Properties of Light

Light is a unique thing. We can think of it terms of both waves and particles called photons. In this post, we are going to look at only the wave nature of light, or in general, all electromagnetic radiation of which light is only a small subset.


 


Electromagnetic radiation is made up of both electric fields and magnetic fields which propagate perpendicular to each other. The wave itself, moves in the third direction, perpendicular to the fields.

where B is the strength of the magnetic field, E is the strength of the electric field, and c is the speed of light (3x10⁸ m/s)

 

So what exactly is a wave? Waves have maxima (crests), minima (troughs), and zero points (nodes).

We can also take a measure of a wave by looking at the distance between successive crests (or troughs) which we call the wavelength, represented by the Greek letter lambda (λ). This can be measured in meters, nanometers (10^-9 m), or Ångstroms (10^-10 m symbolized by Å).

Or we can look at a point in space and how long it takes two successive crests (or troughs) to pass that point in space. This is called the frequency, symbolized as either f or ν (Greek letter nu). This value can be measured in the inverse of seconds (1/s) or Hertz.

The cool thing about the wavelength and the frequency is that you can multiply them together to get the speed of the wave. In our case, for light, the speed is just c.

c= λ*ν

 

Lastly, when we are talking about visible light, we are talking about all light that can be seen by the human eye. They range in color from red to violet (ROY G. BIV), with the longer wavelengths and shorter frequencies closer to the red end of the spectrum and the shorter wavelengths with higher frequencies near the violet end.

For visible light, red is approximately 7000 Å and 430 terahertz (10¹² Hertz). Violet is around 3000 Å and 1 petahertz (10¹⁵ Hertz). 

These will become important later on.

 

 

Planetary Alignment

Every once in a while, a story comes out that a planetary alignment will wreak havoc on Earth and the end of the world as we know it will occur. The most recent example was posted by a friend of mine on Facebook. It basically tells us that a planetary alignment will cause a massive 9.8 earthquake in California on May the 28th. Here is a link for the story if you wish to read it. I admit that I didn't read it because I have one thought on stories like this: they basically come from the back end of a male cow.


These stories are bogus. They are a waste of electrons on the internet. People that don't know science are taken in by these articles because of the gloom and doom that they predict. I'm going to explain why these stories are false.


1. The Sun is the overwhelming largest mass in the Solar System, and by a large margin, contributes the largest component of the force of gravity on the Earth. All the other planets, moons, asteroids, comets, minor bodies, dust particles, alien lifeforms, nanobots, whatever, in our Solar System combined do not contribute much to the force of gravity felt on Earth. Remember, the force of gravity is related to the mass divided by the distance squared (higher the mass, higher the gravity; closer the mass, the higher the gravity as well). Remember, that the Sun contains over 99% of all the mass in the Solar System, and that there are only six objects in Solar System closer to Earth than the Sun (Mercury, Venus, the Moon, Mars, Phobos, and Deimos). They do not have enough mass combined to do anything to our planet. In fact, the only other mass that has an significant affect on Earth is the Moon, and all it does is make water slosh around (see the post on tides).


2. The planets themselves cannot align themselves in such a way that they all are pulling on Earth in the same direction, even if the gravity was strong enough. All the planets are inclined differently to the Sun's equator, and in turn, to the Earth's orbital plane. One planet might be above the Earth's orbital plane, while another below it. Note that it is possible that the planets might all be in the same orbital plane as the Earth as they orbit the Sun, but this is highly unlikely due to all the motions of the planets. But again, it is extremely unlikely that they all would happen to be in the same orbital plane as the Earth, and even then, look at the first point.


I am betting that if you read my blog, you know that these stories are blatantly wrong. However, if by some chance you believe them, just understand, planetary alignments will not hurt the Earth in anyway. I'd be more worried about the people living on the planet.

Why Is the Sky Blue?

Believe it or not, we've actually discussed this before. When we've talked about reflection nebulae, we mentioned how the blue light is reflected towards our line of sight. In the same way, the sky is blue.

Remember, that light is composed of all colors of light. When light from the Sun enters the atmosphere, the dust in the sky easily reflects the blue wavelengths because they are shorter. The longer wavelengths (red, orange, yellow) pass right by the dust particles. The blue light is dispersed over the whole sky and when we look at the sky, we see blue. Of course, when we look at the Sun (which you should never do without proper eye protection), the Sun is yellowish (more yellow light in the Sun's spectrum than orange or red.

