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High above the weather, sunlight strips electrons from gas molecules and leaves a region of charged gas, the ionosphere, that can reflect radio waves back to Earth. In the 1920s almost nothing was known about it over India. Sisir Kumar Mitra set out to measure it with home-built equipment in Calcutta. This post tells his story from the Indian National Science Academy's memoir and works through the physics of bouncing a radio pulse off the sky.
Who was Sisir Kumar Mitra?
The INSA biographical memoir records that he was born in Calcutta on 24 October 1890. His father died while he was still a student, and his mother, a physician at Lady Dufferin Hospital in Bhagalpur, supported the family on a small income.
In 1916 he joined the new postgraduate physics department at Calcutta's University College of Science, where C. V. Raman, D. M. Bose, M. N. Saha and S. N. Bose were also working. He researched the diffraction and interference of light in Raman's laboratory and received a D.Sc. from Calcutta University in 1919. In 1920 he went to Paris, worked under Charles Fabry at the Sorbonne on wavelength standards, and earned a doctorate in 1923.
How did an optics researcher turn to radio?
The memoir says he briefly joined Marie Curie's Institute of Radium, but reports about radio valves kept drawing him. He moved to Professor Gutton's laboratory at the University of Nancy to work on valve circuits, and "his career in radio research was settled."
Radio was not taught in any Indian university then. Mitra wrote to Sir Asutosh Mookerjee proposing that wireless be added to the postgraduate physics course. Back in Calcutta late in 1923, he set up the Wireless Laboratory, which the memoir calls the first postgraduate teaching and research department in India in the subject. He also built a broadcast transmitter, call sign 2CZ, that for a time put out regular programmes.
How did his team detect the E layer?
In 1925-26, the memoir explains, Edward Appleton in England and Gregory Breit and Merle Tuve in America had found direct evidence of ionised layers in the upper atmosphere. Mitra's team, with H. Rakshit building the first Indian equipment, used medium-wave broadcasts from the Calcutta station of the Indian State Broadcasting Service, sometimes transmitted specially outside programme hours. With a receiver about 75 miles from Calcutta, they obtained the first experimental evidence in India of the E layer in 1930. A pulse apparatus followed, letting them measure layer heights directly, and the laboratory joined the Second International Polar Year observations of 1932-33.
The memoir also credits the group with the first successful explanation of how the E layer forms at a height of about 100 km, through ionisation by the Sun's ultraviolet radiation.
How do you find the height of a layer from a radio echo?
This is the pulse method, and you can do it with Class 11 kinematics. Send a short radio pulse straight up, time the echo, and use distance = speed × time. The pulse travels up and back, so:
h = c × t / 2
Worked example, for a layer at the memoir's E-layer height of 100 km:
Round trip distance = 2 × 100 km = 200 km = 2.0 × 10^5 m.
Speed of radio waves, c = 3.0 × 10^8 m/s.
Echo time t = 2.0 × 10^5 / 3.0 × 10^8 ≈ 6.7 × 10^-4 s, about two-thirds of a millisecond.
Reverse it: a sample echo delay of 2.0 ms gives h = 3.0 × 10^8 × 2.0 × 10^-3 / 2 = 3.0 × 10^5 m = 300 km.
The waves themselves were medium-wave broadcasts. NCERT's Class 12 Physics chapter on electromagnetic waves gives the AM band as 530 kHz to 1710 kHz. At 1,000 kHz, the wavelength is λ = c / f = 3.0 × 10^8 / 1.0 × 10^6 = 300 m. And the receiver 75 miles away was about 75 × 1.609 ≈ 121 km from the transmitter.
What mistakes do students make with echo problems?
Forgetting to halve. The echo time covers the trip up and down. Divide by 2.
Mixing units. Convert milliseconds to seconds and kilometres to metres before you divide.
Using the speed of sound. Radio waves are electromagnetic and travel at c.
What did he build after the discovery?
The memoir lists a long run of institutions. Mitra chaired the Radio Research Committee of CSIR from 1943 to 1948. His book The Upper Atmosphere was published by the Royal Asiatic Society of Bengal in 1947; the memoir says its 2,000 copies sold out within three years and a second edition followed in 1952. In 1949 Calcutta University created the Institute of Radio Physics and Electronics, and ionosphere observations moved to the Haringhata field station in 1955.
He was elected a Fellow of the Royal Society in 1958, and the Society later published a biographical memoir of him. He received the Padma Bhushan in 1962. He died on 13 August 1963. A 2017 study in the Indian Journal of History of Science calls him the founder of ionospheric science and radio technology in India.
What can a student take from Mitra's way of working?
Change fields when your interest is clear. He left optics for a subject India did not yet teach.
Build what you cannot buy. His team made its own receivers and pulse equipment.
Use what is around you. A broadcast station became a scientific instrument.
If reflection and wavelength still feel abstract, the Fermi Simulation Lab has interactive simulations for ideas easier to see than read. Our guide to the electromagnetic spectrum covers radio waves, and our post on Jagadish Chandra Bose tells the story of the radio pioneer whose wireless experiments, the INSA memoir suggests, impressed Mitra in his college days.
Try it now: find the echo time for a layer at 250 km and the wavelength of a 1,500 kHz broadcast.
