Heinrich Gustav Johannes Kayser was a smart man who lived a long time ago. He liked to learn about things that were too small to see, like tiny particles that made up light and sound. He was really good at understanding how things moved and how fast they moved.
He found out that different things, like light and sound, could be measured by waves. It's kind of like when you throw a rock into a pond and watch the ripples move out in all directions. These waves can tell us a lot about things, like how hot or cold they are, or how they make us feel.
Kayser was so good at understanding these waves that he was able to make a special kind of camera that could take pictures of them. This allowed him to see things in a way that no one else had ever seen before. He used his camera to study things like the colors of light, how they changed when they bounced off different surfaces, and how they could be made into different patterns.
He was really proud of what he was able to learn, and he wrote many books to help other people understand it too. Even to this day, many people still use the things he discovered to help them learn about the world around us. So, even though he lived a long time ago, Kayser's ideas and work are still important today!