A Question Concerning Coasts

I live by the ocean and when I see a boat departing, I see it dissapearing from top to bottom as if it was sinking into the water.

Conversely, when I am on a boat sailing away from shore, I see the bottoms of the building vanishing beneath the horizon first.

If the earth is flat, why does this happen?

Increasing moisture density over the ocean creates a medium interface whose height increases the farther you are from land. Mirages caused by this medium interface serve to obscure whatever is below the interface, resulting in the obfuscation of the lower parts of the ship at a certain distance. Eventually the medium interface is of sufficient height to obscure the entire ship.

When you see a ship sailing towards you and seems to appear mast first, that is the result of the mast being the first to appear above the medium interface. The ship isn't sailing up over the curve of the earth, but is instead sailing out from under the mirage.

Increasing moisture density over the ocean creates a medium interface whose height increases the farther you are from land. Mirages caused by this medium interface serve to obscure whatever is below the interface, resulting in the obfuscation of the lower parts of the ship at a certain distance. Eventually the medium interface is of sufficient height to obscure the entire ship.

When you see a ship sailing towards you and seems to appear mast first, that is the result of the mast being the first to appear above the medium interface. The ship isn't sailing up over the curve of the earth, but is instead sailing out from under the mirage.

mirages only happen in hot climates

Quote from: "Unimportant"

Increasing moisture density over the ocean creates a medium interface whose height increases the farther you are from land. Mirages caused by this medium interface serve to obscure whatever is below the interface, resulting in the obfuscation of the lower parts of the ship at a certain distance. Eventually the medium interface is of sufficient height to obscure the entire ship.

When you see a ship sailing towards you and seems to appear mast first, that is the result of the mast being the first to appear above the medium interface. The ship isn't sailing up over the curve of the earth, but is instead sailing out from under the mirage.

mirages only happen in hot climates

No.

Increasing moisture density over the ocean creates a medium interface ... The ship isn't sailing up over the curve of the earth, but is instead sailing out from under the mirage.

Here we go again..So explain why the effect is the same, and can be calculated consistant to a nearly spherical earth model, regardless of humidity, temperature, and the angle of the sun.

Quote from: "Unimportant"

Increasing moisture density over the ocean creates a medium interface ... The ship isn't sailing up over the curve of the earth, but is instead sailing out from under the mirage.

Here we go again..So explain why the effect is the same, and can be calculated consistant to a nearly spherical earth model, regardless of humidity, temperature, and the angle of the sun.

I guess the best explanation would be that humidity, temperature, and angle of the sun don't affect the height of the medium interface. If you were asking why height isn't affected, I don't know.

Now wait! You are saying both:

Increasing moisture density over the ocean creates a medium interface

And:

I guess the best explanation would be that humidity, temperature, and angle of the sun don't affect the height of the medium interface. If you were asking why height isn't affected, I don't know.

How can the effect be related to moisture and not have Humidity and Temperature affect it?  Not just the height, but the density of the layer causing the illusion.  If I like the effect to a glass of water, making a pencil bend because of the optical density differeences between air and water, If I change the desity of the material in the glass, switching it for glycerine or alcohol, the angle of the bend changes.  Variations of air density should cause the effect of the illusion to move closer or futher away.

The mirage involves reflection, not refraction, and I don't think reflection is as affected by the density of the medium.

As for the effect of air humidity on the medium density, who knows. If you put a glass of water in a really humid room, the water doesn't get any denser. Or maybe it does, but the affect is so small so as to be negligible. Even then, as I stated above, I don't think altering the densities of the mediums would have any affect on the reflection that occurs at the medium interface, and I can only assume it doesn't affect the height in any measureable manner either.

The mirage involves reflection, not refraction, and I don't think reflection is as affected by the density of the medium.

Mirages are certainly refraction, and reflection is likely to be related to the difference in densities at the interface.  I think there's some relationship between transmitted and refracted energy, and reflected energy, as a function of this differential.

If you put a glass of water in a really humid room, the water doesn't get any denser. Or maybe it does, but the affect is so small so as to be negligible.

It definitely wouldn't.  It might get more water in it, but the density of the water, ceteris paribus, would be the same.

-Erasmus

Quote from: "Unimportant"

The mirage involves reflection, not refraction, and I don't think reflection is as affected by the density of the medium.

Mirages are certainly refraction, and reflection is likely to be related to the difference in densities at the interface.  I think there's some relationship between transmitted and refracted energy, and reflected energy, as a function of this differential.

Yes, the reflection is caused by the difference in density at the interface. That's what causes the mirage. I believe the difference in densities affects how severe an angle the interface is capable of reflecting; if the densities are somewhat close, then light will only be reflected at a very shallow angle; if the angle is too great the light will pass right through the interface, which is where refraction occurs. If the densities are greater, then the angle of reflection can increase too.

