Professor Roy Sambles F.R.S. - Department of Physics, Exeter University
In the early part of the 20th century our understanding of the world was dramatically changed. First Einstein interpreted the photoelectric effect to show that light (convincingly demonstrated by Thomas Young in 1803 to be wavelike) appeared to comprise 'packets' of energy 'photons' and was thus seemingly particulate (as Newton with his 'corpuscular' theory of light had also conjectured in the 17th century). Some 20+ years later, in 1927, Davisson and Germer in the USA and Thomson in the UK showed that electrons (up till then perceived as particles) could diffract - they behaved as waves. This laid the foundations for the now well-established quantum mechanical description of the Universe and 'wave-particle' duality. These historical events together with some of the remarkable consequences of the resulting new understanding of our Universe will be discussed.