Lord Rayleigh, sepenuhnya John William Strutt, Baron Rayleigh ke-3 dari Terling Place, (lahir 12 November 1842, Langford Grove, Maldon, Essex, England — meninggal 30 Jun 1919, Terling Place, Witham, Essex), saintis fizikal Inggeris yang membuat penemuan asas dalam bidang akustik dan optik yang menjadi asas kepada teori penyebaran gelombang dalam bendalir. Dia menerima Hadiah Nobel Fizik pada tahun 1904 kerana berjaya mengasingkan argon, gas atmosfera lengai.
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Strutt menderita kesihatan yang buruk sepanjang masa kanak-kanak dan masa mudanya, dan perlu baginya untuk ditarik dari Eton dan Harrow. Pada tahun 1857 dia memulakan empat tahun pengajian swasta di bawah tutor. Pada tahun 1861 Strutt memasuki Trinity College, Cambridge, dari mana dia lulus dengan gelar BA pada tahun 1865. Dia awalnya mengembangkan minat yang menyerap dalam kedua-dua aspek eksperimen dan matematik sains fizikal, dan pada tahun 1868 dia membeli pakaian alat ilmiah untuk penyelidikan bebas. Dalam makalah pertamanya, yang diterbitkan pada tahun 1869, dia memberikan penjelasan yang jelas mengenai beberapa aspek teori elektromagnetik James Clerk Maxwell, ahli fizik Scotland, dari segi analogi yang akan difahami oleh rata-rata manusia.
Serangan demam reumatik sejurus selepas perkahwinannya pada tahun 1871 mengancam nyawanya untuk sementara waktu. Perjalanan pemulihan ke Mesir dicadangkan, dan Strutt membawa pengantin perempuannya, Evelyn Balfour, saudara perempuan Arthur James Balfour, dalam perjalanan menaiki kapal ke Sungai Nil untuk percutian musim sejuk yang panjang. Pada lawatan ini, dia mulai mengerjakan bukunya yang hebat, Theory of Sound, di mana dia meneliti persoalan getaran dan resonans pepejal dan gas elastik. Jilid pertama muncul pada tahun 1877, diikuti oleh yang kedua pada tahun 1878, menumpukan pada penyebaran akustik di media material. Setelah beberapa kajian semula sepanjang hayatnya dan cetakan semula berturut-turut setelah kematiannya, karya tersebut tetap menjadi monumen kesusasteraan akustik terpenting.
Shortly after returning to England he succeeded to the title of Baron Rayleigh in 1873, on the death of his father. Rayleigh then took up residence at Terling Place, where he built a laboratory adjacent to the manor house. His early papers deal with such subjects as electromagnetism, colour, acoustics, and diffraction gratings. Perhaps his most significant early work was his theory explaining the blue colour of the sky as the result of scattering of sunlight by small particles in the atmosphere. The Rayleigh scattering law, which evolved from this theory, has since become classic in the study of all kinds of wave propagation.
Rayleigh’s one excursion into academic life came in the period 1879–84, when he agreed to serve as the second Cavendish Professor of Experimental Physics at Cambridge, in succession to James Clerk Maxwell. There Rayleigh carried out a vigorous research program on the precision determination of electrical standards. A classical series of papers, published by the Royal Society, resulted from this ambitious work. After a tenure of five years he returned to his laboratory at Terling Place, where he carried out practically all his scientific investigations.
A few months after resigning from Cambridge, Rayleigh became secretary of the Royal Society, an administrative post that, during the next 11 years, allowed considerable freedom for research.
Rayleigh’s greatest single contribution to science is generally considered to have been his discovery and isolation of argon, one of the rare gases of the atmosphere. Precision measurements of the density of gases conducted by him in the 1880s led to the interesting discovery that the density of nitrogen obtained from the atmosphere is greater by a small though definite amount than is the density of nitrogen obtained from one of its chemical compounds, such as ammonia. Excited by this anomaly and stimulated by some earlier observations of the ingenious but eccentric 18th-century scientist Henry Cavendish on the oxidation of atmospheric nitrogen, Rayleigh decided to explore the possibility that the discrepancy he had discovered resulted from the presence in the atmosphere of a hitherto undetected constituent. After a long and arduous experimental program, he finally succeeded in 1895 in isolating the gas, which was appropriately named argon, from the Greek word meaning “inactive.” Rayleigh shared the priority of the discovery with the chemist William Ramsay, who also isolated the new gas, though he began his work after Rayleigh’s publication of the original density discrepancy.Shortly before winning the Nobel Prize, Rayleigh wrote the entry on argon for the 10th edition (1902) of the Encyclopædia. In 1904 Rayleigh was awarded the Nobel Prize for Physics; Ramsay received the award in chemistry for his work on argon and other inert elements. The next year Rayleigh was elected president of the Royal Society.
In his later years, when he was the foremost leader in British physics, Rayleigh served in influential advisory capacities in education and government. In 1908 he accepted the post of chancellor of the University of Cambridge, retaining this position until his death. He was also associated with the National Physical Laboratory and government committees on aviation and the treasury. Retaining his mental powers until the end, he worked on scientific papers until five days before his death, on June 30, 1919.
