Heiligenschein and Albedo
Heiligenschein
In this section we will look at some interesting physics related to the simple law of reflection. The first topic has been readily observable by you since you first started looking at the world around you and yet you may not have ever noticed it.
The concept of heiligenschein (pronounced high-lee-ghen-shine) which means holy shine or holy appearance in German, or something like halo, is closely related to the topic of reflection. It will always be the case that when light is at your back while looking at a surface, that the surface will appear brighter than when the light source is coming at some angle other than parallel to your line of sight.
This means that while walking down the street and observing your shadow, the pavement will be brighter right around your shadow than the same pavement off to your right or left side (assuming your shadow is in front of you). If you've read about such topics, you may be aware that the brain often creates contrasts even when they aren't really present. This is NOT such a case. The relative brightness is real. Please take a look at the Wikipedia site below. If you can't see it, please turn off the safe browsing feature on your browser. The site has nice illustrations of this phenomenon as well as further descriptions of it.
Heiligenschein Article
Brightness of Moon Phases
This notion of heiligenschein can be extended to looking at our moon in the sky. You have likely noticed that a full moon is brighter than all other phases, and you should expect as much. What you probably do not know is that when half of the moon is illuminated from our earthly perspective (called either a first quarter or third quarter moon), it is only 8% as bright as a full moon! But shouldn't we expect it to be half as bright as the full moon? NO! The relative orientations of the sun, earth and moon during a full moon are such that your line of sight toward the moon and the direction of the sun's light are nearly parallel. When we see the moon half illuminated your line of sight is orthogonal to the sunlight incident on the moon's surface. So when half the moon is illuminated, that illuminated half still has lots of little shadows from our perspective whereas when the full surface is illuminated, the sunlight fills the shadows that we'd otherwise see from our perspective.
If this isn't weird enough, the truth is that you have never actually seen a full moon! In fact, if your line of sight is parallel to the sunlight, it leads to a lunar eclipse where earth's shadow covers the moon. The few humans who have truly seen a full moon saw it from a spacecraft with the sun at their backs while viewing the moon ahead of them.
What's remarkable is that the difference between a "full" moon from earth and actually seeing a full moon from space leads to the moon being 30% brighter as seen from space. Having the light traveling parallel to the astronaut's line of sight as compared with viewing from earth where there is a 5 degree angle between our line of sight from earth to moon and the path of sunlight during what we call a full moon makes a notable difference.
Albedo
To describe how well an object reflects light we use a coefficient called the albedo. This coefficient would be zero if the object absorbed all the light incident on it and therefore reflected back none of it. The coefficient would be equal to one if all the light incident on it was reflected back. This albedo includes considerations for heiligenschein, but also includes factors like what color the surface is, etc.
Grass, for example, largely absorbs all colors of light besides green light. This means it certainly doesn't have an albedo of one. Besides its color, much of the light gets reflected off the blades and strike the soil and gets absorbed. This further diminishes its albedo.
Clouds on earth and on other planets as seen from space are in comparison to grass, rather shiny. They do a decent job of reflecting incident light. Please take the time to read at least the parts up to terrestrial albedo in the Wikipedia site below.
Albedo Article
Types of Albedo
One thing that's confusing about albedo is that the standard definition of albedo tells us the fraction of light averaged across the solar spectrum that gets reflected back, but this might not correlate too well with how the object absorbs in visible wavelengths. While visible light is all we see with our eyes, we may be more concerned about infra-red absorption, for instance, if we are studying planetary greenhouse effects and the subsequent warming. When albedo is quoted it is important to know what's being quantified.
In study of astronomical objects we distinguish how well an object reflects only visible light from how well it absorbs across all wavelengths of radiation. Geometric albedo tells us about how an object reflects only visible light and Bond albedo tells us how it reflects all wavelengths including the non-visible ones. Bond albedo is named after the researcher who defined the term, and is not related to molecular bonds or anything. It is the Bond albedo that will be more important in studies of global warming, climate, interior temperatures in office buildings, etc. Geometric albedo would tell us more about how bright a planet would appear from a distance.