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A photograph records how bright each point is, and throws away everything else about the light. A hologram keeps more, which is why it can show depth. The idea came to Dennis Gabor one day at Easter 1947, while he was trying to solve a different problem. This post covers his story, how holography uses the interference you study in Class 12, and a worked example on why it needed the laser.
Who was Dennis Gabor?
Gabor was born in Budapest on 5 June 1900 and died in London on 8 February 1979, according to his Nobel Prize facts page. In his Nobel autobiography he writes that his love of physics "started suddenly at the age of 15", and that he and his brother built a small home laboratory. He chose engineering anyway, because physics was not yet a profession in Hungary, and took his degrees in electrical engineering, finishing his doctorate in 1927.
He joined Siemens & Halske in 1927. When Hitler came to power in 1933 he left Germany, and in 1934 he found work at the British Thomson-Houston Company in Rugby, England. On 1 January 1949 he joined Imperial College London, where he stayed until he retired in 1967. He won the Nobel Prize in Physics 1971 "for his invention and development of the holographic method".
How did he come up with holography?
In his Nobel Lecture, Gabor explains that electron microscopes of the 1940s could not resolve atoms, because their lenses had aberrations. The solution, he says, "suddenly dawned on me, one fine day at Easter 1947": take a poor electron picture that still contains all the information, and correct it later with light.
The problem was that a photograph records only intensity and loses the phase of the wave. His answer was to add a "coherent background", a reference wave. Where the object wave and the reference wave meet, they interfere and make fringes, and the fringes record the phase. Light the developed record with the reference wave alone and the object's wave is rebuilt. He named the record a "hologram", from the Greek holos, "the whole", because it contained the whole information. He published the idea in Nature in 1948 as "A New Microscopic Principle".
His autobiography calls the whole episode "an exercise in serendipity (the art of looking for something and finding something else)". Electron holography did not succeed in his lifetime; optical holography did.
A worked example: why holography waited for the laser
Interference needs coherent light, a topic in the NCERT Class 12 Wave Optics chapter. The JEE (Main) 2026 syllabus lists interference and Young's double-slit experiment in Unit 16.
Part 1: how many fringes? Gabor writes that his best source, a high-pressure mercury lamp, had a coherence length of only 0.1 mm, "enough for about 200 fringes". Check it with a sample wavelength of 500 nm. One fringe needs a path difference of one wavelength, so the number of fringes is about 0.1 mm ÷ 500 nm = (1 × 10⁻⁴ m) ÷ (5 × 10⁻⁷ m) = 200. His figure holds.
Part 2: what the laser changed. He notes that the helium-neon laser of 1962 had a coherence length about 3000 times the mercury lamp's. That is about 0.1 mm × 3000 = 300 mm, or 30 cm. With that much room, Emmett Leith and Juris Upatnieks could send the reference beam around the object instead of through it, and in 1963 they published the first successful laser holograms.
Part 3: how fine are the fringes? Two beams meeting at an angle θ make fringes spaced λ/(2 sin(θ/2)) apart. With sample values λ = 633 nm and θ = 30°: sin 15° ≈ 0.259, so the spacing is 633 nm ÷ 0.518 ≈ 1220 nm, about 1.2 µm, or roughly 820 fringes per millimetre. Compare a Young's double-slit set-up with λ = 600 nm, D = 1 m and d = 0.5 mm, where the fringe width β = λD/d = 1.2 mm. Hologram fringes are about a thousand times finer, so the film must record very fine detail.
Common mistakes students make
Thinking any light interferes. Two independent bulbs do not give steady fringes. You need coherent sources, which is why Gabor had to filter one mercury line through a tiny pinhole.
Mixing up fringe width and fringe spacing on a hologram. β = λD/d is for Young's slits. Fringes from two beams crossing at an angle depend on that angle instead.
Dropping the factor of 2. The spacing λ/(2 sin(θ/2)) uses half the angle between the beams.
Our guide to Young's double-slit problems gives more practice with β = λD/d. Interference is easier to grasp when you can see it change, and the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read.
What can a student take from Gabor's way of working?
Change the question. Instead of building a perfect lens, he asked whether a bad picture could be corrected afterwards.
Stay alert to side results. He set out to improve microscopes and invented holography. Note down surprising results in practice; some are worth more than the answer you wanted.
Expect to wait for tools. His idea was right in 1947 but needed the laser to work well. A method you learn now may pay off much later.
Work out the fringe spacing for two beams crossing at 10° instead of 30°, and explain in one sentence why the answer gets larger.
