“LIGHT” is an electromagnetic wave that travels at the speed of light.
- What is light?
- EM stands for Electromagnetic
- speed of light
- light year
- Anatomy of an EM wave
- The Electromagnetic Spectrum
- EMS Trends
- When light hits matter
Radiant Energy Kahoot to practice your knowledge
If that doesn’t work try THIS link
All light starts with a vibrating charged particle
Imagine holding a long rope that is tied off weight. If you move the rope quickly up and down you would notice a WAVE of energy that traveled down the rope. The more energy you put into lifting and lowering the rope, the more energy you will observe in the waves that travel along the rope.
Throwing a pebble into a still pond makes waves as well. The disturbance of the water sends energy out in every direction of the circle that surrounds the place that was hit by the pebble.
Similarly, if we could shrink down to the size of Ant-Man and grab onto an electron and shake it, we would generate a wave of energy that travels in every direction. This is called an electromagnetic wave or… LIGHT.
Try it yourself with the radio waves PhET
Shaking the electrons on the antennae will send out a wave of energy (usually in all directions) at the speed of light.
The receiving antennae is designed, programmed, or dialed in to be sensitive to only a specific number of waves per second. This is the FREQUENCY of the EM wave.
If I want to listen to a baseball game in St. Louis, I would set my radio to 1120 kHz on the AM dial. Hz stands for Hertz or ONE electromagnetic wave per second. Since KMOX broadcasts in the KiloHertz range, my radio must only receive waves that are vibrating at 1,120,000 waves per second. It’s simply the 1120 times 1,000.
Changing the station to the FM dial means you are looking for EM waves that vibrate MILLIONS of time per second. That’s why the dial will read MHz (MegaHertz) for these stations. Listening to classic rock on 94.7 MHz means my radio must only be sensitive to 94,700,000 waves per second. More waves per second means that the length of each wave must be smaller.
EM waves have both electric and magnetic fields
The “EM” stands for electromagnetic. This means it has both electric and magnetic wave components. They are perpendicular (90 degrees from one another) as they travel forward from the origin.
Electromagnetic waves travel at the “speed of light”
An easy rounding of the speed of light is 300,000,000 meters per second (in a vacuum).
Traveling faster than light speed is fun to think about but at this point it is just science fiction. You might hear it called hyperspace, warp speed, or ludicrous speed but it all refers to objects going faster than light.
Anything that travels at light speed IS LIGHT. It would be a real challenge to convert complicated matter like human DNA into light and then transform it back into exactly the same matter. It usually works great in Star Trek but it is, again, just science fiction for now.
Putting the speed of light into perspective a bit…
How fast are you at the “start /stop” game? Can you hit 0.13 seconds or better? That’s pretty fast for a human.
If we set up satellites to beam a signal of light around the equator of Earth it would only take 0.13 seconds. That’s over 7 trips around the globe in 1 second!!
Sending a beam of light to the moon from Earth takes only 1.5 seconds. During the Apollo missions large reflectors were placed on the surface of the moon. Distances can be measured by bouncing lasers off of the reflectors.
An AU (astronomical unit) is what scientists call the distance from the Earth to the Sun. It is 149 million kilometers (93 million miles). When a photon of light leaves the surface of the sun it travels through the vacuum of space for an average of 500 seconds ( a bit less than 8.5 minutes) before it can get to the Earth for us to see.
To put it another way, if the Sun stopped putting out light at the same moment you hit play on one of the songs listed below , you wouldn’t notice the Earth getting dark until the song came to an end. What would be on your 8 minute and 20 second playlist if you could travel at the speed of light?
Sending a radio message to Mars depends on where both of the planets are in their orbit around the Sun. When they are close, the distance is close to 54.6 million kilometers but when they are on opposite sides of the Sun it is closer to 401 million kilometers. At light speed, a one way trip to the red planet ranges from 3 to just over 22 minutes!
This delay flies in the face of many science fiction movies and tv shows that seem to have a real-time conversation with starships and planets that are significant distances away. It works in StarTrek but in real life you would experience a 3-22 minute lag time between parts of the conversation. Sounds more like a zoom call that froze due to low bandwidth.
Rather than live conversations, it is much more likely that we will record short video messages (vlog) to send long distances. This is demonstrated very well in the 2015 movie based on the book The Martian. Astronaut Mark Watney is accidentally abandoned on Mars and needs to communicate with NASA. The time delay between Watney and Earth is a large part of the plotline as they work to problem-solve his situation.
The closest star to Earth is the Sun (Sol) but the NEXT closest star is called Proximus Centauri. It is found in the southern constellation Centaurus. Light from this star takes a bit over FOUR YEARS to travel to Earth for our telescopes and eyes to see it. What were you doing 4.2 years ago? That’s when that light started its journey just to end up stimulating your optic nerve.
What is a “light year?
When people hear the phrase “light year” for the first time, it is common to have some misconceptions about what is meant. Some think it is a speed. Some think it is a time. A light year actually refers to the DISTANCE that a light would travel in one year of time.
A light year is just under 10 TRILLION kilometers!!! ( 9,460,730,472,580.8 km to be precise)
Our Milky Way galaxy is 100,000 light years across!
This means when you see a star on the other side of the galaxy you are viewing the way that star looked 100,000 years ago! This is as close to time travel as you are going to get.
