americium-241 emits alpha particles. the alpha particles ionize the air, creating a small current. smoke reduces the current, sounding the alarm. the isotope is extremely dangerous if inhaled or ingested. but the EPA says it's okay to just throw away old smoke detectors in the trash.

Is NP-complete in existing models

560 nm – 580 nm

James Clerk Maxwell in 1867 developed the theory of electromagnetism, which predicted the existence of radio waves, electromagnetic waves much slower in frequency and longer in wave length than light.

essentially a helium atom stripped of two electrons. more dangerous but less penetrating than beta and gamma particles.

1,013.25 hectoPascals (hPa), 29.9212 inches Mercury (inHg), or 14.696 pounds per square inch (psi)

530 nm – 540 nm

The human eye with normal vision has three kinds of cone cells that sense light, having peaks of spectral sensitivity in short ("S", 420 nm – 440 nm), medium ("M", 530 nm – 540 nm), and long ("L", 560 nm – 580 nm) wavelengths. These cone cells underlie human color perception in conditions of medium and high brightness; in very dim light color vision diminishes, and the low-brightness, monochromatic "night vision" receptors, denominated "rod cells", become effective. Thus, three parameters corresponding to levels of stimulus of the three kinds of cone cells, in principle describe any human color sensation. these three values compose a tristimulus specification of the objective color of the light spectrum. The three parameters, denoted "S", "M", and "L", are indicated using a 3-dimensional space denominated the "LMS color space", which is one of many color spaces devised to quantify human color vision.

an alpha particle

Heinrich Hertz in 1887

420 nm – 440 nm

about 138 days. alpha decays to the stable polonium-206