826. 1 Some important discoveries at the end of the 20th century 3. The discovery of radioactivity Radioactivity occur naturally and have with us on the earth from the very beginning Henri Becquerel discovered Uranium, Marie curie discovered polonium and radium Ernest rutherford found that the substances emit several distinct types of radiations. One is a penetrating radiation, dubbed a that propagates through several centimeters in air and can even penetrate very thin metal foils. Another less penetrating radiation, dubbed B, is easily stopped by even a sheet of paper Another type, called r, was discovered in 1900 and is much more penetrating than even the a radiation 526.2 The appearance of Plank' s constant h 1. Blackbody Radiation 心 Thermal radiation 800K l000K 1200K 1400K Rb cu The color emitted by atoms after they have exited by heat is characteristic of the particular element they comprise 66 3. The discovery of radioactivity Radioactivity occur naturally and have with us on the earth from the very beginning. Henri Becquerel discovered Uranium, Marie Curie discovered Polonium and Radium Ernest Rutherford found that the substances emit several distinct types of radiations. One is a penetrating radiation, dubbed α, that propagates through several centimeters in air and can even penetrate very thin metal foils. Another less penetrating radiation, dubbed β, is easily stopped by even a sheet of paper. Another type, called γ, was discovered in 1900 and is much more penetrating than even the α radiation. §26.1 Some important discoveries at the end of the 20th century §26.2 The appearance of Plank’s constant h 1. Blackbody Radiation 800K 1000K 1200K 1400K ™Thermal radiation Sr Rb Cu The color emitted by atoms after they have exited by heat is characteristic of the particular element they comprise