Question

Two lamps of 2000 and 1000 candle power respectively are suspended 30 ft above the ground and are 100 ft apart. Find the intensity of illumination at a point on the ground in line with the lamps between them and 40 ft from the base of the more powerful lamp –

• A. 0.48 ft-candle
• B. 0.58 m-candle
• C. 0.58 ft-candle
• D. 0.38 ft-candle

Answer 1

Answer: (C)

0.58 ft-candle

Answer 2

I =$\frac{L_{1}\cos\theta_{1}}{r_{1}^{2}}$+$\frac{L_{2}\cos\theta_{2}}{r_{2}^{2}}$=$\frac{L_{1}h}{r_{1}^{3}}$+$\frac{L_{2}h}{r_{2}^{3}}$

I =$\frac{2000 \times 30}{50^{3}}$+$\frac{1000 \times 30}{(67.08)^{3}}$= 0.48 + 0.1 = 0.58 ft-candle

Therefore the answer is (3).