What physical process converts the collapse of a tiny gas bubble driven by intense sound waves in a liquid into a picosecond flash of visible/UV light, with the bubble's interior briefly reaching temperatures rivaling the Sun's surface or hotter?
#optics
Post a problemWhat causes the brief green (sometimes blue) flash sometimes seen at the upper edge of the sun just as it sets or rises over a clear horizon?
On a clear day, why does the sky directly overhead look a deep, saturated blue while the sky near the horizon looks pale, washed-out blue or nearly white?
Does a faint third-order rainbow, formed by three internal reflections inside raindrops and predicted centuries ago but never convincingly photographed in nature, actually occur?
Has a fourth-order rainbow, formed by four internal reflections inside raindrops, ever been captured in a natural photograph after more than a century of failed attempts?
What optical mechanism produces the colored rings ('glory') seen surrounding the shadow of your head or an airplane cast onto a cloud or fog bank?
A pinhole camera with too large a hole gives a blurry, doubled image from simple geometry, but too small a hole also blurs the image from diffraction: is there an optimal pinhole diameter, and if so, what determines it?
Why do you sometimes see one or more faint, closely-spaced extra bands of color (supernumerary arcs) just inside the primary rainbow, in addition to the usual bow?
Why does a brief flash of green light sometimes appear at the very top edge of the sun in the instant it disappears below a clear, distant horizon at sunset (or reappears at sunrise)?
Why does a ring of light (a halo), or a pair of bright spots on either side of the sun (sun dogs), sometimes appear around the sun or moon when thin high clouds are present?
Diffuse interstellar bands
Research level
What molecules cause the hundreds of unexplained absorption lines (diffuse interstellar bands) seen in starlight passing through interstellar space, and why have only a handful been identified after a century of study?
Gamma-ray lasers (grasers) and superhigh-power lasers
Research level
Can a laser-like device be built that emits coherent gamma rays (a 'graser'), using nuclear rather than atomic transitions, and how far can achievable laser intensities be pushed?
Neutrons, like electrons, have a wavelength that could in principle be used for microscopy: how does the achievable resolving power of a neutron microscope compare to an electron microscope, and what fundamentally limits it?
Why does an airplane passenger or mountain hiker sometimes see concentric colored rings of light (a 'glory') surrounding the shadow of their own head, cast on clouds or fog directly opposite the sun?