The candle power of a lamp placed normal to a working plane is 60 CP. Find the distance if the illumination is 15 lux.

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SSC JE EE Previous Year Paper 15 (Held On: 26 Sep 2019 Evening)
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  1. 1.5 meters
  2. 2.5 meters
  3. 4 meters
  4. 2 meters

Answer (Detailed Solution Below)

Option 4 : 2 meters
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Concept:

Illumination is defined as the luminous flux received by the surface per unit area.

It is usually denoted by the symbol ‘E’ and is measured in lux or lumen/m2 or meter candle or foot candle.

Illumination, \(E = \frac{\phi }{A} = \frac{{CP \times \omega }}{A}lux\)

Solid angle \(\omega = \frac{A}{{{d^2}}}\)

Illumination \(E = \frac{{CP}}{{{d^2}}}lux\)

Calculation:

Given that, candle power (CP) = 60 CP

Illumination (E) = 15 lux

⇒ 15 = 60/d2

⇒ d = 2 metres

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