It is the nearest thing I have witnessed to a miraculous cure for colds. Two hundred and fifty teenagers are coughing like consumptives. But as the water hits the first Petri dish, the hall is suddenly silent.

Colour. What is it? How do we make it? What happens when we mix it? Splash!

It is simplicity itself. Three dishes, an overhead projector, a jug of water and some food colouring. But from the moment the water goes in (“What colour is the water? Transparent? Clear? See-through?”) to the point when the blue and green dye overlap with the cochineal to make black (“This is called subtractive colour mixing”), all eyes are on the screen.

Even the gaggle of giggling lads with the wet-look hair and crimson faces have paused in their perpetual hunt for sexual innuendo (“The green chlorophyll gobbles up the red and blue light...” Phnnaw! Phnnaw! Phnnaw!).

And Zbig has barely begun.

Zbig (that is Dr Zbig Sobiesierski to you) may travel exclusively by public transport, but he comes fully equipped.

Like some itinerant magician producing endless bunnies from a bottomless hat, he reveals the contents of two small suitcases and a 70-litre rucksack. There are some transparencies, three bottles of food colouring, several Petri dishes, two sheets of polarising plastic, three pieces of glass wrapped in sticky tape, a compact disc, a commercial defraction grating, a small green laser and a larger red one, a portable CD player, a light detector, a pair of computer speakers, a digital video camera, a collapsible tripod, a bottle of disinfectant and a comb.

Oh, and there is also a pound;20,000 thermal imaging camera, on loan from a high-tech company in Malvern.

“It’s like in days of old when science was spectacle,” he says. “Not just something that’s done in the Royal Insititution, but something that can travel around.”

And traveling around - “taking science to where the people are” - is what Zbig is good at. Today it is Framlingham in Suffolk - stop number 43 in this year’s Institute of Physics Schools’ Lecture tour. And 15 minutes into his hour-long talk Seeing is Believing? the science is going down a treat with Year 10 students at Thomas Mills high school.

Why is the sky blue? He trickles disinfectant into a water-filled fishtank and watches it disperse like Pernod. Slowly, the white light from a slide projector is scattered, and the water begins to turn the colour of a duck egg. (“My dad didn’t believe me,” said a thank-you e-mail from one incredulous boy after Zbig gave this demonstration in Swansea, his own home town.) Look up now, to see colours dance on the ceiling. They are reflected from the micro-grooved face of a compact disc. “And here (Zbig plays the spectrum over the audience) is what it feels like to be inside a rainbow.”

Stepping over trailing leads, he switches on that thermal-imaging camera and suddenly the lads are in hysterics. For there on the screen, with a black nose and two pieces of coal for eyes, is one of their classmates.

Zbig adjusts the expensive germanium lens and explains about infra red radiation - about why the boy’s nose and glasses appear black (because they are colder than the rest of his face), and how, if we humans lived on a different planet under a different sun, our eyes might have evolved to “see” X-rays or ultra-violet light.

He pans the device acros the audience again, letting the laughter roll. Suddenly, he shows the camera the book he has been clutching, and there on the screen is the after-image of his hand, still glowing warmly on page after page.

Even the boys with the infra-red faces are impressed by that.

The talk is of rods and cones, of nano-seconds and wavelengths. It is solid science, geared to the curriculum. But the action is hotting up. Zbig zaps a green laser across the screen to demonstrate that the beam is pulsing (a teacher in the audience shakes his head at such eloquence). Then he launches into the grand finale.

A red laser (we know it is red because Zbig produces a piece of chalky cloth and begins puffing dust into the path of the beam) fires across the stage at the light detector which is wired to a pair of speakers.

“Click! Whoomph!” go the speakers when he interrupts the beam with his hand. And when he oscillates a plastic comb in its path, the hall is filled with the sound of sawing. “We’re getting sound,” he explains above the sudden laughter, “by changing the amount of light that travels to the detector. But I can go one step further than that...” And he does. Connecting the laser to a CD player, he pops in a disc. “Now,” he says, “the music will change the amount of light that the laser gives out. The sound will then travel at the speed of light into the detector, get turned back into sound, and hopefully, we should hear what is playing over here, over there.”

He flips a switch, and there, quick as a laser, is Robbie Williams.

Zbig lets the music play for a bit, then holds up a hand. “Of course, if you get bored with listening to that, you don’t have to turn the CD player off.” He steps forward into the beam, and the music cuts out.

“But if you’re feeling particularly malicious and you’ve had quite enough of Robbie Williams, you could re-engineer his music by taking this instrument I’ve just been using.”

And without another word, he whips out his plastic comb and sets about sawing up the music like a madman with a chainsaw.

Laughter. Applause. QED.

Later, as the teenagers file out and Zbig begins loading the props back into his luggage, he talks about life as a science communicator.

Since returning from Japan, where he held an associate professorship in quantum electronics at the University of Hokkaido, Zbig has assembled an impressive portfolio of jobs. He spends two-and-a-half days each week teaching in higher education, two days co-ordinating continuing education courses in the physical sciences and half a day tutoring on an Open University masters course called Communicating Science.

He performs science cabaret on the street, in supermarkets and in pubs, and gives talks at OAPs’ clubs and in community halls in the Rhonda. When he rounds off this year’s Institute of Physics tour at the Royal Institution in mid July, he will have given today’s lecture 47 times at 36 venues and reached a total audience of more than 8,000.

“I’m not a preacher,” he says. “I am trying to show that science is relevant for people in a wide range of communities, and that it can be communicated in an interesting manner.”

And with that, he looks at his watch and apologises for his haste. But the taxi is here, and the doctor has a train to catch.

Further details from: The Institute of Physics, 76 Portland Place, London W1N 3DH. Tel: 020 7470 4800. Fax: 020 7470 4848. E-mail: physics@iop.org Website: www.iop.org