August 21, 2019

Why you can't trust your eyes

You watch a nice sunset.  You see the sun crossing the horizon and believe your eyes.

But strangely enough, at the moment you thought the sun had set, it had already set several minutes. ago.

What you experienced was an atmospheric mirage, and there are a lot more like this one.

The trouble is that we are trained to believe our eyes.  And we assume that light moves in a straight line.  That an object is where it appears to be.

But that is often not true, particularly when the atmosphere acts as a lens and causes light to bend.

Light is influenced by our atmosphere.  Where the atmosphere is dense, light slows down.  When it the atmosphere is less dense, the speed of light is faster.  And when light moves between layers of different air density, the direction of the light can change or bend.  You have seen such effect with pencils in water (see below)--now consider what happens at sunset.

The atmosphere is denser near the surface and less dense aloft.  The causes a lens effect that results in objects looking higher than they actually are--something known as a superior mirage.   The figure below shows the situation.  The apparent position of the sun...what it looks like to above the horizon.  The real position is below the horizon...the sun has already set.

The sun is not the only object that isn't where you think it is.  The stars, particularly near the horizon, are also shifted upwards by the lens effects of the atmosphere.  And this superior mirage effect can be seen locally as well when warm air passes over cool Northwest waters.   For example, cliffs on the opposite side of the sound can be elevated into sky as impressive walls of land (see example below).

So the moral of the story is don't always believe your eyes, reality can be different.    Same thing for what you hear.....but that topic belongs in another blog. 😃


  1. Fata Morgana, my favorite, are a little less common than simple superior mirages, but will occur sometimes when you're out on the water around here.

  2. "Where the atmosphere is dense, light slows down. When it the atmosphere is less dense, the speed of light is faster."

    Can you clarify this a wee bit Cliff? The speed of light is a constant that cannot change. So in atmospheric sciences, what would the term be? "Light is impeded by a denser atmosphere and less of it reaches the viewer / Light is less impeded by a thinner atmosphere and more of it reaches the viewer"?

    I have experienced anecdotally in mountaineering that the glare of the sun is far more intense at higher altitudes with less atmospheric pressure, even if we're rock climbing and snow / ice reflection is not a factor.

    1. The speed of light *in a vacuum* is a constant ... but light passing through a transparent medium goes slower. The ratio of the vacuum speed to the medium's speed is called its refractive index. R=c/v (v being the "phase velocity" of light in the medium). Water, for example, has a refractive index of 1.33, and the speed of light through it is about 225,000,000 km/second. The speed of light in a vacuum is about 299,000,000 km/sec. The refractive index for air is cited as 1.0003, so the speed of light through air is barely slowed down to 298,900,000 km/sec. Glass has an index of about 1.5, and diamond is 2.4.
      Two Wikipedia pages that discuss this are: (especially the "in a medium" sub-section) and

    2. c = ~2.99x10^8 m/s, not km/s. c = ~3x10^5 km/s.

  3. Ha ha, but we are really seeing it! I would think of it as the 'real' sunset based on that. I wonder how they calculate sunrise and sunset times, with that thought in mind. It is also interesting to consider that the direction to the sun at that point in time is geometrically true, even though the elevation to the sun is not.
    Martin Paquette
    Land Surveyor

  4. With the sun being 93 million miles away and light travelling through space at 186,000 miles per second, that's 500 seconds for the sunlight to reach us (approx, since Earth's orbit is not a circle). That's a little over 8 minutes. So even if we did not have an atmosphere, I think it's correct to say that when the sun appears right on the horizon, it actually is positioned below the horizon.

    However, I had not even given an ounce of consideration to the light-bending of the atmosphere as you described, so now I am more educated, and trust even less of what I see :)

  5. So, does that mean the sunset "lasts" longer while the sunlight passes through the atmosphere just above the horizon? In other words, the sun moves across the sky at a consistent speed [Earth is rotating at a consistent speed]...but as the sun approaches our horizon, the sunlight lingers as the light bends through our atmospheric lens...correct?

    1. Yes, and this is also why the sun looks slightly flattened at sunset: The "lower limb" is below where it appears to be more than the upper limb is.

  6. Hello Dr. Mass,

    What do you make of the fires in the Amazon? Perhaps you can address that issue in an upcoming blog.



  7. Cliff, I'd like to suggest that this would also be a good place to discuss the "Green Flash" which of course stems from the different wavelengths of light being bent differently. I have looked and looked for this, and only seem a very faint effect. Any hints... sorry this comment is late: I was roaming around in the Rockies...


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