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If you put a glass of water in a really humid room, the water doesn't get any denser. Or maybe it does, but the affect is so small so as to be negligible.

It definitely wouldn't.  It might get more water in it, but the density of the water, ceteris paribus, would be the same.

-Erasmus

That's the point I was hoping to make.

Yes, the reflection is caused by the difference in density at the interface. That's what causes the mirage. I believe the difference in densities affects how severe an angle the interface is capable of reflecting; if the densities are somewhat close, then light will only be reflected at a very shallow angle; otherwise the light will pass right through the interface, ergo refraction occurs. If the densities are greater, then the angle of reflection can increase too.

I reiterate my claim that it is a refraction that occurs, and not a reflection.  Refer either to Wikipedia or to this page on atmospheric optics.

As for angle of reflection, as far as I know, it's always equal to angle of incidence.

-Erasmus

I reiterate my claim that it is a refraction that occurs, and not a reflection.  Refer either to Wikipedia or to this page on atmospheric optics.

As for angle of reflection, as far as I know, it's always equal to angle of incidence.

-Erasmus

The angle of reflection is always the angle of incidence, but at a certain point the light hitting the medium won't be reflected, it will just pass through. This is why mirages will generally appear in the distance closer to the horizon; if you look closer to yourself, the angle of incidence will be high enough that the light won't be reflected (or refracted back up above the medium).

My understanding of mirages is and has always been as follows: Take a pane of glass and hold it in front of you, so you can see through it. Slowly tilt the pane of glass away from you until it is nearly level. At some point you will no longer be seeing through the glass, but instead a reflection of what is above the glass.

This is what happens in a mirage, and the definition of wave behavior as being refraction instead of reflection is, in my mind, semantic.

It is quite clear that the mirage in this photograph is creating a reflected image of the car above. Whether the light bends (refraction) or bounces (reflection) when it hits that medium interface is, frankly, immaterial. You're right; all the academic sources I've found cite refraction as the cause of the image produced, but the effect is similar enough to reflection so as to be a non-issue.

Sorry to correct you, but does this somehow explain the issue of a ship giving the illusion of "sinking" into the ocean when it gets furhter away? I am sure it does and I was just not able to recognize it from the thread. Please excuse if it is my fault, but could you explain?

If you put a glass of water in a really humid room, the water doesn't get any denser.

There is a difference between a liquid and a gas when dealing with density.  Liquids maintian a specific density, thus hydrolics work.  Gasses do not, so you can have a tank of pressurized air.

Take a pane of glass and hold it in front of you,... At some point you will no longer be seeing through the glass, but instead a reflection of what is above the glass.

This is what happens in a mirage, and the definition of wave behavior as being refraction instead of reflection is, in my mind, semantic.

It is quite clear that the mirage in this photograph is creating a reflected image of the car above. Whether the light bends (refraction) or bounces (reflection) when it hits that medium interface is, frankly, immaterial.

You're right; all the academic sources I've found cite refraction as the cause of the image produced, but the effect is similar enough to reflection so as to be a non-issue.

No, it is a large difference.  A reflection happens at a specific plane, a mirror reflects, and incidence equals reflection.  The point of relfection on a mirror is at the back of the plane of glass where the coating has been applied.

In refraction the light is bent.  the difference in optical density affects the degree of bending.  To use your glass plane example, close obervation would show that it does not go from a clear image to a reflection, the light bends in varying degrees.  For an easily demonstraitable experiment, take a laser pointer and a half empty bottle of water.  hold the bottel horizontal.  Shine the laser pointer at an angle through the middle of the bottle, and note where it strikes the surface below, remove the bottle and unless you were pointing sraight down the point where the laser strikes will change. Now, replace the bottle, and move your pointer in a slight arc, again remove the bottle.  The point will move, in relationship to the angle which the light strikes the flat surface of the water.

Your example of the hot air over the road way is a perfect demonstraition, If you are not within a thin range of angles to the "Mirrage" there is no refraction.  It is cause by the heated air above the roadway being a different density than the air you are looking through.  A short distance above the road way, the air cools enough for the effect to go away.

Ok, so similar question as the original poster had.  Why do tall buildings and mountains also exhibit a similar effect when you are driving towards them?

If you're on land you're going to have a tough time proving it's not just a hill. This is why the ocean has been adopted as the official backdrop for the conversation.

Not all that hard.  If you don't trust topographical maps (not all of them are made by the government), a barometric altimeter can give you an accurate enough idea.  You just have to take enough trips to account for variations caused by localized weather systems.

If you're on land you're going to have a tough time proving it's not just a hill. This is why the ocean has been adopted as the official backdrop for the conversation.

You never answer why the effect occurs with the Rocky Mountains.  I can prove there's no hill, because water flows from the mountains down to the plains.  If you are down stream, there can't be an interposing hill.

