Transparent materials such as glass and water have the ability to let light through as if they were made of nothing. Yet some transparent materials, such as glass, are very dense. They are full of atoms.

For light to pass through such material without scatter, every photon has to meander through the material in a fashion identical to every other photon.

Slalom

A way to envision this is to think of photons as slalom skiers, and the glass as a slope full of evenly spaced poles in all directions, with the poles being atoms.

When photons enter such materials, they are guided by their pilot waves. They make a half roll past the first atom. Then they continues with full rolls past every subsequent atom until they makes their final half roll past the last atom on exiting.

For photons entering the material at an angle, the first half roll will either be larger or smaller than average, depending on the angle of entry and which side of the first atom they enter. However, this is perfectly balanced on exit with a corresponding deviation from the average.

The result of this is that photons leave transparent materials in the exact same direction that they entered it, provided the first row of atoms are parallel to the last row of atoms.

Pilot waves

The photons’ pilot waves don’t only guide the photons through the transparent medium, they also set up organizing structures in the form of standing waves. The whole space becomes an organized mesh that guide the photons past each other so that they don’t collide with each other nor with atoms in transparent media.

The speed of light never changes

Since photons travel at the exact same speed regardless of their size. They always travel at the speed of light. However, the length of the path travelled by a small photon and a big photon will not be identical.

Small photons roll past atoms with their geometrical centre closer to the atom than the bigger photons, so even when large photons and small photons take the same path through a transparent medium, the smaller ones end up travelling a shorter distance.

Send a red photon and a blue photon through a piece of glass at the exact same time, and the red one ends up exiting the glass ahead of the blue one. The red one has less energy than the blue one. It’s smaller, and is therefore rolling past the atoms in the glass at a shorter distance from the atoms’ centre than the blue one.

This explains why blue light takes more time to travel through transparent media than red light.

Light moving through a prism

This also explains why white light is sometimes diffracted into all the colours of the rainbow, with blue light always at the most acute angle from the prism, and red light at the least acute angle.

Being larger than red photons, blue photons take more time rolling past the first atom. This makes the initial half roll more acute for blue photons than red photons. It also makes the subsequent full rolls and the final half roll more acute.

The magnitude of the rolls are defined by the photons’ sizes compared to the atoms in the medium. The bigger the photons, the more they roll from side to side, and this will under certain conditions lead to diffraction on exit from a prism.

Note that the photons don’t divert from each other in their overall direction through the medium. Photons of different colours take the same route when entering the same spot at the same angle. The only difference is the magnitude of their rolls past atoms.

Diffraction happens on exit only

It isn’t until the final half roll that diffraction happens, and it only happens under very specific conditions. We know this because optical lenses don’t diffract light even thought they are prisms of sorts, with entry angles different from exit angles. This is because diffraction is in fact a boundary condition. It occurs only at angles so acute that the media is about to flip from being transparent to reflecting in respect to the exiting light.

Diffraction happens only when the angles of entry and exit are different and so acute that the organizing mesh set up by pilot waves no longer manage to keep things together. When this happens, it is the biggest photons with the biggest rolls that veer off the most to the side. The smaller ones veer off less so in order of their sizes.

Blue light diffracts the most, with red diffracting the least.

This is why white light remain sharp and focused even through the thickest of glass sheets, while the smallest of prisms split white light just as well as big ones. The size of the prism is irrelevant. Only its shape and position relative to the incoming light is of importance.

Note also that this has nothing to do with wavelength. All that matters is the size of the photons.