![]() ![]() A mirror has high specular reflectivity and low diffuse reflectivity. ![]() In contrast, diffuse reflectivity measures how much light gets reflected in all directions. Specular reflectivity measures how much of the incoming light gets reflected by the object in the direction given by the mirror angle. ![]() There are two main types of reflectivity: specular reflectivity and diffuse reflectivity. Therefore, a planet that is covered with clouds, such as Earth or Venus, is generally brighter than a rocky moon or planet that has no atmosphere. Snow, rough ice, and clouds are highly reflective. The amount of sunlight incident on a moon or planet that gets reflected depends on the materials in its surface and atmosphere as well as its surface roughness. The strongest source of light in our solar system is the sun, so usually we see planets and moons because they are reflecting sunlight. Since planets and moons do not emit light, the only reason we can see them is because they reflect light from some other source. If a planet somehow became large enough to initiate nuclear fusion and begin glowing, it would no longer be a planet. In contrast, planets and moons do not generate their own visible light*. In terms of astronomical bodies, stars are the main objects that create significant amounts of visible light, and therefore are some of the brightest objects in the universe. Examples include campfires, light bulbs, candle flames, and computer screens. Objects that create light tend to also reflect ambient light, so that they tend to be the brightest objects around. Either the object creates new light or it reflects light that already existed. There are two main ways that an object can direct light into our eyes. In general, we can see objects because they direct light into our eyes (or into cameras which record information that is later used by display screens to direct light into our eyes). Photo of the moon and earth when illuminated directly by sunlight, as taken by the DSCOVER spacecraft on July 16, 2015. ![]()
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