# The Naked Scientists Forum

### Author Topic: How high would a light shine from 50 miles away?  (Read 7958 times)

#### DoctorBeaver

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##### How high would a light shine from 50 miles away?
« on: 27/03/2008 19:44:01 »
Seany's post about a laser his friend bought prompted this question.

http://www.thenakedscientists.com/forum/index.php?topic=13568.0

Assuming this device really does have a range of 50 miles and was shone horizontally from the waist (say 3ft above the ground), at what altitude would the beam be 50 miles away?

#### Seany

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##### How high would a light shine from 50 miles away?
« Reply #1 on: 27/03/2008 22:47:47 »
Nice question Doc! LOL

#### Karen W.

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##### How high would a light shine from 50 miles away?
« Reply #2 on: 27/03/2008 22:49:36 »
Yes indeed!

#### DoctorBeaver

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##### How high would a light shine from 50 miles away?
« Reply #3 on: 27/03/2008 23:04:34 »
The relevance of diegoqing's reply is...?

Post by Seany click to view.

#### Seany

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##### How high would a light shine from 50 miles away?
« Reply #4 on: 27/03/2008 23:05:53 »
Shrunk
Ermm.. Like.. Because

If you buy some Nike trainers, you feel more lively and jolly and moveable.. So you jump up and down with Michael Jordan's nike shoes and you see how high the laser is..

I donno!!

#### Karen W.

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##### How high would a light shine from 50 miles away?
« Reply #5 on: 27/03/2008 23:14:24 »
Well I think his shoes allowed me to give a good kick!! LOL!

#### DoctorBeaver

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##### How high would a light shine from 50 miles away?
« Reply #6 on: 28/03/2008 07:31:59 »
Anyone reading this will wonder what the hell we're talking about. They won't realise a post has been deleted. Well, they will now because I've mentioned it. But they won't know what it said.

Post by Seany click to view.

#### Seany

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##### How high would a light shine from 50 miles away?
« Reply #7 on: 28/03/2008 10:41:27 »
Shrunk

#### turnipsock

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##### How high would a light shine from 50 miles away?
« Reply #8 on: 28/03/2008 11:45:40 »
It will be 0.35 of a mile, plus 3 ft.

#### Seany

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##### How high would a light shine from 50 miles away?
« Reply #9 on: 28/03/2008 12:46:11 »
It will be 0.35 of a mile, plus 3 ft.

How did you work this out?

#### Seany

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##### How high would a light shine from 50 miles away?
« Reply #10 on: 28/03/2008 12:46:49 »
Seany's post about a laser his friend bought prompted this question.

http://www.thenakedscientists.com/forum/index.php?topic=13568.0

Assuming this device really does have a range of 50 miles and was shone horizontally from the waist (say 3ft above the ground), at what altitude would the beam be 50 miles away?

Doesn't it also differ with the quality of the laser? Better lasers spread less than others//

#### turnipsock

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##### How high would a light shine from 50 miles away?
« Reply #11 on: 28/03/2008 13:35:25 »
It will be 0.35 of a mile, plus 3 ft.

How did you work this out?

The circumference of the earth is 360x60=21,600 miles. That is because one mile equals 1 minute of a degree.

From that you can calculate the radius of the earth.

Then it is just a right angled triangle after that where one side is the radius of the earth, another side is 50 miles and you can calculate the length of the hypotenuse. The altitude is the difference between the hypotenuse and the radius of the earth.

A similar thing occurs when ships have to calculate at what distance they should be able to see a lighthouse.
« Last Edit: 28/03/2008 15:04:56 by turnipsock »

#### ukmicky

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##### How high would a light shine from 50 miles away?
« Reply #12 on: 28/03/2008 19:10:34 »

#### DoctorBeaver

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##### How high would a light shine from 50 miles away?
« Reply #13 on: 28/03/2008 20:39:19 »

The circumference of the earth is 360x60=21,600 miles.

No it isn't. The equatorial circumference is 24,901 miles, and the polar circumference is 24,859 miles.

#### Seany

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##### How high would a light shine from 50 miles away?
« Reply #14 on: 28/03/2008 22:42:21 »

#### Bored chemist

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##### How high would a light shine from 50 miles away?
« Reply #15 on: 29/03/2008 16:41:46 »
There's another complication. The air is denser at lower altitudes and so it has a higher refractive index. This means that the light bends downwards slightly.

#### Pumblechook

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##### How high would a light shine from 50 miles away?
« Reply #16 on: 29/03/2008 17:02:48 »
If h is in meters, that makes the distance to the geometric horizon 3.57 km times the square root of the height of the eye in meters (or about 1.23 miles times the square root of the eye height in feet).

= about 500 metres.  This is ignoring any bending and the fact that it is rarely clear enough.  A power light ot laser will help.  I saw the hills of North Wales (incl Great Orme) from near sea level at Blackpool last year at the distance is pretty well 50 miles.

#### DoctorBeaver

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##### How high would a light shine from 50 miles away?
« Reply #17 on: 30/03/2008 08:38:57 »
Wouldn't the Earth's gravitational field pull the beam down too?

#### Bored chemist

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##### How high would a light shine from 50 miles away?
« Reply #18 on: 30/03/2008 14:33:03 »
The direct effect of gravity is pretty small but the effect of the refraction is significant. If the earth's atmosphere weree replaced by SO2 which has a slightly greater refractive index, the curvature due to refraction would be roughly the same as the curvature of the earth and light would go right round (ignoring any mountains).

#### DoctorBeaver

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##### How high would a light shine from 50 miles away?
« Reply #19 on: 30/03/2008 21:08:03 »
The direct effect of gravity is pretty small but the effect of the refraction is significant. If the earth's atmosphere weree replaced by SO2 which has a slightly greater refractive index, the curvature due to refraction would be roughly the same as the curvature of the earth and light would go right round (ignoring any mountains).

Oooh... that could be fun. I could shine a torch on my own ars back!

#### turnipsock

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##### How high would a light shine from 50 miles away?
« Reply #20 on: 30/03/2008 23:56:41 »
The direct effect of gravity is pretty small but the effect of the refraction is significant. If the earth's atmosphere weree replaced by SO2 which has a slightly greater refractive index, the curvature due to refraction would be roughly the same as the curvature of the earth and light would go right round (ignoring any mountains).

Would that mean if the Doc fired a laser, 3ft above the ground, in an atmosphere of SO2, it would hit him in the ars back? I saw a good laser in Thunderball.

#### Karen W.

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##### How high would a light shine from 50 miles away?
« Reply #21 on: 31/03/2008 03:25:40 »
Anyone reading this will wonder what the hell we're talking about. They won't realise a post has been deleted. Well, they will now because I've mentioned it. But they won't know what it said.

He was spamming the site.. in a couple places.. LOL!

#### The Naked Scientists Forum

##### How high would a light shine from 50 miles away?
« Reply #21 on: 31/03/2008 03:25:40 »