Why greasy foods make paper bags translucent

You might have wondered why a cheesesteak bag or a pizza box becomes slightly see-through when touched by its greasy contents.

To understand why, we need to understand how light interacts with matter. The colours we see are different energies of visible light waves. We see these waves as the different colours of the rainbow — red, orange, yellow, green, blue, and purple.

When a light wave hits an object, a few things could happen, depending what the object is made of. The wave could be absorbed by the object, it could be reflected (light bounces off at the same angle it hit), it could be scattered (bounced around or reflected in many different directions), it could be refracted (bent), or transmitted (passed through making the object transparent).

The colour of the object that we see is the colour of light that is reflected. A banana is yellow because yellow light is reflected back to our eyes and other wavelengths of light are absorbed.

Snow is white because it reflects and scatters all the different colours of light equally. Snow is made of ice crystals with tiny pockets of air between those crystals. When light hits snow, the light is scattered and reflected as it passes through all these different crystals.

The same thing happens when light hits a piece of paper. Paper is made of fibres and there are little pockets of air between those fibres. When oil, grease, or fat comes in contact with paper, tiny droplets of it fill all the little gaps between the fibres of the paper.

As a result, “Light doesn’t have to do all that bouncing and scattering,” says Larry Scheckel, author of “Ask a Science Teacher: 250 Answers to Questions You’ve Always Had About How Everyday Stuff Really Works. “It only has to pass from air through the grease. Light does not have to pass from paper back to the eye. The paper is transparent, or, to be more technically correct, translucent. We can read words right through the paper.”

Similarly, ice appears transparent because it does not have those pockets of air so light goes right through.

The scattering of light by an object like a paper fibre depends on its size and shape, but also on the difference of the amount of light that’s refracted, known as the index of refraction, between the fibres and its surroundings, explains Michael Patterson, a professor of physics at McMaster University in Ontario. Generally, the smaller the difference, the less scattering.

“The oil, grease, or fat has about the same index of refraction as paper,” says Scheckel. “So the amount of scattering is kept to a minimum. Most of the light that would be scattered from the not-oiled paper is now transmitted through the paper.”

Water has a lower index of refraction than paper fibres, which is why it generally does not make paper transparent.

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