# Can light go around a planet?

1. Nov 28, 2015

### Robin04

I found a very interesting exercise for high school students: The atmosphere of a planet changes in height so that if we point a laser in a tangential direction, the light goes around the planet. This can be easily solved (at high school level) by applying the Fermat principle, but is this possible in real life? Isn't it the base of geometric optics that light travels in a straight line? I've been thinking about that if it's possible then this could be a very interesting problem to deal with. :)

2. Nov 28, 2015

### tech99

By using a sphere with graded refractive index, an incident ray can be made to emerge in the direction from which it came, forming a retro reflector. It is the principle of some radar reflectors used on vessels.

3. Nov 28, 2015

### 256bits

Shortwave radio - is that what you are thinking of.

4. Nov 28, 2015

### tech99

In the case of radio waves and the ionosphere, it is not the same. A wave arriving from outside will be reflected off and will not propagate around the planet.
I must mention that all waves longer than a certain limiting value are reflected by the ionosphere; the magic of short wave propagation is that the absorption is also small. This happens when we use waves that are just longer than the limiting value.

5. Nov 28, 2015

### A.T.

Here with laser light:
http://www.nature.com/nphoton/journal/v7/n11/abs/nphoton.2013.247.html

6. Nov 28, 2015

### 256bits

The usual term "reflection off the ionosphere" has a good deal of refraction associated with it due to the gradient of electron density in the upper atmosphere. Back propogation of the signal would involve reflection.

Such qualities of the ionosphere is used by the SuperDARN radar network to probe the atmosphere at high altitudes at over the horizon distances farther than direct line of sight signals.
The radar depends upon the refractive and reflective properties of the upper atmosphere. You yourself can set up a station with a minimal outlay of funds.
http://vt.superdarn.org/tiki-index.php?page=sd_tutorial

LF waves, or waves of similar wavelength, can also hug the surface of the earth due to the difference in refractive index for the earth and the atmosphere and thus follow the curvature of the earth for considerable distance.

As for a natural phenomina of an emf wave travelling in a curved path, there is direct evidence for support.
The earth has its own natural graded refractive index.

7. Nov 28, 2015

### DaveC426913

I've never heard of such a device. The only radar er ... reflectors ... I'm aware of on ships are reflector types. Can you point me to a refractor?

[ EDIT ] Well, you learn something new every day.

8. Nov 28, 2015

### tech99

Such a device is called a Luneburg Lens, and one company offering these is as follows. They give the most uniform reflection pattern of all the radar reflector types. http://www.radar-reflector.com/

9. Nov 28, 2015

### tech99

So far as I am aware, the predominant modes of propagation beyond the horizon for LF waves are the surface wave and the ionospheric path. At VLF the ground/ionosphere is often considered to be a waveguide. The surface wave is not traditionally considered to be dependent on the atmosphere in any way, although I would agree there is uncertainty about the exact mode involved.

10. Nov 28, 2015