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Q & A: Umbras and Penumbras

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Most recent answer: 09/20/2016
Q:
Light Why is there a shadow with both umbra and penumbra when the light source is larger or there are more than one light source?
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- Anonymous (age 13)
Toronto, On
A:
Umbras and penumbras are the names for two kinds of regions in a shadow that have different amounts of light in them. An "umbra" is the part of the shadow where all of the light from the source is blocked by the shadowing object. A "penumbra" is that region around the umbra where the shadow is only partial, or imperfect. You get these when the light source is larger than a single point. These form because while some of the light from the source gets blocked by the shadowing object, not all of it does. If you are in the penumbra looking towards the light source, you will see part of it visible, and part of it blocked. A point light source will either be all visible or all blocked, but an extended source can be partially viewable beyond the edge of the shadowing object. The same is true of multiple sources.

Photographers often used point light sources when they want shadows in their pictures with sharp edges on them (that is, without penumbras), and multiple or extended sources to make the shadow edges "fuzzier" or "to fill in the shadows with light". The sun is a very large source of light and it does not cast sharp shadows because of its size. Here is a diagram of the umbra and penumbra regions of the sun and the moon.


Umbra and Penumbra regions of the sun and moon

In a solar eclipse, if you are standing in the umbra region, the light of the sun will be blocked out and it will be very dark. If you are in the penumbra region, you will see part of the sun ("a 'bite' has been taken out of the sun!" where the moon is in the way).

Tom
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(published on 10/22/2007)

Follow-Up #1: lunar eclipse

Q:
what is a lunar eclips? Ammar Karachi.Pakistan A; lunar eclips occurs when the sun casts a shadow of the Earth on the moon.It happen when the sun , the Earth comes in b/W the sun and the moon , Blocking the light from the sun to the moon.
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- Ammar (age Ahmed )
Karachi,Pakistan
A:
Fine, but I'm not sure exactly what our contribution to this discussion is supposed to be.
Mike W.
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(published on 03/25/2011)

Follow-Up #2: shadow analogy

Q:
Is this a good analogy? One paintbrush soaked with paint (a point source) is flicked at a wall but your hand is up between the brush and the wall. The result is a crisp outline of your hand (umbra). Next, five paintbrushes soaked with paint are in front of your hand then are flicked at the wall (diffuse source). The result is a penumbra. Some of the paint made it around your hand to produce fuzzy edges. The individual paint droplets represent photons.
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- Evelyn (age 40)
Ohio, USA
A:
Sure, that captures it.

Mike W.
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(published on 07/27/2012)

Follow-Up #3: merging shadows

Q:
I have a light bending question for you. I have observed an interesting phenomenon with shadows and have not been able to come up with an explanation for why it occurs. On a sunny day hold your hands at two different heights from the ground (around a foot different). Watching the shadows of your two hands on the ground very slowly move your hands so the two shadows touch, keeping your hands at different heights. Strangely the shadow of the lower hand seems to bulge out too meet the other shadow. It seems that the light is diffracting, but in my limited understanding of diffraction I would expect the shadow to shrink not grow.
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- Brian (age 32)
Newmarket, Ontario, Canada
A:

Well, this was an easy experiment to reproduce. It's not really that the lower hand shadow reaches out toward the upper one. Whichever shadow has fingers pointing toward the broad part of the other one seems to reach out. What that means is just that the small gap between the shadows fills in, and our brains do the "reaching out"  interpretation. 

So now to the physics. Diffraction isn't important here because your hands are very big compared to the wavelength of light and the distance to the ground isn't big enough for a tiny angular spread of the light to matter. What does matter is that the sun isn't a point, it has some spread. There's a region of the shadow that's dark because your hand blocks off light from all parts of the sun. There's a "penumbra" around that which is not so dark because your hand blocks off light from only part of the sun. When the penumbras overlap, you get a region where one hand blocks off light from part of the sun and the other blocks off light from the rest, forming a dark shadow.

Mike W.

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(published on 06/12/2013)

Follow-Up #4: sharp edge of umbra

Q:
Why are the edges of Umbra sharp?
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- Vandana (age 16)
Muscat
A:

As you move into the region of the shadow of the Moon in a total eclipse, to pick an example, there is an exact point at which the Moon just barely starts to block of some light from the Sun. That's the edge of the penumbra. Farther in, there's an exact point at which the Moon blocks off the last bit of light from the Sun. That's the edge of the umbra. Although these edges are sharp, the change in light intensity is gradual. Just outside the umbra, light only reaches the shadow from a little bit of the Sun, so it's still very dark. Just inside the penumbra, light still reaches the shadow from most of the Sun, so it's not very dark.

Mike W.

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(published on 09/20/2016)

Follow-up on this answer.