Photo 51 is the nickname given to an X-ray diffraction image of DNA taken by Raymond Gosling in May 1952, working as a PhD student under the supervision of Rosalind Franklin, at King’s College London in Sir John Randall‘s group. It was critical evidence in identifying the structure of DNA. 
James Watson was shown the photo by Maurice Wilkins without Rosalind Franklin’s approval or knowledge and along with Francis Crick, Watson used characteristics and features of Photo 51 to develop the chemical model of DNA molecule. In 1962, the Nobel Prize in Physiology or Medicine was awarded to Watson, Crick and Wilkins. The prize was not awarded to Franklin; she had died 4 years earlier, making her ineligible for nomination. 
The photograph provided key information that was essential for developing a model of DNA. The diffraction pattern determined the helical nature of the double helix strands (antiparallel). The outside linings of DNA have a phosphate backbone, and codes for inheritance are inside the helix. Watson and Crick’s calculations from Franklin’s photography gave crucial parameters for the size and structure of the helix. 
Photo 51 became a crucial data source that led to the development of the DNA model and confirmed the prior postulated double helical structure of DNA, which were presented in the articles in the Nature journal by Raymond Gosling.