It is believed that light travels in straight lines. However, light bends very slightly at the edges of objects. Thus, the shadows formed will be a little smaller than if it were a simple straight line. This effect is called diffraction.
The effects of diffraction of light were first carefully observed and characterised by Francesco Maria Grimaldi. Grimaldi also coined the word ‘diffraction’ from the Latin word diffringere, meaning, ‘to break into pieces’, referring to light breaking up in different directions. Grimaldi’s observations were posthumously published in 1665. A more conclusive study was done by Augustin-Jean Fresnel who made his calculations on diffraction public in 1815 and 1818.
The discovery of diffraction supported the wave theory proposed by Christiaan Huygens. According to the wave theory, light is a stream of waves, with each wave made up of smaller wavelets. When light hits a glass at an angle, the wavelets that reached first would slow down. This causes light to bend. As the wave theory conflicted with Newton’s ideas, it was not accepted until the 19th century.
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