Curiosity

A few weeks ago, when the first news on the Higgs boson came out, a friend of mine sent me an email requesting that I write a blog entry explaining what it’s all about (that blog entry will be coming next week on Laser Mom). He said that understanding this part of physics was hard for him as an engineer since he did not have much background in modern physics. He also asked,  “Why do we care about what gives us our mass?”

His question made me think about research in general. Of course, as scientists and engineers, we all write proposals to get funding, and papers for scientific journals, and give conference talks, and spend quite a bit of time explaining why what we do is useful and will save the world and make us all rich someday. But really, why do we really do our research? I think that most us can honestly say (hopefully!) that we do it because it’s really cool. It’s fun. Why do we want to find out what gives us our mass? Because we can. We are curious and want to understand how things work and what new things we can accomplish.

One of the things that I have done in the recent past for research is to shine laser beams into glass or polymers (i.e. plastic) and see what happens. It turns out you can use the energy in the laser beam to change the properties of the materials for practical engineering reasons – creating small feature sizes so that we can have better, faster, smaller computers and to write optical waveguides so that we can send information (internet) easier and faster from place to place. But I did not really study this just for practical reasons – it’s just fun! (The practical reasons are, of course, important to get funding to get paid to do what I love.)

I have to wonder if the first scientists to write optical wires in glass with high power lasers were not thinking (at least a little bit), “This laser is so cool. I wonder what would happen if we focused it down into that piece of glass? Will the glass vaporize? Let’s try it!” (I believe Davis et al. were the first to publish on this topic.)

Perhaps they were much more organized and focused in their pursuits, but I have to admit that most of the interesting things I have accomplished in the lab have started out with, “I wonder what would happen if…”

My one year old daughter is a natural experimentalist. She loves to try out new things and do things just to see what happens. If I drop this, what will happen? What if I pull on this handle? What will happen if I pull myself off of this landing (Ow!)? It is amazing to see how quickly she learns new things just by exploring and trying everything that comes to mind.

We have all (hopefully) mastered the basics of gravity at this point, but our curiosity and desire to try out new things is what makes us good scientists. Shining a laser beam into a light sensitive polymer and watching what happens just fascinates me. There are so many interesting things going on, from basic light propagation in glass/plastic to chemical reactions in the material.

Then there are a the cool things you do when you put two laser beams together – using different colored laser beams of different shapes to activate different chemicals in a material and making super tiny little dots in the material. There were three papers in Science a few years ago (May 15 2009 issue) discussing different ways of combining optics and chemistry to create features that were smaller than anything you could do before with these lasers (By the way, you can get all three of those papers for free if you sign up on the Science website.)

Oh sure, there are lots of good practical applications of shining lasers into materials (faster internet and computers?) and those are probably the things I will tell you if you ask me to give a talk on this research and certainly what I will tell you if I need to write a grant proposal.

But why do I really work with lasers, shine them into materials, use them for microscopy? Why are the scientists at CERN looking for the Higgs Boson?

I think the Mars rover’s name covers it: Curiosity.  Anyone who watched the video of Curiosity’s landing (YouTube – watch between minutes 5 and 6) can see the excitement and joy that scientists and engineers get when they have successfully completed a mission, discovered a new particle, built a new gadget or made measurements that no one before them has ever been able to do.

As scientists and engineers, we are a curious folk. Sometimes we get so wrapped up in funding, papers, etc. that we forget why we do what we do. That’s one of the reasons I am excited to go to an optics conference this fall. Conferences are a great opportunity to explore and enjoy that curiosity. We can go to talks outside our field, talk to other scientists we have never met, and get outside our normal routine. This setting always re-energizes me and gives me lots of new ideas of things I would like to explore when I get back to the lab.

Of course, with an infant at home, I do not really need to go anywhere. Babies are natural experimentalists and seeing her intense concentration in trying something new and her joy at discovering new things reminds me every day why science is so cool. We are all born curious, and hopefully we keep that curiosity as we grow older though we each tend to focus it in different directions.

Lightning Strikes

I was on vacation with my family last week and did not quite manage to get this entry up on time and so missed last week. But I enjoyed the time spent with my family, visiting and traveling to new places.

