Q.21)-Suitable impurities are added to a semiconductor depending on its use. This is done in order to
  • increase its life
  • enable it to withstand higher voltages
  • increase its electrical conductivity
  • increase its electrical resistivity
Q.22)-Of the following natural phenomena, tell which one known in Sanskrit as 'deer's thirst'?
  • Rainbow
  • Earthshine
  • Halo
  • Mirage
Q.23)-One thousand microns is equal to
  • 10-3m
  • 10-6m
  • 10-9m
  • 10-12m
Q.24)-Sound travels at the fastest speed in
  • steel
  • water
  • air
  • vacuum
Q.25)-Superconductors are substances which
  • conduct electricity at low temperature
  • offer high resistance to the flow of current
  • offer no resistance to the flow of electricity
  • conduct electricity at high temperatures
Q.26)-Light travels at the fastest speed in
  • glass
  • water
  • hydrogen
  • vacuum
Q.27)-Optical fibre works on the
  • principle of refraction
  • total internal reflection
  • scattering
  • interference
Q.28)-Light from the star, Alpha Centauri, which is nearest to the earth after the sun, reaches the earth in
  • 4.2 seconds
  • 42 seconds
  • 4.2 years
  • 42 years
Q.29)-Radio telescopes are better than optical telescopes because
  • they can detect faint galaxies which no optical telescope can
  • they can work even in cloudy conditions
  • they can work during the day and night
  • All of the above
Q.30)-Light Emitting Diodes (LED) is used in fancy electronic devices such as toys emit
  • X-rays
  • ultraviolet light
  • visible light
  • radio waves