Light
rallain | May 17, 2010Grades are in. So, let me just say a couple of trends that I saw on the physical science final exam.
Gravity on the moon
I asked the question: "why is the gravitational force on an astronaut less on the moon than on Earth?"
The simplest answer is that the gravitational field on the moon is smaller than on Earth (I would accept that answer). Why is this? It is because the moon as a much smaller mass even though it also has a smaller radius (that idea is rather complicated for this class - that gravitational force depends on both mass and radius). I would also take "the mass is smaller" as…
rallain | May 13, 2010Yes, green laser pointers are cool. Especially when you use them to make stuff fluoresce. Ok, what about a blue laser pointer? They are getting surprisingly cheap (Amazon has a 10 mW for pretty cheap). Still not cheap enough for me. But, you know what? Some of the physics majors here at Southeastern Louisiana University purchased a couple of these. Physics major Daniel let me borrow his.
First, they don't look too bright. This is probably because our eyes are not too sensitive to this wavelength. The blue 10 mW does not look anywhere near as bright as the 5 mW green that I used in…
rallain | May 4, 2010So you have seen these color filters (or gels as they are also called). When you look through a red filter, everything looks red. What do they do to the light? I am not going to tell you the answer. However, I will show you some examples so that you can figure out the answer yourself.
In this video, I am going to use a red and a green laser pointer. The nice thing about laser pointers is that they essentially create only one color of light.
rallain | May 4, 2010It is my duty as a blogger to mention lasers in this time of international laser celebration. This May is the 50th anniversary of the first lasers. Everyone knows a laser that they love, right? We all use them. So, instead of talking about lasers, I am going to post some great links to other laser stuff (including some of my stuff).
the history of the first lasers (AIP)
The above is the American Institute of Physics's presentation of the history of the laser. Really, this does a great job of giving all the details you would want. I highly recommend it.
Uncertain Principle's Laser…
rallain | April 12, 2010This was a great question. When you come inside after playing in the sunny outside, why is it so dark?
Simple answer: because your eyes are smart. When you are outside, there is a lot of light. Really, it is too much light. To compensate for this, your pupils (the part of your eye that light goes through) closes some. And then, when you go back inside your pupils are still small. Inside (even with the lights on) is not nearly as bright as outside. Not enough light is getting through your pupils and so everything looks "dark".
Here is a simple demo. While inside, take a flashlight (not…
rallain | February 11, 2010Do you have a green laser pointer? You should. They are relatively inexpensive now. What if you take your laser pointer and aim it at stuff in your house? The laser dot always looks green, right? This is actually a pretty important point about color. Suppose I have the lights on in a room and I look at a blue book. White light is basically all the colors of the visible spectrum, so a diagram like this might be appropriate:
So, the book looks blue (since that is what your eye sees). Note that I just showed blue light coming off the book, really the other colors reflect some too but…
edyong | February 3, 2010"That is Shadowfax. He is chief of the Maeras, lords of all horses, and not even Theoden, King of Rohan, has ever looked on a better. Does he not shine like silver, and run as smoothly as a swift stream?" - Gandalf
In the Lord of the Rings, Gandalf rides upon a magnificent white stallion called Shadowfax. White horses have been greatly prized in human societies as a sign of wealth and dignity, largely because their bright coats are both pretty and rare. There are reasons for that. In the wild, the same conspicuousness that inspires legendary tales also makes white horses vulnerable to…
rallain | January 14, 2010Here is a picture of something you have seen before.
These are two pictures of the same location in my house. The one on the left is taken when the Sun was up outside and the other one when it was dark outside. For both pictures, I had the same lights turned on inside. So, why does it do this? Why can you see stuff outside when it is bright outside, but you don't see a reflection of the stuff outside? Why when it is dark outside, does the opposite happen? You know what I am going to do next, right? Diagram. Here is a diagram for when it is dark outside.
The person can see the blue…
rallain | January 13, 2010Check out this picture.
These camera flashes are just awesome (well, they look awesome). They are not a very useful thing for pictures in this case. Here is the problem. Well, first here is what the flash on a camera is supposed to do. Suppose there is a camera and a ball in a dark room. In order to get light to reflect off the ball and be detected by the camera, you need light. The flash is that light. You can think of it like a flashlight that is only on for a moment. It doesn't need to be on for long since it doesn't take very much time to get an image.
Notice that I drew red…
rallain | December 20, 2009This is one of my favorite demos. I like it because anyone can do it at home and people usually find the results surprising. Here is the situation:
How much of yourself could you see in a small mirror? What if you move farther away from the mirror, can you see more of yourself? If you want to do the demo yourself, now would be a good time. All you need is a small mirror (I used a platter from a 10 GB hard drive - they make awesome mirrors). It will help out a lot if you can mount the mirror on a surface that is very near to vertical.