There have been a lot of fires in CO this year. The dry winter, dry spring and very hot, dry and windy summer weather have made for perfect conditions for fires to spread rapidly in the forested areas of the mountains. Because of this, there is an extreme fire ban in CO to prevent more fires. This was the quietest 4th of July I can remember because many of the towns around where I live cancelled the fireworks celebrations. No one wanted to start another fire.

Unfortunately, mother nature has not been informed of the fire ban. Many of the fires this year were started by lightning strikes during one of our many dry thunderstorms – lots of lightning and thunder and little or no rain.

A few weeks ago, I went out for a walk with my daughter and looked up to see a huge plume of smoke in the sky. I immediately looked up the news to try to figure out where the fire was – not close enough to be a danger to us (other than the horrible smoke) but close enough to affect many of my friends. In the news article, it stated that the fire was probably started by lightning as 51 lightning strikes had been detected in the area.

51? Really? The curious geek in me forgot all about the fire and started to wonder how they could possible know that. I spent some time looking into lightning detectors. There are several different types, but this article mentioned ground detectors, so I wanted to know how those work.

Lightning is really cool, and of course, a little scary too. But I have always been mesmerized by watching lightning flashes on a stormy day. In order to understand how ground detectors measure lightning, let’s first think about why we can see lightning – what is causing that huge bright flash?

Lightning is caused by huge static discharges. In an electrical storm, there is a huge charge difference between the clouds and the ground. This sets up a very large electric field across the air. This basically means that there is a huge amount of energy stored that has the potential to affect the air around it.

If the charge stored is enough, the electric field starts to ionize the air around it. This means that the strong electric field can actually tear electrons off of the atoms in the air. These electrons then become free to move around (no longer tightly attached to the atoms) and this causes a current to flow through the air like it would through a metal wire. (How Stuff Works: Lightning).

Very cool! And sort of scary…something powerful enough to rip atoms apart certainly is going to do a lot of damage if it hits me!

As this energy moves through the air, this electrical energy and atom ionization emits electromagnetic radiation – light. So we see a bright flash of light. You can see a pretty picture of the lighting and its spectrum (what colors are contained in this white light) here.

The lightning also emits a characteristic lower frequency electromagnetic radiation. In order to detect the lightning, antennas are set up to detect this lower frequency radiation. For cloud-to-ground detection, a set of three antennas are set up to triangulate the signal. All three antennas will detect the signal from the lightning and by measuring the time it takes to reach each antenna, the location of the lightning strike can be determined. Visible light detection (the big bright flash!) is also used to make sure that the lower frequency radiation is really coming from lightning and not some other source. (Wikipedia)

I never knew that these types of detectors were set up around us. I think it’s really cool that lightning detectors work so well. You can even make your own basic lightning detector if you like home electronics projects.

Despite the high level of technology and engineering that goes into really effective lightning detectors, I must admit that I am still more in awe of the amazing photographers out there that can take beautiful pictures like this (Wikipedia Commons):

During our vacation, we landed twice very near a thunderstorm. Despite being quite exhausted from travel and delays with an infant, I was in awe of the beauty and power of the lightning viewed from the sky. It is an amazing thing.

Fortunately, the weather has improved since the worst of the fires in CO. While it is still dry here, daily thunderstorms have brought some rain to help things from getting too dry. Of course, these storms also tend to bring lightning. Hopefully the wetter conditions will prevent more fires, but it is still important to be careful in the late afternoons, especially in the mountains. Lightning is beautiful but also quite powerful and dangerous.

The Great Nightlight Mystery, Part I: Spectroscopes

Spring is here and whether or not the weather is beautiful and sunny where you are, the days are getting longer and the nights are getting shorter everywhere in the northern hemisphere. April is a month of increasing light. So this month we are going to talk about different types of light sources, including the sun, and how to tell them apart and measure them.

My motivation for investigating a variety of sources of light is my daughter’s nightlight. When we first moved into our new home, my daughter had a terrible time sleeping at night. We realized that at least one problem was that it was too dark in her new bedroom. I went out and bought a nightlight right away to help with that problem. I purchased an Energizer Automatic LED Nightlight.

