This was a puzzling thing, and I didn’t see any reference to anyone else talking about it online right away but I think this is pretty simple once you hear it.
First though we’re going to do a quick thought experiment to get in the mood.
Let’s use Sam, the square from flat land. As it often discussed when talking about flat land, we can see inside Sam’s body and inside rooms which are closed and locked. Now that may be the case but we have to remember that part of the reason Sam can’t see inside his friends and walls is that the light which operates in flat land doesn’t penetrate things the same way our light can be blocked by a parasol or a roof. So when we look at flat land, we should only be seeing things illuminated which are actually illuminated in Sam’s world. For the same reason that it is dark inside your stomach, it is also dark inside of Sam.
So if we look at Sam in his flatland we can see that when we look towards his insides we find it is all dark. Now we can shine a light on Sam from our dimension and light up his insides. Think that the light Sam can see must be on the same plane. Light which bounces out of that plane is not visible to Sam. If he even uses light at all to sense things.
So if we illuminate Sam from the inside they can see that their insides are brighter if they happen to be canulated. And unless the flashlight we shine is only directly within his insides there will be photons that hit things outside of same which will gain and expel photos in all directions.
This means the objects, including Sam, which get light shined on them will heat up and also glow at some level, light a light was shining on them. If Sam were to record a video inside a dark box then they would later see the inside of the box glowing.
So in Sam’s world when someone from a 3D universe shines a flashlight on their car, the car will glow with light from there, but because that light is from a higher dimension it will mostly bounce out of that plane and very little will reflect into the 2D plane where Sam lives. Alternatively is Sam does not use light as we know it but a currently light that cannot exist in more than 2 dimensions (and not just using a cross section of the same light as is in our universe) then our light might still raise the energy level of things and cause them to appear to glow but the light won’t reflect on anything.
Now obviously we just lift this layer one dimension up and where you have a 4 Spatial-Dimensional creature which is viewing in part one of our universes then they are able to see inside of the things we cannot. If, as previously postulated, they have a collection of 3-D image receptors (like we have 2-D retnias) which are wrapped into 4sDs (like a hypersphere made of many eyeballs as we know it) and they use the same photons for illumination that we use then they should be blind to the insides of things unless they have a way to cast a light source from 4sDs so that they can light up something here completely, inside and out (while heating it).
The other idea would be that the 4sD-al light is inherently different from the photos that we use and the light without illumination that results from a 4sD-al being shining this light on us would appear like the object was brightly lit while not shining light onto the things around it, like on the ocean or a ship or any specific thing that someone wants to look at closely.