Here is my version of this demo (in case you can't…
rallain | December 14, 2009There was a question from Jerry in a previous post:
"A simple question for which I can't find an answer: Why do you have to collimate light spectroscopy? What would happen if you didn't collimate it?"
The basic idea of spectroscopy is to look at the different colors of light coming from some source. Typically light from a source (like an excited gas) is passed through a diffraction grating that makes different colors bend different amounts (like a prism, only better).
Collimated light is basically light that is all going the same direction. How about I go ahead and draw a picture. This is…
rallain | December 7, 2009So, it snowed here (in Louisiana). Sure, it wasn't a lot, but it was still a big deal. The following day, there was still some snow on the house roofs. I took some pictures. Here is a shot looking at the North side of a house.
And here is a view of the South side of the same roof.
What is so cool about that? All the houses were the same way. North side of roof = snow. South side = no snow. At first, I was going to use these pictures to talk about flux. Basically, since the Sun is lower in the South sky the southern side of the roof gets more solar energy flux. Then I realized my…
rallain | December 1, 2009One of the roads near my house was just redone. They added some awesome retroreflectors in the middle. Here is a shot. No wait, I don't have a picture of that. I tried to take one, but it just didn't turn out very well. Oh, you know what it is supposed to look like. It looks like little tiny lights in the middle of the road.
What makes these things so cool? Why do they look like they are battery powered or something? Maybe it would be helpful to compare retroreflectors to some other materials. I can group stuff in the following manner:
Shiny stuff
Non-shiny stuff
Retroreflectors
What…
rallain | November 5, 2009Very simply, parallax is an apparent motion of an object due to a change in observation position. Let me start with an example. Here are two photos. I took a picture of the cabinet in the background from two slightly different positions. In the foreground is a clone trooper that did not actually move.
I added the dotted line so you could see how the clone trooper appeared to change positions with respect to the background. Here is a diagram of the camera in the two positions along with the toy.
Since the camera changed positions, the object that is closer appears to have moved with…
ejohnson | September 28, 2009
I have photographed jugglers several times in the past for physics text books. I have been impressed with the level of skill some jugglers can obtain. It is difficult enough to juggle three balls, four is more difficult, and fire is a another story. When objects move in a circle they can undergo some fairly complicated motions. What would be the best way to show this motion in a still image?
In this case the camera is panned by the juggler at a constant rate on a computer controlled pan head. When the juggler is about the center of the frame a flash is set off. The image shows…
rallain | September 17, 2009Yes, this can be very complicated. But what should a middle-school student understand about light? You see stuff in textbooks that is either wrong or just a bunch of disconnected factoids (I like the word factoid). So, what do I think is important about light (not at the Maxwell's equations level)
What is a wave
If you want to talk about light, you need to talk about waves. So what is a wave? I like to start with an example. Suppose you are in a sports stadium - maybe a football game. Some inspired fan decides to start a wave. If you look at the individual people, the wave might be…
rallain | September 9, 2009I love this question:
Why is it warmer in the summer than in the winter (for the Northern hemisphere)?
Go ahead and ask your friends. I suppose they will give one of the following likely answers:
The tilt of the Earth
The tilt of the Earth makes us closer to the Sun
We are closer to the Sun
Really, that should be at least 85% of the answers. To really answer the question, think of the following key points:
When it is warm in the Northern Hemisphere, it is cold in the Southern Hemisphere
The Earth's orbit around the Sun is very close to being circular
The seasons depend on the time of…
bnsullivan | August 3, 2009We see this happen all the time here in Hawaii: Tourists go snorkeling -- sometimes for the first time in their lives -- and they are excited by what they see. They decide they want to take pictures of all the pretty fishies and corals to show their friends back home. They buy a single use waterproof camera, they snap away, and they are sorely disappointed when they see the result. Most of the photos are blurry, and though they thought they were shooting in color, all of the images are monochrome -- blue monochrome.
For quite a few reasons, taking photos underwater is very different from…
rallain | July 21, 2009This comes up everytime I teach physics for elementary education majors. The curriculum I use (Physics for Everyday Thinking - which is awesome) says that the colors in white light are ROYGBV (Red-Orange-Yellow-Green-Blue-Violet). Typically, I will get a student that says "Hey! What about indigo? Shouldn't it be ROYGBIV?" My first reaction to this was "huh?" Really, does it matter? Here is the spectrum you would see looking at a white light source.
You could break this into as many or as few colors as you like. So, it doesn't really matter. But this leads to a great question: Who…
rallain | July 8, 2009What do you see when you are in a completely dark room with no lights?
That is a great question to ask. It can bring out some interesting ideas. Anyway, here is an easy demo to show the color black. The basic idea is to build a box that has a small opening. Here is what it looks like from the outside:
As you can see, just a basic box. I have a door on the top, and I put a paper towel tube for a window. To make it look pretty, I covered it with black paper (so you couldn't tell where I stole the box from).
To demo this to students, I first go around and let everyone look inside. I ask…