The nightlight is light sensitive and only turns on when the room is dark. Interestingly, if I turned the overhead lamp on in the middle of the night to change and feed my daughter, the nightlight would remain on. However, during the day, even with the shades drawn, the nightlight would turn off. The room seemed brighter to me with the overhead light on than in the morning with just a little light coming from the window, so I was confused. Brightness did not seem to be what turned the nightlight on and off. I had also put a second nightlight in a different room and that nightlight turns off if the overhead lamp is turned on. Curious…is sunlight different from my light bulbs? Are light bulbs in different rooms different from each other?

How could they be different? Light is light, right? They look the same. Well, sort of. Sunlight is a purer form of ‘white light’ than the room lights. White light is made up of all the colors of visible light, but our eyes can be tricked into thinking something is white light if you add up several colors (but not all of them). So how do I know what I am looking at? Is it true white light or something else? There is sunlight, my LED nightlight, and even the bulbs in the house lights are different.

I decided to buy a spectroscope to investigate further. I have used these often in classes that I have taught and they are great, inexpensive tools for investigating light. Today’s blog is going to discuss how these spectroscopes work. Over the next couple of weeks, I will talk about different types of light sources and what causes them, and hopefully by the end, we will be able to solve the mystery of the nightlight.

My spectroscope (photo below) is just a black box that is not easily taken apart, so I do not know exactly what is inside, but I will take an educated guess about what might make this little black box work.

The picture above shows the full spectroscope with scale in inches (left), one end with viewing window/diffraction grating (middle), and the opposite end with slit on left and scale on right (right).

Here is a diagram of what the inside of this spectroscope looks like followed by an explanation of how it works:

First, you aim the slit in the spectroscope at the light source you would like to measure, like the sun. It is important to block all other light sources from getting into the spectroscope so that you have a pure measurement. Unfortunately, white light is generally incoherent. That means that all the light (the individual photons or waves of light) from the source are random and have nothing in common. Sending the light through a slit gives you a nice single beam of light that all comes from a single location (the slit) and is traveling in the same direction and blocks off any other light. This single, more coherent, beam of light travels through the spectroscope to the diffraction grating on the opposite end.

A diffraction grating has the same function as a prism – it splits white light into its many colors. Diffraction gratings consist of a periodic structure, such as a set of very closely positioned slits, that can reflect light at different angles according to the wavelength. The angle, Q, at which the light is reflected is described by

where λ is the wavelength of the light and d is the distance between the slits of the diffraction grating. Blue light, with a wavelength of about 400 nm is reflected at a much smaller angle than red light, with a wavelength of about 650 nm. If you look at the reflected light on a screen a distance away from the diffraction grating, you see all the colors spread out like below:

This is a picture of what you would seen when looking through my spectroscope at sun light. As you can see, there is a scale that indicates the wavelength of the light. The numbers refer to 100’s of nanometers – ‘7’ means 700 nm. For those of you who are familiar with the visible spectrum, you will probably notice that the scale is not quite right (Click here for a more accurate version of the spectrum from Wikipedia). Alas, an ~$10 spectroscope is not a perfect instrument, but we can still see that there is a continuous spectrum of colors that make up the sun light.

Using the spectroscope to look at the light in our kitchen, at night so that no sunlight snuck in to the spectroscope, we saw this:

The light was quite bright, making the scale a little difficult to read, but you will notice clear lines of color in the violet, blue, green, orange and red. Since colors from across the spectrum are represented, the combined result appears white, but when you look at the spectrum, it is clear that the light is not made up of all the colors of the rainbow, just a few distinct lines of color. The kitchen light has much more violet than the sunlight and less of the higher wavelength reds.

So my little spectroscope can show me what colors make up the different lights around my house and from a quick look, they are clearly different. Why? Why is sunlight made up of all the colors of the rainbow while the light in my kitchen is made up of only a few? Are all the lights in my house the same or do they vary by type as well?

Tune in for the next couple of weeks for answers to these questions and a solution to the mystery of my daughter’s nightlight…

Special thanks to my wonderful husband who takes most of the photographs for this blog. His support of my silliness and amazing photography skills help make this blog colorful.

CT Scans

Someone I know had a CT scan last week. These scans are very commonplace these days, but I was wondering how many people know they work or even what CT stands for. I learned about this when was in graduate school and studying a method of imaging that is very similar to CT scans, but using laser light instead of x-rays.

