How can you make paper translucent?
Answers
Explanation:
Clean dry paper is a mix of cellulose fibers, with an index of 1.469, and air gaps with a refractive index of 1.000. Refractive index is the ratio between the speed of light in a vacuum and the speed of light in the medium. The speed of light in a vacuum is 299,792 km/s, and the speed of light in cellulose is 204,079 km/s.
When you look at a plain glass filled with water, you can see that the light changes direction as it passes through an interface between two different materials. Light does this because it's a wave. When the wavefront changes speed at an interface, it also changes direction, according to Snell's law. Some of the light is also reflected off these changes in index of refraction.
If you look at light going through a plain glass filled with ice cubes, it's a lot harder to make sense of the image that you see, because once there are several surfaces refracting, you can't easily undo their distortion in your head. Look at a plain glass filled with shaved ice, and the distortion gets so complex that all you really see is the average color of all the light that's arriving on the surface of the ice.
The cellulose fibers in paper are even smaller than the ice crystals in a Slurpie. Light is refracted and reflected back out of the paper over a distance scale so small that the surface appears solid. Also, cellulose bends light more than ice because it has a larger index of refraction. The index of ice is 1.31.
Now let's say you dump safflower oil onto the paper. The surface tension of the oil is lower than cellulose, so the oil wets the cellulose, driving the air out of the matrix and forming a smooth surface at the two sides of the paper. Safflower oil has a refractive index of 1.466, which is a pretty good match to cellulose.
There is some significant complexity in the calculations involved, because reflections and refractions depend a lot on the angles at the surfaces. To give an idea of the magnitudes involved, the reflection of light at normal incidence to a air/cellulose surface is ((n1-n2)/(n1+n2))^2 = 3.6% = 36000 ppm, but the reflection of light at normal incidence to an oil/cellulose surface is 1 ppm. (In the equation, n1 and n2 are the indices of refraction of the two materials.)