Final answer:
The amount of light that reaches us per unit area from a luminous object is measured as illuminance in lux, which decreases proportionally with the square of the distance from the light source. In astronomy, this is described as flux, the energy received per area per second from a star or celestial body. The inverse square law governs how illumination changes with distance, affecting the amount of light planets in our solar system receive from the Sun.
Step-by-step explanation:
The amount of light reaching us per unit area from a luminous object is typically measured as illuminance, represented in units of lux (lumens per square meter). This value describes the intensity of light that falls on an object at a specific distance from the light source. For example, if you are near a light that is radiating uniformly in all directions, the illuminance at a distance can be determined by the inverse square law, which states that illuminance decreases proportionally to the square of the distance from the source. Therefore, if you double the distance from the light source, the illuminance will decrease by a factor of four.
In the context of astronomy, when discussing stars or celestial bodies, we refer to this as the flux, which measures the power of light received over an area (energy per area per second). Astronomers distinguish this from luminosity, which is a measure of the total energy output from the source and does not depend on the distance. The flux can inform us about the power of light we receive from a star and helps in determining its distance from us, among other properties.
Applying this to our solar system, Venus and Mars receive different amounts of sunlight per square meter than Earth does because of their respective distances from the Sun. The inverse square law dictates that Venus, being closer, receives about twice the illumination per square meter, while Mars, being farther, receives about half the illumination per square meter as compared to Earth.