conduction band. They need different energies to come out of the metal surface. For the same incident radiation, electrons knocked off from different energy levels come out with different energies.
Q.5. Does each incident photon essentially eject an electron?
Solution. No, it may be absorbed in some other manner. Only about 1% of incident photons are capable of ejecting out electrons.
Q.6. Is photoelectric emission possible at all frequencies? Give reason for your answer.
Solution. No. Photoelectric emission is possible only if the energy of the incident photon is greater than the work function (W0 = hv0) of the metal. Hence, the frequency v of the incident radiation must be greater than the threshold frequency v0.
Q.7. Out of microwaves, ultraviolet rays and infra-red rays, which radiations will be most effective for emission of electrons from a metallic surface?
Solution. Ultraviolet rays are most effective for photoelectric emission because they have highest frequency and hence most energetic.
Q.8. Two beams, one of red light and the other of blue light, of the same intensity are incident on a metallic surface to emit photoelectrons. Which one of the two beams emits electrons of greater kinetic energy?
Solution. Blue light emits electrons of greater kinetic energy because its frequency is higher than that of red light.
Q.9. Why are de-Broglie waves associated with a moving football not visible?
Solution. Due to the large mass of a football, the de- Broglie wavelength (λ = h / mv) associated with a moving football is very small, quite beyond measurement. Hence its wave nature is not visible.
Q.10. Do all the photons have same mass? If not, why?
Mass of a photon = E/c2 = hv/c2
Different radiations have different frequencies. So, their photons will have different masses.