Optical density

Optical density is like how easy or hard it is to see through something. Imagine a piece of paper with a picture on it. If you hold the paper up to a bright light, you can probably see through it a little bit. But if the picture is really dark and there's a lot of ink on the paper, it will be harder to see through it.

Now think about a glass filled with water. When you look at the glass, you can see through the water and see what's on the other side. But if you add a lot of sugar or something else to the water, it becomes harder to see through it. This is kind of like how optical density works.

Scientists use optical density to describe how much light can pass through something. The more light that can pass through, the lower the optical density. The less light that can get through, the higher the optical density.

There are devices called spectrophotometers that measure optical density. They shine a light through a sample and measure how much of it comes out on the other side. Then they can use that information to calculate the sample's optical density. People use optical density to figure out how much of something is in a sample, like how much sugar is in a glass of soda, or to see how much bacteria is in a test tube.