Loading…
NEET-XII-Physics
22: Photometry
- #4The relative luminosity of wavelength 600 nm is 0.6. Find the radiant flux of 600 nm needed to produce the same brightness sensation as produced by 120 W of radiant flux at 555 nm.Ans : Given,
Relative luminosity = 0.6
Let P be the radiant flux of the source.
∴ Luminous flux = P`` \times ``685
Again, relative luminosity`` =\frac{\,\mathrm{\,Luminous\,}\,\mathrm{\,flux\,}\,\mathrm{\,of\,}\,\mathrm{\,a\,}\,\mathrm{\,source\,}\,\mathrm{\,of\,}\,\mathrm{\,given\,}\,\mathrm{\,wavelength\,}}{\,\mathrm{\,Luminous\,}\,\mathrm{\,flux\,}\,\mathrm{\,of\,}\,\mathrm{\,a\,}\,\mathrm{\,source\,}\,\mathrm{\,of\,}555\,\mathrm{\,nm\,}\,\mathrm{\,of\,}\,\mathrm{\,same\,}\,\mathrm{\,power\,}}``
`` ``
`` \therefore 0.6=\frac{\,\mathrm{\,Luminous\,}\,\mathrm{\,flux\,}\,\mathrm{\,of\,}\,\mathrm{\,a\,}\,\mathrm{\,source\,}P\,\mathrm{\,watt\,}}{685P}``
`` \Rightarrow 685\times P\times 0.6=120\times 685``
`` \Rightarrow \,\mathrm{\,P\,}=\frac{120}{0.6}=200\,\mathrm{\,W\,}``
So, 200 W radiant flux is needed to produce the same brightness sensation for 600 nm wavelength.
Page No 455: