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<   No. 3221   2012-04-01   >

Comic #3221

1 {photo of a swimmer admiring the sunrise while towelling off after a morning dip in a tidal pool}
1 Caption: Golden Hour

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Yellow Water Dawn
Yellow Water, Kakadu National Park, Northern Territory. Sunrise.
The best time of day to take photographs is roughly the hour either side of sunrise or sunset. And the best weather is when it's partly cloudy.

The time immediately after sunrise or immediately before sunset is known in photography jargon as the golden hour. At these times, the sun is very low in the sky. And rather than being the blinding yellow-white orb that glares into the corner of our eyes, it takes on a rich red or deep orange hue. It also appears dimmer, while the rest of the sky is still quite bright. This combination illuminates the landscape and the people around you with a warm, soft glow. The harsh shadows and stark contrast of the midday sun are replaced by a more gentle, even light that makes everything look just unusual and different enough to be more beautiful than normal. You can, literally, see everything in a different light.

The presence of some clouds in the sky enhances the effect tremendously. They catch the pink and orange hues of the rising or setting sun. Their edges are lined with bright colours that fade gracefully into their interiors. You need just the right sort of cloud for this. Thick cloud, or a band of cloud that blocks the sun is no good because it interferes with the light. Big fluffy white clouds are good. Small, dappled clouds are the best of all, as the rainbow of light washes over them, producing the most gorgeous coloured ripples across the sky.

The clouds also reflect some of this coloured light down to Earth, where it amplifies the golden illumination from the sun. A blank sky above tends to add blue light to the mix, reducing the warm hue a bit and making the golden hour shorter.

Light scattered off molecules of air.
The light from the sun strikes the Earth only after passing through its blanketing atmosphere of air. Near the middle of the day, when the sun is high in the sky above you, it shines down through only a relatively thin layer of air. Most of the light makes it through unimpeded. A small fraction of the sunlight, however, interacts with the molecules of the various gases that make up the air[1]. In a process known as Rayleigh scattering[2], some of the light bounces off the molecules, and ends up heading not down to where you are, but off in some other direction.

Where does that scattered light end up? A lot of it still ends up coming down to illuminate the surface of the Earth, just not in a straight line from the sun. If you look up into a clear sky away from the sun, what do you see? If you were on the moon, where there is no atmosphere, and your eyes were adjusted to the darkness of the sky (as opposed to the brightness of the surface)[3], you'd see the inky blackness of space, and stars - even if the sun was up. But on Earth there is atmosphere, and that atmosphere is deflecting light from the sun. So what you see is deflected sunlight. The daytime sky is bright. Not as bright as the sun, to be sure, but bright enough to see easily if the direct sunlight is blocked, say by a cloud.

Now the thing about Rayleigh scattering is that it's caused by an electrical interaction of the rays of light with the molecules of the air. Light, remember, is electric and magnetic fields moving along together through space. Those fields can interact with the electrons in an atom. When they interact, you essentially get a collision, and the collision can produce a change in direction, in a similar way to when a cue ball interacts with another snooker ball[4].

There's something else about light: The amount of energy carried by a ray of light depends on its wavelength (or frequency). This is an important property of light, and one we'll return to in more detail in the future, but for now it suffices to say that the higher the frequency of light, the more energy it carries. So radio waves, having low frequencies, also have low energy. Light has higher frequencies and higher energy. X-rays have higher frequency and energy still. And gamma rays have the highest frequencies and energies of all, which is why they are so dangerous to living things. Within the visible spectrum of light, red has the lowest frequency (or longest wavelength), and hence the lowest energy, while blue and violet have higher frequencies and energies.

Different colours in the sky soon after sunrise.
This is important for Rayleigh scattering, because the amount of interaction between a ray of light and a molecule depends on the energy in the electric field of the light. The more energy, the more likely the light is to interact with the electrons in the molecule[5]. So blue/violet light interacts more strongly with air molecules than red light does. Which means that the blue and violet light is more likely to collide and deflect off the air molecules than red or other colours. So a lot of violet and blue light is deflected through the air, a bit less green light, less again yellow, less again orange, and less again red. All of the colours are there, but the bias is strongly towards the blue end of the spectrum. So the overall colour of light deflected by the atmosphere is a light blue shade that we know—for good reason—as sky blue.

And that's why the sky is blue! Oh wait, we weren't asking that question today?

Right, now another thing about Rayleigh scattering is that the amount of light scattered depends on how much air it passes through. When the sun is high in the sky, only a small portion of the light is scattered, roughly one percent or so. The remaining 99% just shines straight through, making the sun far too bright to look at safely. However, when the sun is low on the horizon, in the minutes after sunrise or before sunset, the rays of light are hitting the Earth at a glancing angle. This means they have to pass through a much greater thickness of atmosphere to reach you. So a much bigger fraction of the light is deflected by the atmosphere, again most of it towards the blue end of the spectrum. The result is twofold: firstly, the sun appears much dimmer, and secondly, the sun appears much redder.

