How does bright yellow food colouring turn tissue transparent? To understand why, it’s essential to consider the reason things look opaque in the first place. The bits of our body – cell membranes, proteins, fluids – all cause light to refract, or bend.

If light bends just once – think of a beam of sunlight hitting a sheet of glass – the image it carries is still mostly clear. But as light refracts over and over, off fluids, proteins and other cellular miscellany, it scatters in lots of directions. All that scattered light, Rowlands said, makes it hard to see through – “like watching TV through a glass of milk”.

In 1897, the science fiction writer H.G. Wells published The Invisible Man, the tale of a scientist who invented a serum to alter how the body’s cells refracted light, turning himself invisible. That’s conceptually similar to what the Stanford researchers did.

By applying textbook physics principles, the researchers were able to screen for molecules that they predicted would, when absorbed by the body, change how biological tissues refract light. They hit on tartrazine, dissolved in water. But the proof was in the experiment. They soaked a slice of raw chicken in a tartrazine solution and found that the chicken turned clear as they increased the amount of tartrazine. When they rubbed that solution onto the skin of mice, they saw internal organs come into view. The tartrazine reduced the amount of refraction, the light scattered less and the tissue appeared clear.

When the dye was washed off, the tissue returned to normal and the scientists reported “minimal systemic toxicity” in the mice. Even though tartrazine is used as a common food dye, this technique hasn’t been tested in humans, and it isn’t the sort of effect that would occur at the minuscule concentrations that happen when people get a little dust from flavoured chips on their hands.

Hong said his lab is not working with human tissues or subjects, and noted that experiments on humans require ethical approval, so it is unclear when researchers might try this technique on people. Rowlands said he was intrigued by the possibility of identifying other dyes that do the same thing at lower doses.

Francesco Pavone, a physicist who specialises in biophotonics at the University of Florence, said the study would start a race within the field to find applications.

“The final goal is to use on a human, and so far, we are limited,” Pavone said. “The biggest breakthrough I see here is it could be finally accessible to humans.”