Quote from: "Unimportant"

If you're on land you're going to have a tough time proving it's not just a hill. This is why the ocean has been adopted as the official backdrop for the conversation.

You never answer why the effect occurs with the Rocky Mountains.  I can prove there's no hill, because water flows from the mountains down to the plains.  If you are down stream, there can't be an interposing hill.

There can be a hill, as long as it is not as high as the source of the water.  As long as the potential energy of the water at the start of the stream is higher than the potential energy of the water at the top of any interveening hill.

Quote from: "Doubter"

Quote from: "Unimportant"

If you're on land you're going to have a tough time proving it's not just a hill. This is why the ocean has been adopted as the official backdrop for the conversation.

You never answer why the effect occurs with the Rocky Mountains.  I can prove there's no hill, because water flows from the mountains down to the plains.  If you are down stream, there can't be an interposing hill.

There can be a hill, as long as it is not as high as the source of the water.  As long as the potential energy of the water at the start of the stream is higher than the potential energy of the water at the top of any interveening hill.

The banks of the river before the hill would have to be as high as the hill or the water would pour over the bank rather than over the hill.  With the exception of limited and extreem conditions (such as a "haystack" in white water or a glacier being forced over something) , water does not flow up hill, but goes to the lowes energy level it can find.

Like I said, there can be an interveening hill.

Like I said, there can be an interveening hill.

Not unless the banks of the river are as high as the top of the hill, or the velocity of the water is grerat enough to force it up and over, and if you've travelled in Colorado you have had a good chance to see several rivers where neither of these conditions could occur.

Quote from: "TheEngineer"

Like I said, there can be an interveening hill.

Not unless the banks of the river are as high as the top of the hill, or the velocity of the water is grerat enough to force it up and over, and if you've travelled in Colorado you have had a good chance to see several rivers where neither of these conditions could occur.

Like I said, there can be a hill.

Also, as long as: (kinetic energy at the bottom of the hill > potential energy at the top of the hill) the water would not pool at the base of the hill.  No need for high banks, the water would just go over the top.

Also, as long as: (kinetic energy at the bottom of the hill > potential energy at the top of the hill) the water would not pool at the base of the hill.  No need for high banks, the water would just go over the top.

Not quite, if the kintic energy at the bottom of the hill was greater than the potential energy of the banks, it would also overflow the banks.

But the rivers are not runnining fast enough.  To make the illusion of the horizon between the plains of Colorado and the Rocky Mountains, you would need many miles of river going up hill, the force behind the water to push it up the hill at enough speed to create the effect.

I have been there, I lived in Denver for a while.  The rivers are not that fast.

Not quite, if the kintic energy at the bottom of the hill was greater than the potential energy of the banks, it would also overflow the banks.

The conservation of momentum would prevent this.  If the water has momentum going forward, it would not suddenly shift its momentum to the left or right to go over the banks.

You site conservation of momentum as that it won't change direction, but you are only considering direction in 2 planes, x and y, it will also have momentum in the z-direction (ie down), and since it is obviously going at quite a pace down if it is meant to go over a hill, it will "burrow" into the ground and cause a back log of water (Which would break bank) before it attempted of go over the hill.


Also on the illusion of things disappearing over the horizon. How do you explain that if you go up 100 feet you can see X miles, but if you go up 200 feet you can still only see X miles (plus a small amount) instead of the distance that Pythagoras' theorem would predict if the earth were flat.


I truely hope that your not going to claim Phythagoras is wrong, because i'm currently in a rather tall building and i don't want it to fall down around me.

You site conservation of momentum as that it won't change direction, but you are only considering direction in 2 planes, x and y, it will also have momentum in the z-direction (ie down), and since it is obviously going at quite a pace down if it is meant to go over a hill, it will "burrow" into the ground and cause a back log of water (Which would break bank) before it attempted of go over the hill.

This would only happen if all the energy of the water went into burrowing through the hill.  This would happen if there was no trough between the stream and the hill. Take a look at an actual river.  There are often big rocks protruding out of the river.  The water has enough kinetic energy to go over the rock and doesn't burrow though it (macroscopically anyway).  The river doesn't have to rise until the water level is higher than the top of the rock in order to traverse over it.

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Not quite, if the kintic energy at the bottom of the hill was greater than the potential energy of the banks, it would also overflow the banks.

The conservation of momentum would prevent this.  If the water has momentum going forward, it would not suddenly shift its momentum to the left or right to go over the banks.

Except that the momentum has to over come bothe "gravity" and friction for at least 60 miles, and rivers do not travel a straight path, so with the velocity needed, it would overflow the bank of the first curve it reaches.

And since the bed of a river is not smooth, the water is not all traveling with exactly the same vector, so again it just doesn't work.

Too bad though, it would be the wildest white water river ever seen.