1. How a Small Hole and a Prism Made a Long Band of Light
Newton made a window in a room completely dark and made a small hole in it. He placed a prism in the path of the single beam of light coming through the hole [1]. On the wall, he saw a band with all the colors of the rainbow. However, Newton cared about the shape, not just the number of colors. The hole was a small circle, but the image on the wall was not round. It was a long, thin shape, about 5 times longer than it was wide [2]. This experiment is said to have happened when Cambridge University was closed due to a plague outbreak. The exact year varies in records, with some sources saying 1664 to 1666, and others saying around 1665 to 1666 [3][1].
2. Why the Band Was Long: Each Color Bends by a Different Amount
At that time, people thought colors came from white light changing. They believed light got colored when it passed through things like glass. People from Aristotle to Descartes held this view [1]. Newton thought the opposite. He explained that the image became long because different colors bend (refract) by different amounts [1]. He changed the thickness of the prism and the size of the hole, but the result was the same. So, he concluded that sunlight itself is a mix of many colors [1].
3. How Newton Built a Telescope That Avoided Color Blur
Telescopes that use lenses have a problem. Because light bends differently for each color, a color blur (chromatic aberration) appears around the image. Newton thought this was hard to fix with lenses, so he made a reflecting telescope that uses mirrors to gather light instead [5]. Note that modern lens telescopes have ways to fix this blur, but Newton did not have those options at the time. The first reflecting telescope was finished in 1668. A second version made in 1671 is kept at the Royal Society [4]. The mirror was made of an alloy of copper and tin [5]. The second telescope had two tubes that slide into each other to focus. It was about 20 cm (about 8 inches) long. Its magnification is said to be about 30 times or about 40 times, depending on the source [3][5].
4. The Criticism Newton Received After Publishing His 1672 Letter
In a letter dated February 19, 1672, titled "A New Theory about Light and Colors," Newton wrote about testing color phenomena with a triangular prism. This letter was published in the Royal Society's journal, becoming his first paper [2]. Robert Hooke and others criticized this paper [6]. Newton replied publicly, but by May, he wrote that he decided not to send new papers [6]. Even after this argument, Newton stated in his book "Opticks" that he would show light through experiments and reasoning [7].
5. Why Newton Published "Opticks" in 1704
"Opticks" was published in 1704 [3]. In the preface, Newton wrote that he delayed printing to avoid getting involved in arguments [7]. This was more than 30 years after his 1672 paper. In the same preface, he wrote the goal of the book. He wanted to show and prove the nature of light using reasoning and experiments, not just hypotheses [7]. The book shows through experiments that white light is a mix of colors, that mixing colors returns to white, and that the color of objects relates to how their surfaces reflect light [3].
6. How to Try Making a Light Band Yourself at Home
On a sunny day, close the curtains to make the room dark. Let a thin beam of light come through a gap and hit a clear glass with water in it. Project this onto white paper. Depending on the conditions, you may see a faint colored band. Look at the shape and order of the colors. If you do not see it, try changing how the light enters. Whether it looks like the shape Newton saw depends on the shape of the hole in the window and the glass. For safety, do not look directly at the sun, and handle the glass with an adult [7][1].