From: veritasium
The fundamental question, “What is light?”, has puzzled scientists for centuries [00:00:59]. Historically, the greatest minds in science debated this very issue [00:01:03]. Initial public perception of light often focuses on its observable qualities, such as brightness [00:00:14], its ability to illuminate a room [00:00:21], and its different colors [00:00:24].
Historical Debate on the Nature of Light
The nature of light was a hotly contested topic. In the late 1600s, Isaac Newton, in his treatise Opticks, proposed that light was a stream of particles or corpuscles [00:01:06]. However, at the same time, Dutch physicist Christiaan Huygens argued that light behaved as a wave [00:01:14]. This debate persisted for many years [00:01:19].
Young’s Double Slit Experiment
The long-standing debate was eventually settled by Thomas Young’s double slit experiment [00:01:19]. To ensure accuracy in recreating this experiment, the original source material was consulted: Thomas Young’s handwritten notes from 1803, found in the vault beneath the Royal Society in London [00:01:27], [00:01:37].
Young’s original notes described the setup and observation:
“I brought into the sunbeam a slip of card, about one-thirtieth of an inch in breadth, [00:01:41] and observed its shadow, either on the wall or on other cards held at different distances. [00:01:46] Besides the fringes of color on each side of the shadow, [00:01:52] the shadow itself was divided by similar parallel fringes, of smaller dimensions.” [00:01:55]
This experiment is simple enough to perform at home, though it is “fiddly” to achieve with sunlight [00:02:01].
Experiment Setup and Observations
A recreation of the experiment involved an empty box with a small eyepiece and a hole [00:02:19]. A slide containing two very narrow, side-by-side openings—a “double slit”—was placed over the hole [00:02:24]. The box was then tilted towards the sun so that sunlight hit the double slit directly [00:02:33].
Before looking, people commonly predicted seeing two distinct lines [00:02:41], [00:02:44], or perhaps even one line [00:02:48]. However, the actual observation was surprising:
“I see dots.” [00:03:05] “Oh, there’s one.. there’s one in the middle, the strongest, two either side.” [00:03:09] “The two on the outside are multicolored, and the one in the middle… is just white.” [00:03:12] “It looks kind of a rainbow.” [00:03:17] “Quite a few colors and lots of little dots.” [00:03:20] “And there are more dots appearing…spreading along.” [00:03:22]
This unexpected pattern left observers “really confused” [00:04:05].
Explaining the Phenomenon: Wave Interference
The observed pattern makes no sense if light behaves solely as particles [00:03:51]. If light were particles, passing through two slits would simply produce two bright spots underneath [00:04:14].
However, if light behaves as waves, then the waves from each slit can interact with one another [00:04:23]. This phenomenon can be demonstrated with water waves:
- A single source of ripples creates circular wavefronts [00:04:33].
- Adding a second source of ripples, similar to the two slits, creates an interesting pattern [00:04:46]. This pattern is formed by the ripples interacting [00:04:56].
This interaction leads to two types of interference:
- Constructive Interference: Where wave peaks meet peaks, and troughs meet troughs, the amplitude of the wave increases [00:05:02].
- Destructive Interference: If a peak from one wave meets a trough from another, they cancel each other out, resulting in no wave [00:05:09]. This is analogous to “light canceling itself out” [00:05:36].
This is precisely what occurs with light in the double slit experiment [00:05:17]:
- When light waves from the two slits constructively interfere (peaks with peaks, troughs with troughs), they produce a bright spot [00:05:20].
- When light waves destructively interfere (trough from one meets peak from the other), no light is observed [00:05:27].
The result is a series of bright and dark spots [00:05:50].
Wavelengths and Color Differences
A slight complication arises because sunlight is composed of many different colors, and each color has a different wavelength [00:05:54]. Because of their different wavelengths, colors will constructively interfere at slightly different points [00:06:00]. This phenomenon explains the “rainbowing effects” observed further from the central bright spot [00:06:00]. For example, red light will meet up at different places than blue light [00:06:07].
Therefore, the difference between blue light and red light, or any colors, is their wavelength [00:00:24], [00:06:14]. Our perception of color is fundamentally linked to these different wavelengths [00:06:22].
Unanswered Question
It was noted that the light pattern observed at the bottom of the box appeared as “kind of round blobs” rather than defined slits [00:07:00]. The reason for this specific shape remains an open question for further consideration [00:07:08].
Conclusion
The compelling results of Young’s double slit experiment led the scientific community to conclude that light must behave as a wave [00:07:21].