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Photometry — Physics STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 SciencePhysicsPhotometry1 Mark
Question
Mark the correct options.
  1. The luminous efficiency of a monochromatic source is always greater than that of a white light source of same power.
  2. The luminous efficiency of a monochromatic source of wavelength 555nm is always greater than that of a white light source of same power.
  3. The illuminating power of a monochromatic source of wavelength 555nm is always greater than that of a white light source of same power.
  4. The illuminating power of a monochromatic source is always greater than that of a white light source of same power.
✓

Answer

  1. The luminous efficiency of a monochromatic source of wavelength 555nm is always greater than that of a white light source of same power.
  2. The illuminating power of a monochromatic source of wavelength 555nm is always greater than that of a white light source of same power. 
Explanation:
  1. The luminous efficiency of a monochromatic source may be less than that of the white light if the former emits wavelength far away from 555nm.
  2. Yes, it is true that our eyes mostly respond to colours close to the wavelength of 555nm and detect them bright. So, luminous efficiency is unity (highest).
  3. It is true because white light distributes its energy amongst certain colours that our eyes cannot detect as brightly as they detect a 555nm light.
  4. It is not necessarily true. If the monochromatic light radiates in a wavelength that is far away from 555nm, our eyes will not perceive it as bright. So, it will have lesser illuminating power.

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