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Shadows look sharp, so it is easy to believe light travels only in straight lines. The first person to describe carefully where that fails was Francesco Maria Grimaldi, a Jesuit priest in Bologna, and he gave the effect the name we still use: diffraction. Our posts on Thomas Young and Augustin Fresnel cover how the wave theory later explained it. This post is about what Grimaldi saw, why his book had so little direct effect, and how to calculate diffraction yourself.
Who was Francesco Grimaldi?
According to his MacTutor biography, Grimaldi was born in Bologna on 2 April 1618 and died there on 28 December 1663. He and his brother Vincenzo joined the Jesuits on 18 March 1632. He taught rhetoric and humanities at the College of Santa Lucia in Bologna from 1638 to 1642, earned a doctorate in 1647, and was ordained a priest in 1651.
For years he was the experimental hand of the astronomer Giovanni Battista Riccioli. He dropped weights from Bologna's Asinelli tower and timed them with a calibrated pendulum. Riccioli's *Almagestum novum* (1651) credits him with more than 40 experiments and includes his map of the Moon. The Linda Hall Library notes that the naming system on that map is the one still in use, so the Sea of Tranquility, where Apollo 11 landed, first appeared with that name there. The Galileo Project records Riccioli's praise of his skill at devising, building and operating instruments.
MacTutor points out that nothing was published under Grimaldi's own name while he lived.
What did Grimaldi see in the dark room?
MacTutor describes the experiment from his book. Grimaldi let sunlight through a small hole into a darkened room, where it spread as a cone onto a screen. He held a thin rod in the cone and measured its shadow. The shadow was larger than it should have been if light went in straight lines from the hole past the rod.
He also saw coloured bands next to the shadow. Each band had a broad white middle, a narrow violet edge on the side nearest the shadow and a narrow red edge on the far side. He also described what happened with obstacles of other shapes.
He chose the name from the Latin for breaking apart, because the light reminded him of a stream of water splitting around a stick. He also let light in through two holes and looked where the two patches overlapped. MacTutor quotes his proposition, in English, that "a body actually enlightened may become obscure by adding new light to that which it has already received". Adding light could make a place darker.
Why did his discovery travel so slowly?
The book, *Physico-mathesis de lumine, coloribus, et iride*, appeared in Bologna in 1665. MacTutor explains that it argued both sides of whether light is a substance, leaned towards light being a quality, and argued against light being made of tiny particles. It was little read. The Jesuit Honoré Fabri described the experiments in his *Dialogi physici* (1669), and Isaac Newton said he learned of diffraction from Fabri.
In England the word arrived through a book review. A. Rupert Hall's article Beyond the fringe (1990) records that Henry Oldenburg reviewed the book in the *Philosophical Transactions* of 22 January 1671/2. Oldenburg reported that light spreads "not only directly, and by refraction, and reflexion, but also by diffraction", which brought a new technical word into English.
How big is the effect? A worked example
The NCERT Class 12 chapter Wave Optics gives the single-slit result: dark minima at angles θ ≈ nλ/a, where a is the slit width. Here are sample numbers.
A slit of width a = 0.20 mm is lit with light of wavelength λ = 600 nm, and the screen is D = 2.0 m away.
Convert: a = 2.0 × 10⁻⁴ m, λ = 6.0 × 10⁻⁷ m.
First minimum: θ = λ/a = 3.0 × 10⁻³ rad.
Distance on the screen: y = Dθ = 2.0 × 3.0 × 10⁻³ = 6.0 × 10⁻³ m, or 6.0 mm.
Width of the central bright band: 2y = 12 mm.
Now widen the opening to 2.0 cm. Then θ = 3.0 × 10⁻⁵ rad and y = 0.06 mm. NCERT notes that the wavelength of light is much smaller than most obstacles, which is why we rarely notice diffraction. Grimaldi needed a narrow beam, a dark room and careful measurement.
Colour follows too. With the same slit and screen, red light at 700 nm has its first minimum at 7.0 mm and violet at 400 nm at 4.0 mm. Longer wavelengths spread further, which fits the red edges Grimaldi saw on the side of each band away from the shadow.
Which mistakes should you avoid?
Leaving a in millimetres. Convert every length to metres before dividing.
Halving the central maximum. Its full width is 2λD/a, twice the distance to the first minimum.
Mixing up single and double slits. Young's fringes have width λD/d, where d is the slit separation. Our post on Young's double-slit problems covers that case.
The JEE (Main) 2026 syllabus lists diffraction due to a single slit and the width of the central maximum. Check the syllabus for your exam year.
If you want to watch the pattern change as the slit narrows, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read.
What can a student take from Grimaldi's way of working?
Measure what looks obvious. Everyone had seen shadows. Grimaldi measured one and found it was the wrong size.
Record the details. The order of violet and red in his bands is still checkable centuries later.
Separate observation from explanation. His theory of light did not last, but his observations did.
Try it now: hold two razor blades edge to edge, as NCERT describes, look through the gap at a straight bulb filament, and watch the bands widen as you close the slit.