Swimmer and boats
Mindil Beach, Darwin. Sunset.
You'll also notice at this time that the sky around the sun is no longer blue. Instead it too shows a reddish or orange tinge. This is because of another feature of Rayleigh scattering. Because blue and violet light interacts more strongly with the air molecules than red light, it not only is more likely to be deflected, but also when it does deflect it's more likely to deflect again. The result is that the blue and violet light ends up deflected by a greater angle than the red and orange light. So the sky close to the sun is dominated by the colours of light that deflect the least: red and orange. (In broad daylight this effect is swamped by the overwhelming blue everywhere.)

So that's why sunrises and sunsets are so red.

The other really good time to take photos is just before the sun comes up, or just after it has set. This time is known as twilight, or in photography jargon, the blue hour. At this time, the sun is just below the horizon, so you don't get the bright light of day. But it's close enough to the horizon that the sky above has sunlight passing through it. And of course, we know that sunlight passing through air ends up being partially deflected by Rayleigh scattering, and that most of the light deflected is blue/violet. The result is that the sky has a faint glow with a very deep blue colour. Near the horizon right above the sun you may get changes of colour according to the closeness of the line of sight to the direction of the sun, resulting in thin bands of yellow, orange, and pink glows.

Under the Arch
Sydney Harbour. Sunset blue hour.
The blue hour is a good time to take photos in cities, where there is plenty of artificial lighting. The lights illuminate architecture and people (hopefully in an attractive way), while the deep blue sky above provides a contrast in colour and sometimes a dramatic backdrop effect. This effect is missing in photos taken later at night, when the sky simply appears black in photos.

On the other hand, while the golden hour can also be used to good effect in cities, it often shows its best face in landscape or seascape photography. An unrestricted view of the horizon near the sun lets you see the full majesty of the glowing, shifting colours, and allows them to spill over the landscape. Even mountains and trees can get in the way. The best place to capture the golden hour is on a coastline, looking at the sun over water. Those fortunate enough to live in a western coast can take advantage of glorious sunsets. Not only are they easier to be awake for, you can also see a truly spectacular sunset coming by the patterns of cloud in the sky during the afternoon.

Those, like myself, who live on an eastern coast have to deal with sunrises. The east coast photographer gets up an hour or two before sunrise, checks the cloud cover as best can be done in pitch blackness, then heads out to the beach or the sea rocks and hopes. Many times the weather is cruel. I have spent many a dawn sitting in a car waiting for the light sprinkle of rain to stop, or staring at thick grey banks of cloud out at sea that simply blot out all trace of the sun and the morning light.

Friday Morning
Rock shelf at Curl Curl. Sunrise.
But sometimes it's worth it.
[1] For shorthand I (and other writers) might refer to a "molecule of air" or an "air molecule". This really means "a molecule of one of the gases that makes up air". For example, a molecule of nitrogen, or a molecule of oxygen, or a molecule of carbon dioxide, or a molecule of water vapour, etc.

[2] Named after Lord Rayleigh, the English physicist who first described it. Rayleigh also co-discovered the noble gas element argon, and supervised the Ph.D. work of J. J. Thomson, who we've met before with respect to the structure of the atom.

[3] When the sun is up on the moon, the moon's surface is very bright. If you're looking around at things on the surface, your eyes would adjust to that brightness, and you'd actually see nothing in the sky but the sun surrounded by inky blackness. Your eyes would be too light-adapted to see any stars. But if you looked up into the dark sky and shielded your eyes from the bright surface for a minute or two, your eyes would become dark-adjusted and you'd see the stars. Incidentally, this is why in photos on the surface of the moon taken by Apollo astronauts, you can't see stars in the sky. Because the camera exposure was set to correctly expose the bright objects on the surface, the much fainter stars in the sky didn't have the necessary exposure time to register on the film, moon-landing-hoax conspiracy theorists notwithstanding.

[4] Or pool or billiard ball, if you prefer. And the analogy is not perfect - two snooker balls are pretty much identical physically, whereas an atom and a ray of light are very different beasts.

[5] This is a little bit of a simplification. The "higher the energy, the more likely it is to interact" is true for light around the visible part of the electromagnetic spectrum, but as you go to higher and higher energies such as x-rays and gamma rays, the chances of interaction go down again, because very different physical principles come into play. The frequencies of these types of radiation are essentially too high to interact much with the electrical fields in molecules.

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