NEET-XII-Physics
22: Photometry
Qstn# iv-16 Prvs-Qstn
- #16Two light sources of intensities 8 cd and 12 cd are placed on the same side of a photometer screen at a distance of 40 cm from it. Where should a 80 cd source be placed to balance the illuminance?Ans : Total intensity of the 8 cd and the 12 cd light source is 20 cd.
Let d be the distance of the 80 cd source.
∴ Illuminance due to the 20 cd source`` \left({E}_{1}\right)`` is:
`` {E}_{1}=\frac{20}{{\left(0.4\right)}^{2}}`` .....(1)
∴ Illuminance due to the 80 cd source`` \left({E}_{2}\right)`` is:
`` {E}_{2}=\frac{80}{{d}^{2}}`` .....(2)
As, E1 = E2
`` \therefore `` `` \frac{20}{{\left(0.4\right)}^{2}}`` = `` \frac{80}{{d}^{2}}``
⇒ d = 0.8 m = 80 cm.