I have a question about light diffraction. Take a look at these images of the Pantheon oculus.

enter image description here

enter image description here

Now what I don't understand, in the first picture, the light is coming in from overhead and forms a really huge light diffracted light cone on the floor.

In the second picture, the light enters at another angle, the cameraman also is at another location BUT the beams that pass through the hole look straight and not cone-like.

Why does the light beam become thinner as it enters at the slightly different angle, compared to the first picture?

Also at different times of the day, the casted light's radius keeps shrinking as the sun sets. Can you guys provide a comprehensive explanation/ visual demonstration why this happens so?


1 Answer 1


Neither image displays diffraction. They both illustrate shadowing.

The second photo was apparently shot on a clear sunny day. All of the rays come from the sun. Some are blocked, those that aren't go straight down to the floor.

The second was probably taken on a hazy day. Rays originate from the haze; they come from all directions, and make their way to the floor.

  • $\begingroup$ It's not diffraction of light? I thought it was diffraction because it seems similar to the diffraction in a circular aperture where light diverges into a cone as it passes through a circular slit ( the hole in the dome in this case) ? $\endgroup$ Commented Jan 29, 2016 at 1:56
  • 1
    $\begingroup$ No, diffraction from a circular hole is noticeable only when the size of the hole is small. Even an inch-sized hole is large enough to make diffraction insignificant in most situations. (You'd need a careful lab setup to observe diffraction from an inch-sized hole.) $\endgroup$
    – garyp
    Commented Jan 29, 2016 at 2:11
  • $\begingroup$ Interesting. Okay I have another question. If the sun rays are traveling in a near parallel line, why does (in most of the Pantheon images on google) the hole above the building seem to act like a "spotlight" and diverge the light? If you look at the floor, you can see how the cast light is almost 200% times bigger than the opening on the roof. Isn't the cast light supposed to be the same size as the oculus above the dome? $\endgroup$ Commented Jan 29, 2016 at 2:16
  • $\begingroup$ Near parallel. They are actually diverging from the sun which 1.) is very far away so the angle of divergence is small but not zero and 2.) has the form of a disk in the sky so that those slightly diverging rays originate from different points on the disk. So we don't have a perfect shadow, just as your own shadow on the ground does not have perfectly sharp edges. $\endgroup$
    – garyp
    Commented Jan 29, 2016 at 3:52

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