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  • From this photo it's easy to see that not all parts of a flame are made the samethey

  • can be brighter around the edges and darker in the middle. Which is weird, because a flame

  • is normally *brightest* in the middle, since that's where you're looking at the most

  • hot burning gas.

  • I mean,

  • Look at the sunactually, don't do that without a solar filter – but, it's a

  • big ball of hot glowing gasand it's definitely brightest in the middle. In fact, not only

  • does it get darker towards the edges, it gets redder. These dark, red edges of the sun are

  • calledlimbs,” sincelimbusis latin foredge”. And the limbs of the

  • sun get darker and redder near the edge for three simple reasons:

  • First, the sun is cooler closer to itssurfaceand hotter deeper in.

  • Second, hotter stuff glows brighter and yellower, which you've probably seen with a hot poker

  • or candle flame.

  • And third, if you look across the edge of a sphere, you have to look through more of

  • the sphere for your line of sight to reach a given depth than when you look directly

  • into the middle. If the sphere is only partially transparent (like the sun), when looking at

  • the edge, you won't see as deep into the depths before your vision is blocked.

  • So, when you look directlythrough a solar filter – into the middle of the sun, you

  • see deeper in to hot gas which glows bright and yellowAnd when you look at the edge

  • of the sun, you see shallower, cooler, gas which glows darker and redder.

  • The same effect in reverse explains why some flames have bright edges and darker centers

  • these flames are clearly hotter and brighter near their surfaces, likely because that's

  • where the best mixture of oxygen and fuel is, so when we look at their edges, our line

  • of sight passes through more hot light-emitting gases than when we look straight-on. Take

  • a candelabra and turn it side-on, and you'll see exactly what's going on.

  • The optical effect I describe in the video is called limb darkening

  • or limb brightening.

  • It also explains the theory the appearance observe nebulae

  • which are actually hollow expanding shell of glowing gases

From this photo it's easy to see that not all parts of a flame are made the samethey

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