Unlike most metals, gold is yellowish rather than silver because fast-moving electrons reduce the size of its atoms and alter how they absorb light.

Although gold has been prized since its use in 40,000 BCE for decorative purposes, it has more recently served as a replacement for mercury in the production of PVC plastic. Because gold does not react with air or corrode, it has remained in circulation and reused in electronics and biomaterials.

As electromagnetic waves interact with particles in a transparent medium, particles vibrate and generate their own electromagnetic waves. The interference of the original and generated waves, which are slightly delayed, results in ensemble light with an effective slower speed but unchanged color.

Gold is a pretty expensive topping. Nonetheless, luxury menus around the world are offering food staples—think bagels, chocolate, and burgers—wrapped or dusted with gold. What's the deal with this fascination?

This peak intensity is produced alongside a spectrum of other colors, which, when combined, make the sun appear white in space. Since Earth's atmosphere scatters bluer and greener wavelengths of light more than red and orange wavelengths, sunlight reaching Earth's surface appears yellow, while the sky appears blue.

Out of the total above-ground stock of gold (weighing roughly 216,265 metric tons), the largest share is found in jewelry (97,149 metric tons).

When heated, electrons absorb energy and move to excited states within an atom. They promptly drop back down and release light energy corresponding to the size of the drop, which is unique to each element.

When electrons within an atom drop to a lower energy level closer to the nucleus, they lose energy and produce light with energy equal to the difference between the two energy levels. Because these levels and the corresponding emitted light are unique to each element, the light serves as a unique signature that can be compared to known lab samples for identification.

In principle, this provides price stability as the amount of gold (and therefore the value of the currency) is limited, but also restricts the ability to modulate the money supply during economic downturns.

Earth's core, containing 99.999% of all known gold, is leaking trace amounts of precious metals into the mantle. Advanced isotopic methods have detected this leakage at volcanoes in Hawai'i, providing new insights into the movement of material beneath Earth's surface.

For centuries, the things people used as money had intrinsic value, such as precious metals like gold. In medieval Italy, giant wheels of parmesan cheese were once used as a store of value.

All forms of electromagnetic radiation, from radio waves to gamma rays, travel at the speed of light in a vacuum, but their wavelengths decrease as their frequencies increase. The light-sensitive nerve cells in the human eye are sensitive to a narrow range of values, which we observe as the spectrum of visible light.

When objects emit or reflect light, it takes time for the light to reach an observer and be detected. During the travel time, the object may have changed, limiting detections to describing the conditions of the object when the light was first emitted or reflected, before it was detected.

Just as fire ranges from red to blue with increasing temperature, lightning's blueish-white comes from the superheating of air. The emission of light from the predominantly nitrogen- and oxygen-rich atmosphere produces a purple tint, with additional color caused by light scattering from humidity, hail, dust, and ash.

"Gold bugs" often argue that fiat money allows governments to engage in financially reckless behavior (think: chronic borrowing). They also say the price of gold rises as the value of fiat currency falls.

Gold is valued for its stability and reliability, serving as a hedge against inflation and economic uncertainty. Bitcoin shares both of these characteristics, but also provides near-instant liquidity and isn’t subject to government regulation.

As space stretches out, the wavelength of light traveling through it also expands, causing its frequency to decrease to maintain the speed of light constant. Since we perceive frequency as color, a lower frequency would produce a redder color, allowing astronomers to correlate reddening with the rate of expansion.

About 216 million metric tons—about 0.4% of the Great Pyramid of Giza's volume—have been extracted and used primarily for jewelry. The remaining reserves are estimated based on chemical analysis of rock samples extrapolated to the entire mass of Earth's crust.

It’s typically kept in the form of bars or ingots. Governments and central banks often keep bullion reserves on hand; they can use it as a hedge against inflation. One can invest in bullion via ETFs, among other financial vehicles.

However, critics argue that globalization has made the gold standard impractical for modern economies, as it limits a nation’s ability to have a flexible monetary policy—hence, none of today's governments have a gold standard.

Light-dependent reactions during photosynthesis convert sunlight into chemical energy, stored within the bonds of molecules such as ATP and NADPH. Rather than being absorbed for these reactions, most green light is reflected, causing most plants to appear green.

Although light energy is captured to power the reactions that produce the building blocks of starch, cellulose, glucose, and other sugars, chlorophyll a and b are most sensitive to violet, blue, orange, and red light, leaving green light mostly unabsorbed. This green light is instead reflected, giving most plants this color appearance.

Observing high-angle deflections when alpha particles were fired at gold foil led to an atomic model with a dense, positively charged nucleus. Firing those particles at nitrogen resulted in the emission of hydrogen nuclei—protons—leading to the discovery of the subatomic particle.

Particles like electrons can exist in isolation, enabling them to move more freely. Within circuits, this allows electrons to carry energy across wire surfaces quickly. Contrastingly, light created in the sun's core is delayed by a million years on its way to Earth due to collisions with free electrons.

Just as one becomes exhausted as they walk up a flight of stairs against gravity, light is predicted to lose energy as it travels out of a gravitational well created by an object warping spacetime. This is observable through a decrease in the frequency of light toward longer, redder wavelengths.

The troy ounce is the standard measurement for precious metals like gold and silver and it’s heavier than a standard ounce. The term “troy” is widely believed to come from the town of Troyes in France, which was a popular trading hub for merchants all across Europe in the Middle Ages.