Light & Coherence part 2: Spatial Coherence (and the Double Slit Experiment)

Summary

This video delves into the concept of spatial coherence, following up on the previous episode about temporal coherence and light's spectral properties. Using a pond analogy and numerical simulations, the presenter illustrates how waves become organized and spatially coherent over distance. The video explores the relationship between spatial coherence and geometry, explaining the formula for coherence area and applying it to the Sun and Antares, a star in the Scorpio constellation. It also touches on the historical misunderstandings in science communication, the double-slit experiment with white light, and the Fourier transform effect with multiple slits. The script concludes by pondering the classical versus quantum perspectives on light, setting the stage for further discussion in a future sequel.

Takeaways

• 📚 The video is a sequel focusing on spatial coherence, continuing from a previous episode about temporal coherence and its relation to the spectral properties of light.
• 🌌 The presenter uses a pond in Hilversum, Netherlands, named after Hendrik Antoon Lorentz, to illustrate spatial coherence by observing the behavior of waves in water.
• 🌊 Waves in the pond become more organized and linear as they move away from the chaotic splash zone, demonstrating how spatial coherence develops with distance from the source.
• 🔍 A numerical simulation by Nils Berglund is introduced to show in detail how waves develop over time and how spatial coherence grows with distance from multiple wave sources.
• 📉 The simulation highlights areas of strong wave amplitude separated by boundaries of low amplitude due to destructive interference, illustrating spatial coherence and decoherence.
• 📐 The spatial coherence of waves can be quantitatively understood through geometry, with the coherence area being related to the distance from the source, wavelength, and source diameter.
• 🌞 The formula for coherence area is applied to the Sun, revealing that its spatial coherence on Earth is quite small, comparable to the cross-section of a human hair.
• 🔬 Thomas Young's famous experiment on light and colors used sunlight and a pinhole to create a spatially coherent beam, despite common misconceptions about a double slit configuration.
• 🌈 A true double slit experiment with white light is demonstrated, showing how spatial coherence affects the interference pattern and the visibility of colors.
• 🌠 The video discusses the coherence area for stars like Antares, which is much larger than that of the Sun, emphasizing the uniformity of the field despite the star's immense size and distance.
• 🤔 The presenter reflects on the different perspectives on light between classical and quantum mechanics, questioning whether light itself is quantized or if only its interaction with matter is probabilistic.

Q & A

What is the main topic of the second video on coherence?

The main topic of the second video is spatial coherence, explaining how it arises and its implications for the properties of light.

What is the significance of the pond named after Hendrik Antoon Lorentz in demonstrating spatial coherence?

The pond, with its fountain and water jets, serves as a real-life example to illustrate how waves become spatially coherent as they move away from the source of disturbance.

How does the video script use the analogy of waves in a pond to explain spatial coherence?

The script describes how waves in the pond near the fountain are chaotic, but as one moves away, the waves become more organized and linear, demonstrating the concept of spatial coherence.

What role does the numerical simulation play in the explanation of spatial coherence?

The numerical simulation, created by Nils Berglund, provides a controlled environment to observe the development of waves over time and how spatial coherence grows with distance from the source.

How does the script relate temporal coherence to the frequency changes of waves?

The script mentions that the frequency of waves changes with time in the direction of wave propagation, linking this phenomenon to temporal coherence, which was covered in a previous video.

What is the formula given in the script for calculating the area of coherence for waves in 3D space?

The area of coherence is proportional to the distance from the source squared, the wavelength squared, and inversely proportional to the diameter of the source squared, applicable when the distance R is much larger than the size of the emitter.

How does the size of the emitter affect the area of coherence according to the formula?

The area of coherence is inversely proportional to the diameter of the source squared, meaning that a smaller emitter results in a larger area of coherence.

What historical figure's work is discussed in the script, and how does it relate to the understanding of light?

The script discusses Thomas Young and his experiments with light and colors. It highlights the historical misunderstanding around the double-slit experiment and emphasizes the wave nature of light.

How does the script use the double-slit experiment to demonstrate the effects of spatial coherence in light?

The script describes an experiment with two slits and white light, showing how a light source with a larger area of coherence produces an interference pattern with colored lines due to different wavelengths going in and out of phase.

What is the significance of the star Antares in illustrating the concept of spatial coherence?

Antares, being a massive star with a large area of coherence, demonstrates that even though it emits vast amounts of random emissions, the resulting field on Earth is almost uniform within a large area, showcasing the concept of spatial coherence.

How does the script differentiate between the classical and quantum mechanical perspectives on light?

The script suggests that a classical mindset views light primarily as an electromagnetic phenomenon, while a quantum mechanical perspective focuses on the emission and absorption processes involving discrete energy transfers.

What is the script's stance on the quantization of light?

The script questions whether experiments like the photoelectric effect and Compton scattering truly show that light is quantized into discrete energy packages or if they only demonstrate the probabilistic nature of electromagnetic radiation's interaction with matter.