Placeholder Image

Subtitles section Play video

  • 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

Subtitles and vocabulary

Operation of videos Adjust the video here to display the subtitles

B1 sun darker sphere flame limb brighter

The Limb of the Sun

  • 29 2
    Summer posted on 2020/09/10
Video vocabulary