As the 19th century approached its end, the physicists of the time felt that physics was almost a completed subject. Its primary ingredients were absolute space and time, the causal laws of mechanics, electricity and magnetism, embodying a wave model of light, and a picture of matter as consisting of discrete and indivisible particles obeying these laws. But such complacency was shattered in less than ten years by the discovery of the electron, radioactivity, the quantum of energy and special relativity as each of them, in its own way, called for a drastic revision of our picture of the physical world. The discovery of X-rays, which are electromagnetic waves, like light but of a much shorter wavelength had a great influence on the course of physics. J J Thomson's discovery of electron, negatively charged particle with a far smaller mass than any particle previously known, changed the old idea forever that atoms are indivisible. The other constituents that went into the structure of atoms, namely protons and neutrons were discovered later.