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

Question

Question

This question was previously asked in

SSC JE EE Previous Year Paper 15 (Held On: 26 Sep 2019 Evening)

Answer 1

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