Imagine for a moment that you were instantly transported to a planet that was 65 million light years from Earth. You look back towards Earth with a powerful telescope. You are now seeing the light from our Sun that reflected off of Earth …but this light has been traveling for 65 million years! Maybe you could get a glimpse of the last day of the dinosaurs!!
The following is one of my favorite videos EVER! The entire thing takes place within one quarter of the Andromeda galaxy as seen by the Hubble telescope. Be impressed…
Keep in mind that each point of light is a star…a cluster of stars…or maybe another galaxy that you are seeing through Andromeda.
Which of these stars has a solar system of planets? It’s estimated that there are 500 billion planets in the Andromeda Galaxy. That means there are approximately 125 billion planets passed over in this video!
The SPACE between each point of light will vary but remember that our closest star is over 4 light years away from the Sun.
Notice the concentration of stars increases when you get closer to the center.
As the video ends, remind yourself that EVERY star you just saw was located in
…one quarter
…of one galaxy
…in one tiny section
…of the night sky that we can see. Simply AMAZING!
Below is the remastered version (but I like the music on the first one better)
The Anatomy of an Electromagnetic wave
All EM waves travel at the speed of light (in a vacuum) but each type of wave has a different sized wave. We call it the WAVELENGTH and it is the distance from the crest of one wave to the crest of the very next wave.
Wavelength can also be measured from trough (the lowest point) to the next trough. For that matter you can pick any point on the wave and measure to that same point on the next…it all called a wavelength. It’s like the bar that connects the wheels of a train. The length of the bar doesn’t change when the wheels turn.
The FREQUENCY of the wave is the number of times a wave passes by a fixed point. Imagine sitting by the side of the road in traffic counting the cars that go by. That would be the frequency of cars.
For light, one vibration of the electron each second will generate ONE wave of electromagnetic radiation per second. This would be a frequency of 1 Hertz.
If the electrons are vibrating at 1000s of times per second the resulting ripples of light would be measured in kiloHertz (AM radio)
Millions of waves per second would be MegaHertz (FM radio) while billions of of vibrations per second would generate light in the GigaHertz (Bluetooth and WiFi)
The frequency and wavelength of the light will determine which type of light we will classify it as.
Below is an example of the relationship between frequency, wavelength, and speed. We can calculate the speed of light using PEEPS in a microwave?!
All of the wavelengths of light are classified on the ELECTROMAGNETIC SPECTRUM
A SPECTRUM is created when we have a wide range of a certain variable. For example we could line everyone up in the classroom based on height, age, eye color, hair length or limitless other variables that we all share.
Electromagnetic waves all share some basic things. They all originate from a vibrating charged particle. The all travel at the speed of light. They all come in waves. The variable that makes them so different is the WAVELENGTH of light. Some wavelength are invisible to us but very useful while others form the colors of the rainbow or a fantastic sunset. Below we will investigate the different forms of light in the EMS or electromagnetic spectrum.
This is the portion of the EMS that our human eyes can receive and do something with. While other types waves can enter our eyes, we don’t have receptor cells to interpret those wavelengths. The colors are always arranged in a rainbow according to their wavelengths. We use the acronym ROY G. BIV to remember their order.
Red, Orange, Yellow, Green, Blue, Indigo, and Violet (ROYGBIV)
The colors represent a very small portion of the entire spectrum of electromagnetic waves. In fact, the light we can “see” only makes up 0.000001% of the entire EMS!
For perspective, if you wrapped the entire EMS around the globe , the portion of light that we call “visible” would only be 40 cm long .
“COLOR” is simply the wavelengths of light that are reflected off of objects to our eyes. As depicted in the picture below. Light is reflected off of a white surface while a black surface absorbs all of the light. Black and white are technically not colors. White is what we see when your eyes collect all colors at once while black is the absence of light. When light hits a “red” object, all wavelength of light that are not red are absorbed into the surface. Red light is reflected off to our eyes so we see “red”.
TWO important trends found with the EMS.
1. As the wavelength DECREASES, the frequency INCREASES (think about the reverse as well…)
Imagine three lanes of traffic all moving at the same speed. Count the number of vehicles that pass by the red line per second. How many trucks? How many cars? How many motorcycles? As expected the smaller vehicles have a higher frequency across the line. Larger vehicles have a LOWER frequency across the line.
Similarly, large wavelengths like radio have low frequencies compared to the shortest waves like gamma rays.
2. As the wavelength DECREASES, the energy of the wave INCREASES
Less energy invested in particle vibration will give a LOW frequency but a LONG wavelength.
It takes more energy to make more waves per second. HIGH energy waves have SHORT wavelengths and HIGH frequencies.
When light hits an object there are THREE possible actions:
TRANSPARENT – ALL light could pass through the object. A clear window or eyeglasses are best when they allow all light through. Cleaning a window makes sure that nothing gets in the way of viewing the scenery.
TRANSLUCENT – SOME light could pass through the object. Sunglass lenses and theatrical gels only allow some wavelengths to pass through. Sunglasses typically lower all wavelengths evenly while the theatrical gels absorb different colors while letting others get to our eyes. Shining a high powered light into your hand stops most of the wavelengths however, some of it can still get through.
OPAQUE – NO light passes through the object. These blocks are creating shadows where light has been stopped. Analysis of a shadow provides evidence of the brightness and angle of the light projected.