However, at sunrise or sunset, the sky around the Sun is orangish-red. Why is this? There are two reasons:

1.      The blue light is scattering away from our line of sight so all we see from the Sun are reds and oranges.

2.      The amount of atmosphere the light travels through to reach our eyes is more at sunrise and sunset than when the Sun is directly overhead. More dust (and pollution) for the blue light to interact with and be reflected away from us.



 

Hohmann Transfer Orbit

A Hohmann transfer orbit is a method of getting a spacecraft or satellite from one circular orbit to a lower or higher circular orbit. It involves using thrusters at particular points in the orbits to either speed up or slow down the craft.

The reasoning behind using this type of orbit is that for circular orbits, the speed of the craft does not change as it is a constant distance from the center of the orbit (the center of the planet or moon that it is orbiting). A Hohmann transfer orbit is a semi-elliptical orbit with the focus of the ellipse at the center of the system.

To go from a lower orbit to a higher orbit, the spacecraft must increase its orbital speed. This increase will put it on the elliptical orbit that will transfer the craft to the higher orbit. Once the craft reaches the desired orbit, the craft must then again fire its engine to speed the craft up to the correct speed for the orbit, putting it into a new circular (higher) orbit. This is the method in which spacecraft dock with the International Space Station.

The reverse also works, but in this case, the craft's engines slow it down to get to the lower orbit, again slowing the craft at both the higher orbit (to transfer) and the lower orbit (to insert it into the new orbit).

In the picture below, the blue orbit is the lower circular orbit, the yellow/green dashed path is the Hohmann transfer orbit, and the red orbit is the higher orbit. At point A, the engines are fired to increase the speed of the craft and at point B, they are fired again to insert it into orbit. To transfer from the higher to the lower, the engines are retro-fired at B to slow the craft down and again at A to insert it into the lower orbit.



 

Space Elevator

There has been discussion of finding the best way to get people and cargo into space. Right now, we strap people into a rocket and use as much thrust as can be mustered on that rocket to achieve low-Earth orbit. One of the ways that is physically possible, though the engineering may be a little off, is a space elevator, first proposed by Russian Konstantin Tsiolkovsky.


A space elevator is basically what it sounds like. It is a way to lift people and cargo into space into a geostationary/geosynchronous orbit using some sort of transport vehicle moving along a track into space. The question is what kind of propulsion can the transport use and what material will be used for the track?


First of all, what makes a space elevator a space elevator? The space elevator will have two end stations, one at the equator and one at a geostationary orbit above the Earth-bound station. Remember from our discussion on geostationary orbits that these are orbits that keep a satellite in the same location above the Earth at all times. More on these orbits can be found here. The station on Earth has to be at the equator because the station in space has to remain in the same location, and geostationary orbits require points above the Earth's equator. It is possible to have way stations along the path, but these way stations must have continuous propulsion to remain in place.


Space elevators can be made out of a material called carbon nanotubes (which are theoretical). These tubes would have the property of being both lightweight and strong. The track material must withstand large tensions and torsional forces to prevent breaking. The preferred method of propulsion would be magnetic forces, much like a maglev train uses magnetic repulsion to move them at high speeds.


The station on the end of the elevator in space will have to be at 35,786 km above the equator because it is a geostationary object. This also means that if somehow the elevator did break, it would wrap around the Earth more than once since the circumference of the Earth is only ~25,000 km.


So the physics is there to make space elevators a reality, but unfortunately, the engineering has not quite caught up. If we were able to build one, the cost of launching objects into space would be decreased immensely.

MESSENGER (Mercury Mission)

The MESSENGER probe (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) was a probe that studied Mercury for four years.


It was launched in 2004, performing flybys in January of 2008, October of 2008, and September of 2009 before entering orbit in March of 2011. From MESSENGER, we learned a lot about the surface of Mercury, taking a lot of images of the surface. The mission ended on April 30, 2015 when it crashed onto the surface of Mercury.

First Image (left) and Final Image (right)

The first image is from March 29, 2011 and is the first image taken after orbital insertion

The final image was taken just before it crash landed on the surface of Mercury.

Both photos are courtesy of NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

For more information about the MESSENGER mission, check out the mission sites:

• JPL: MESSENGER: John Hopkins University
• NASA: MESSENGER: NASA