The luminous efficacy of monochromatic radiation of frequency 540×10¹² Hz, Kcd, is to be ____ when expressed in lumen per watt.

Correct option is B

The correct answer is (B) 683
Explanation:
• The frequency 540×10¹² Hz corresponds to 555 nm (green light), where the human eye’s sensitivity is maximum.
• SI defines luminous efficacy at this wavelength as exactly 683 lumen/watt.
• This is used as a reference for all photometric measurements.
• The candela (cd) is defined using this luminous efficacy standard.
Information Booster:
• Luminous flux unit: lumen (lm).
• Luminous intensity unit: candela (cd).
• Human eye responds most strongly to green light.