# Horizontals in a grid do not cast shadow while verticals do. Why is this?

A friend shared this image and asked about the shadow. As it appears the horizontals are not casting a shadow while the verticals are. It seems that the light source is on the left hand side wall, based on the vertical shadows. So how can this be? (https://i.sstatic.net/pHYB7.jpg)

Edit 1 - Not very clear but an image with the source of the light. https://i.sstatic.net/WzhoT.jpg

Edit 2 - A gif from a short video of the table under the light. https://giphy.com/gifs/2uInVzY9vOBwbmh4Nw

Edit 3 - Another friend suggested that it might have something to do with the principles behind Young's double slit experiment (an answer says its not, I agree)

Edit 4 - https://giphy.com/gifs/jVEUnP6fiah53zdMcp A video made in to a gif with the grid being moved around and the source of light shown in relation to the object

• I do not believe those shadows are being cast by those slats. Nor is it obvious to me that the light is coming from the upper left. If it is, perhaps those are shadows of slats in a window blind.
– pwf
Jul 10, 2018 at 18:02
• I have added an image with the source of light. Jul 10, 2018 at 18:17
• I half suspect those lines aren't even shadows, but colored lines on the wood. The easy way to figure out where shadows are coming from is to start sticking your hand various places see where the shadow shows up until you can trace your way back to the responsible light source. Without being able to do that ourselves here there's not much we can say, I think. Jul 10, 2018 at 18:40
• Well it is not lines on the wood I suppose, even so what about the shadow of the grid which should be there for a table under the light? Jul 10, 2018 at 19:15
• It's not double slit. Double slit only works with slits that are small (or at least not too large) in comparison to the wavelength of light. You can detect diffraction around larger object but it is very subtle, nothing like this. sciencedemonstrations.fas.harvard.edu/presentations/…
– user93146
Jul 10, 2018 at 19:57

If the light source is like a long fluorescent light, shadows in one direction (along the length of the fluorescent tube) will be blurred and shadows in the other direction will be fairly sharp. Can you ask your friend to send you a photo of the light source (not of light reflected off the wall)?

• This seems to be the answer from what I can see the latest video. Thank you very much. Jul 11, 2018 at 15:50
• So, shadows along the length (parallel to the tube) are clearly visible and the lines perpendicular to the tube have multiple shadows from multiple light sources thereby cancelling each other out. Jul 11, 2018 at 17:25
• @ArjunSatheesh, that is correct. Jul 11, 2018 at 18:45

The light should be one of those standard long white neon tubes. In the animated gif you can also see the very light shadows in the other direction.
The wooden bars parallel to the lamp cover the light in its full length, while those orthogonal to the light tube cover only a small width. That's why the first are much more visible.
If you turn the table by 45° the shadows should form a rather even grid.

It doesn't really have much to do with a double slit experiment, which is used to prove a wave character. This appearance can also be explained purely with the particle character of the light, as light from the lamp can reach every part of the low shadow area in a straight line.
For the double slit experiment the slits have to be sufficiently narrow and on the screen behind it you will find maxima in places where there is no straight path to the light source.

• On tilting the grid, light shadows did appear on the surface below. It is light from a fluorescent tube. Thank you for the explanation. Jul 11, 2018 at 16:17

Shadows perpendicular to the fluorescent tube light source are not visible as they appear to be producing multiple shadows from multiple light sources thereby cancelling their own shadows.

Shadows parallel to the fluorescent tube light source have a point light source on them and this causes sharp shadows to fall on the surface below.When the grid is tilted and lines no longer remain perpendicular,new shadows are formed as is visible in edit 4.

Sharing an image drawn by a friend of a friend who asked this question. I believe the question has been answered.