“I understand the sun is a really long distance from the earth, such that the paths photons take that strike the earth are pretty much in parallel. So why, when I see “rays of sunshine”, produced (I assume) by the sun shining through differing cloud densities, are they radial with their point of origin being at the apparent location of the sun in our sky?”
Seen poking through a cloud, trees or other opaque materials, sunbeams are one of the most surprising natural phenomena, when you think about it. There’s always scattered, ambient sunlight in all directions, and the bright sunshine is never visible as a ray when there aren’t clouds. Moreover, the light almost always appears to diverge away from the beam’s point of origin, rather than seeming to be the parallel rays you’d expect. So why is this the case? Why aren’t rays of sunshine parallel, like you’d expect? The fact of the matter is that the rays actually are parallel, even if they don’t appear so to your eyes.
NASA Kepler’s Scientists Are Doing What Seems Impossible: Turning Pixels Into Planets
“It isn’t the image itself that gives you this information, but rather how the light from image changes over time, both relative to all the other stars and relative to itself. The other stars out there in our galaxy have sunspots, planets, and rich solar systems all their own. As Kepler heads towards its final retirement and prepares to be replaced by TESS, take a moment to reflect on just how it’s revolutionized our view of the Universe. Never before has such a small amount of information taught us so much.”
When you think about exoplanets, or planets around stars other than the Sun, you probably visualize them like we do our own Solar System. Yet direct images of these worlds are exceedingly rare, with less than 1% of the detected exoplanets having any sort of visual confirmation. The way most planets have been found has been from the Kepler spacecraft, which gives you the very, very unimpressive image of the star you see featured at the top. Yet just by watching that star, the light coming from it, and the rest of the field-of-view over time, we can infer the existence of sunspots, flares, and periodic “dips” in brightness that correspond to the presence of a planet. In fact, we can figure out the radius, orbital period, and sometimes even the mass of the planet, too, all from this single point of light.
Shadows are fascinating to all living creatures; the subtle fear and curiousness of an entity that always follows you around everywhere you go makes shadows extremely thrilling to explore.
How are shadows formed ?
Shadow is just a region where no light of rays are able to enter.
Relative sizes of the planets and the sun
When one pictures the sun one might be tempted to think (from its small apparent size in our skies) that it as a point source of light, but the size of the sun is HUGE!
If it were only a point source of light, one would never get the familiar umbras and penumbras that we see during an eclipse.
In addition to the Umbras and Penumbras, there is another classification to these shadows – Antumbras
If you were standing in the Antumbral region, you would experience an annular eclipse, in which a bright ring is visible around the eclipsing body
This is NOT the ‘shadow’ of the ISS
Photo credit: NASA/Joel Kowsky
As you can see from the ray diagram, the ISS DOES cast a shadow and if you were on the ISS you would most certainly see it (orange box – umbra region)
what you are witnessing from Earth in that amazing ISS photobomb of the 2017 Solar
eclipse is NOT a shadow but just the outline of the ISS against the
surface of the sun: An annular eclipse of the ISS.
On disappearing shadows of Birds and airplanes
The shadows of airplanes and birds when they are closer to the land are definitely a common sight.
Try bringing two of your fingers closer in the back drop of a light source and you would observe this:
Long before your fingers actually touch, the edges magically seem to touch each other. How is this even possible?
Transit of Venus
When scientists were observing the transit of Venus from Earth i.e when the planet Venus passes directly between the Sun and Earth,they faced a similar problem.
At the moment when Venus should
nearly touch the edge of the sun, the circular planet began to elongate.
And they noticed the same phenomenon for Mercury as well (which has no atmosphere).
What is causing this optical phenomenon?
The physics behind this beautifully bizarre optical phenomenon will be revealed tomorrow on FYP!.
But since this is something that you can all try at home, we strongly encourage you to play around with this and get a feel for it. It requires only your hands and a source of light.
Once you do, try to hypothesize a solution for this behavior.
The black drop effect
Let’s take a closer look at this optical phenomenon by projecting your hand on to a screen and bringing the fingers closer together.
Observe the behavior of the penumbral region of the shadow i.e the less darker region:
Ahhaa… Notice that even though my fingers are not touching each other in the last image, if you see the shadow it seems as though they are!
This is because of when two penumbral regions of the shadow overlap, you get a much darker region in the middle. Here’s an illustration:
Yes in reality, you do observe gradients of darkness in between the two objects like so:
BUT our eyes are not that great at handling such fine contrasts in darkness; It clips off the less darker regions between the two shadows and replaces it with the surrounding darker region.
A similar response is rendered by the camera’s noise suppression algorithm too. That’s why you get that bulge connecting the two umbras irrespective whether you view it through your eye or through the camera.
In the case of the camera, this can be rectified using the appropriate tools and processing, whereas in the case of the eye you are stuck with it.
The case for Venus Transit
You can observe the same clipping phenomenon (called Black drop effect) that we talked about if we project the image onto a camera instead of a screen:
If the light source were the sun, the object were Venus instead of your fingers, and the screen were your eyes, you get this fuzzy shadow behavior commonly observed during the transit of Venus.
This is not a droplet, but the black drop effect observed during the transit of Venus
The case for Diffraction (OPEN DISCUSSION):
When I posed this question to a lot of my friends, their first response was Diffraction but honestly I couldn’t visualize this phenomenon with Diffraction.
I spent a lot of time playing with a laser module trying to figure out how Diffraction would fit into this explained but I am unable to come up with a reasonable argument for it.
If you have a valid explanation of this using Diffraction(or other), please enlighten me and the rest of the community. I personally would really love to know.