How different things absorb different colors

HostIt's funny how we take the colors around us for granted. You look at a bright red apple or a deep blue shirt and you just accept that's how they are. But they're both sitting in the same white light from the sun or a lamp.

HostWhy does one thing swallow up some parts of that light while another thing lets them bounce off?

GuestTo get why that happens, we have to look at light as a kind of fuel or energy. White light from the sun feels like one thing, but it's actually a big mix of every color in the rainbow. It's like a crowded bus full of different people. When that light hits an object, like that red apple, a very specific trade happens. The skin of the apple is made of tiny building blocks, and inside those blocks are even tinier bits called electrons. These electrons are picky. They only want to grab onto very specific amounts of energy. If the energy in a certain color of light matches what they need, they swallow it whole.

HostSo if the apple looks red, does that mean red is the color it likes to eat the most?

GuestIt's actually the opposite. This is the part that trips most people up. When you see a red apple, you're seeing the one color the apple didn't want. The apple skin is full of bits that are perfectly sized to soak up blue light and green light and yellow light. Those colors hit the apple and get trapped. They turn into a tiny bit of heat. But the red light? The electrons in the apple don't have a use for it. They can't soak it up. So that red light just bounces right off the surface and hits your eye. We only see the leftovers of the light that the object rejected.

HostThat feels backwards. If I'm wearing a black shirt, is it rejecting everything or soaking everything up?

GuestA black shirt is the hungriest thing in your closet. It has bits in it that can swallow almost every single color in that white light. Since almost no light bounces back to your eye, your brain sees that lack of light as black. This is also why you get so hot when you wear black on a sunny day. All that light energy from the sun is being taken in and turned into heat. A white shirt is the lazy one. It doesn't want any of it. It bounces almost all the colors back at once, and when all those colors hit your eye together, you see white.

HostI'm trying to picture these little bits in the apple or the shirt. You said they're picky. Why is a blue bit different from a red bit?

GuestIt comes down to the gaps. Think of an electron like a person standing at the bottom of a ladder. To move up to the next rung, they need a very specific amount of energy to make the jump. Not a little more, not a little less. Each kind of thing in the world, whether it's the paint on a car or the leaf on a tree, has its rungs set at different heights. In a green leaf, the rungs are spaced just right so the electrons can grab the energy from red and blue light to make their jump. But the energy in green light isn't the right amount to help them reach the next rung. It's the wrong size. So the green light gets ignored and bounces away.

HostBut if it's just about the gaps in the ladder, why don't we see things changing color all the time? If I hit a blue wall with a hammer and move the bits around, it's still blue.

GuestThat's because those gaps are part of the atoms themselves and how they're hooked together. You would've to actually change what the thing is made of to change the color it soaks up. When a leaf turns from green to brown in the fall, that's exactly what's happening. The green stuff, which we call chlorophyll, is breaking down. Those ladders are being taken apart. New bits with different gaps take over, and they start soaking up different parts of the light. The shape of the tiny bits determines the color we see.

HostWhat about something like glass? It seems like it doesn't have any ladders at all because the light just goes straight through.

GuestGlass actually has ladders, but the rungs are way too high. The light we can see with our eyes doesn't have enough energy to help those electrons make the jump. It's like trying to throw a ball to someone on a roof that's ten stories up. You just can't get it there. So the light doesn't get soaked up and it doesn't bounce off. It just passes right through the gaps and comes out the other side.

HostSo when we look at a sunset and see all those deep oranges and reds, is the air itself changing its ladders?

GuestThe air is staying the same, but the path of the light is changing. During the day, the sun is right above you and the light has a short trip through the air. But at sunset, the light has to travel through a lot more of the blanket of air around the earth to reach you. All the blue light gets bumped around and scattered away by the tiny bits in the air before it can get to your eyes. By the time the light reaches you, only the long, red waves are left.

GuestThe most amazing part is that most of the light in the world is hitting things with ladders we can't even see, using colors that our eyes aren't built to catch.

HostThe blue of the sky and the red of the apple are just the few scraps of light that our world decided it didn't have any use for.