Capsule : Le spectre électromagnétique de la lumière

What is Light? Its Nature and Discoveries

The Early Understanding of Light

• In 1666, Isaac Newton isolates a ray of sunlight through a small opening in a dark room and uses a glass prism to refract the light into a spectrum of colors: red, orange, yellow, green, blue, indigo, violet.

• Although indigo does not exist in the spectrum, Newton included it to maintain the mystical significance of the number seven during the Enlightenment period.

Advancements in Light Discovery

• In 1800, William Herschel discovers infrared light by measuring temperature variations across different colors in sunlight; he finds that temperatures rise beyond red.

• Johann Ritter later identifies ultraviolet light in 1801 when he exposes silver chloride photographic plates to sunlight and observes reactions beyond violet.

Understanding Wavelengths

• Thomas Young interprets colors as manifestations of light wavelengths around 1 micron (one-thousandth of a millimeter) through his interference experiments conducted in 1803.

• Heinrich Hertz demonstrates electromagnetic waves in 1885 by creating sparks between two metal spheres and observing simultaneous sparks at a distance without wires.

Expanding the Electromagnetic Spectrum

• Hertz concludes that these are electromagnetic waves with properties similar to light; he measures their wavelength at about one meter.

• By 1895, Wilhelm Röntgen discovers X-rays while studying electric currents in low-pressure bulbs; these rays can penetrate various materials including human bodies.

Radioactivity and Further Discoveries

• The discovery of shorter wavelength radiation involves key figures like Henri Becquerel (radioactivity), Marie Curie (isolating radium), Paul Villard, and Ernest Rutherford who categorize radioactivity into alpha, beta, and gamma types by 1900.

• Rutherford establishes the electromagnetic nature of gamma rays by 1914; this completes much of our understanding of the electromagnetic spectrum but leaves some boundaries unclear until microwave technology emerges post-WWII.

Modern Applications and Instruments

• During WWII, radar technology is developed for detecting enemy bombers using microwaves; this leads to innovations such as microwave ovens discovered incidentally by American engineers in the 1950s.

• It takes nearly three centuries to fully explore electromagnetic radiation. Our eyes perceive only a tiny fraction; however, advanced detectors now allow us to visualize invisible lights for cosmic exploration.

Cosmic Exploration Through Light

• Modern astronomy utilizes satellites that capture images across various wavelengths rather than just visible light. This enables detailed mapping of celestial objects like dust clouds, stars (both visible and invisible), galaxies, neutron stars, black holes, and supernovae.