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Photometry — Physics STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 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 $555\ nm$ is always greater than that of a white light source of same power.
  3. The illuminating power of a monochromatic source of wavelength $555\ nm$ 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 $555\ nm$ is always greater than that of a white light source of same power.
  2. The illuminating power of a monochromatic source of wavelength $555\ nm$ 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 $555\ nm$.
  2. Yes, it is true that our eyes mostly respond to colours close to the wavelength of $555\ nm$ 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 $555\ nm$ light.
  4. It is not necessarily true. If the monochromatic light radiates in a wavelength that is far away from $555\ nm$, our eyes will not perceive it as bright. So, it will have lesser illuminating power.

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