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Homework Help: Questions about diodes, relativity, radiations

  1. Sep 18, 2009 #1
    Given below are some questions which I have attempted and all I want to know now is whether they are correct or wrong and what the correct approach should be. So please check all my answers and point out the mistakes.

    1. Why do ordinary silicon diodes not emit light?
    A. Because they have a low value of forward bias as compared with gallium arsenide LEDs.

    2. How does the motion of an electron in n-type substance differ from the motion of holes in p-type substance?
    A. Theres no difference in the motion of the two types of majority charge carriers in their respective regions (n/p). However, motion of holes takes place in valence bands while the motion of electrons takes place in conduction bands.

    3. What are the measurements on which two observers in relative motion will always agree upon?
    A. Speed of light (according to the special theory of relativity).

    4. Does the dilation mean that time really passes more slowly in moving system or that it only seems to pass more slowly?
    A. According to time dilation formula, t=(gamma)to, time dilation means that time does pass more slowly in a moving frame of reference having velocity v relative to a frame of reference which is at rest.

    5. If you are moving in a spaceship at a very high speed relative to the Earth, would you notice a difference (a) in your pulse rate and (b) in the pulse rate of people on Earth?
    A. The pulse rate of a person who is traveling in a spaceship is not changed with respect to clock inside the spaceship. But he will notice a change in pulse rate of people on Earth, according to time dilation equation. His clock is moving slow due to the large velocity of the spaceship than the clock on Earth.

    6. As a solid is heated and begins to glow, why does it first appear red?
    A. Because when a body is heated, it starts to radiate and the radiations which come out first have lower energy (frequency) and those coming later on due to an increase in temperature will have more energy (frequency). Thus as red has the least energy among the visible light spectrum, it will be the first to be radiated and thats why the solid first appears red.

    7. What happens to total radiation from a black body if its absolute temperature is doubled?
    A. According to Stefens Law, Energy is directly proportional to the fourth power of absolute temperature, therefore, E=2^4=16. Thus, the total radiation emitted by black body increases 16 times.

    8. A beam of red light and a beam of blue light have exactly the same energy. Which beam contains the greater number of photons?
    A. Number of photons depends on the intensity (amplitude) of a certain wavelength or frequency wave and not on frequency. Thus both the beams will have same number of photons.

    9. Which photon: red, green or blue carried the most (a) energy (b) and momentum?
    A. (a) Blue (because it has the highest frequency)
    (b) (Because p=h/wavelength) Blue

    10. Which has lower energy quanta: Radio waves or X-rays?
    A. Since E=hf, the frequency of radio waves is very small as compared with X-rays, therefore, quanta of radio waves have less energy than quanta of X-rays.

    11. Does the brightness of a beam of light primarily depend on the frequency of photons or on the number of photons?
    A. Number of photons because more the photons, higher will be the light intensity and therefore brighter the light. Increasing frequency will change the wavelengths and therefore change the colour of light.

    12. When ultraviolet light falls on certain dyes, visible light is emitted. Why does this not happen when infrared light falls on these dyes?
    A. UV consists of photon of higher energies as compared to those of visible light. Due to this, photons of UV have the ability to excite the atoms of dyes, when these atoms are de-excited, photons of frequencies lying within the range of visible light are emitted.

    13. Will bright light eject more electrons from a metal surface than dimmer light of the same colour?
    A. Yes, because more the intense light used in photoelectric demonstration, greater was the amount of current which flowed in the circuit due to greater number of ejection of electrons from the metal surface. (More photons will attack the atom and cause more electrons to be ejected)

    14. Will high frequency light eject a greater number of electrons than low frequency light?
    A. No. The higher frequency light will not eject greater number of electrons than the lower frequency light. However, the high frequency light will eject electrons of large K.E.(max) as compared to low frequency light.

    15. Why can red light be used in photographic dark room when developing films, but a blue or a white light cannot?
    A. AS wavelength of red light > wavelength of blue light, therefore, E of red < E of blue light.
    Hence, photographic films and the material concerned are less affected in the presence of red light. Thats why red light preferrd.

    16. Why dont we observe a Compton effect with visible light?
    A. Compton effect only becomes noticeable for photons of wavelengths that are not too large compared to the Compton shift, which is the case for x-rays and gamma rays. Otherwise the wavelength change is too small to measure. At visible and ultraviolet wavelengths, the photons can be totally annihilated in the photoelecric effect, whereby an electron is emitted from the atom's electron shell.

    17. Can pair production take place in vacuum? Explain.
    A. No, because in vacuum theres no matter (nucleus) which essential for pair production to take place. A nucleus helps the pair to obey the law of conservation of momentum (particle and antiparticle move in one direction and the nucleus in opposite direction)

    18. Is it possible to create a single electron from energy? Explain.
    A. No, because two antiparticles are essentially produced during the conversion of energy thus obeying the law of conservation of charge.
    Last edited: Sep 18, 2009
  2. jcsd
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