These scans are usually called either CT (Computed Tomography) or CAT (Computed Axial Tomography) scans. The two terms seem to be used interchangeably. The ‘computed’ part is easy to understand – a computer is used to construct the final image. ‘Tomography’ refers to the method of taking multiple pictures of an object from different points of view in order to build up a two-dimensional image from one-dimensional pictures. ‘Axial’ means that these pictures are taken by rotating a camera in a circle around a center axis.

So how does this work in practice? While the method does not specify what type of light is used, doctors usually use x-rays for these scans. What are x-rays? They are part of the electromagnetic spectrum, which most of us just refer to as ‘light.’ We know all about the many colors of visible light because we can see them – from violet to red and everything in between. But as the wavelength of light gets shorter (and the frequency gets higher), the energy of the light increases. You can think of this as a wave on a string that oscillates faster and faster. As this happens, the wavelength (distance between peaks) gets smaller and smaller, like in the simple graphic below:

X-rays have shorter wavelengths & higher frequencies than the visible light we see. Much, much shorter. Here’s a graph of the different wavelengths of light and how they compare to visible light:

Light generally can not be used to measure things that are much smaller than the wavelength of that light (there are a few interesting exceptions). So x-rays can measure things that are much smaller, or with much better resolution, than microwaves or radio waves or even visible light. Of course, x-rays have wavelengths on the order of a nanometer – that is 100,000 times smaller than the width of a human hair! So that resolution probably is not needed for typical imaging in the body.

How do these x-rays interact with your body? A beam of x-rays is sent through the body, and some parts of your body absorb more of the light, and some absorb less. What you see on the camera on the far side is a shadow showing what parts of your body absorbed the light and which did not. Visible light is not good for this because it stops at your skin – you can see a shadow of your outside shape, but no features on the inside.

The fact that x-rays get absorbed throughout our bodies is one of the reasons that they can be so dangerous and why we should not be exposed to them too often. In addition to helping to diagnose cancer with CT scans, excessive exposure can cause cancer. Those high energy waves are absorbed by our cells and that energy can be used to alter and damage the cells. Short exposures of low doses are considered safe, but long term exposure can cause serious problems. Sadly, many of the early scientists who discovered and studied x-rays died of cancers caused by this research because they did not yet know of the dangers or how to protect themselves.

Now that we know what x-rays are, let’s get back to these multiple views. Why do we need to look at the body from different angles? This is the tomography part. If you have ever had a CT scan, you know that you need to lie still while an x-ray source and detector spin around you to get different views of your body. We know now that the pictures are basically shadows of your insides, so let’s look at some shadow pictures of some household objects to see how important it is to get multiple views.

Below are two pictures of two different objects in my house. The pictures were taken by shining a bright light on the object and then photographing the shadow created on the wall.

These two objects look to be almost exactly the same, and there is not a lot of information available on what the objects even are.

I then rotated the objects 90°, and these are the new shadows on the wall:

Now we can see that the object on the left is a paring knife and the object on the right is a serrated knife. Multiples views of the object give us more information about its shape and size in different dimensions.

Below are a few more fun shadow pictures from my house. As you look at the pictures, think about the following questions:

  • Can you tell what the objects are?
  • Do you need all the views to understand what you are looking at?
  • Are some views more useful than others?
  • How does having the multiple views help you better understand what you are looking at?

This last object is partially transmissive – light travels through the wings, but not the edges. This is similar to a picture of your body where the x-rays are not completely absorbed by some parts of your body. This gives us more information about the internal structure of the object being viewed.

So CT scans use x-rays, which are short wavelength light, and operate by taking multiple shadow pictures of our bodies at many different angles to reconstruct a full image. The picture below shows how this is done.

A computer adds up all the data on all those different lines, giving us a full circle cross section picture of the body. I will skip the details of the computer programming for now and just look at the results. I do not have any pictures myself of CT scans, but here is a really cool set of cross sectional scans of a human brain from Wikipedia’s page on Computed Tomography.

It’s really amazing that we can get so much information about how our body works from what are essentially shadow pictures. I frequently have students in my introductory Physics courses who are interested in being doctors. I think I should add a day on CT scans to the unit on light to see if I can convince them that Physics really is something useful for them to learn. If anyone out there has some more medical applications you would like me to explore in the future, I am all ears – there are tons of fun ones!