A battery-operated torch is configured to emit an almost parallel beam of light. When the light strikes a wall 2 m away, it produces an illuminance of 40 lux . The illuminance produced when it strikes a wall 4 m away is approximately

Step-by-step solution

For a parallel beam of light, the cross-sectional area of the beam remains constant as it propagates through space. Illuminance E is defined as the luminous flux incident per unit area A , given by E = A . Since neither the luminous flux nor the cross-sectional area changes with distance for a parallel beam, the illuminance remains independent of the distance from the source. Therefore, the illuminance at a distance of 4 m will be the same as at 2 m , which is 40 lux .