The Conclusion to the Great Nightlight Mystery

Three weeks ago, we embarked on the Great Nightlight Mystery: Why does my daughter’s nightlight turn off in sunlight, but not when I turn on the overhead light?

First, we looked into how a spectroscope works so that we could investigate the different spectra of the lights in my house. We discovered that incandescent light bulbs contained all of the colors of the rainbow, much like the sunlight that comes through my windows. This is because sunlight and incandescent bulbs are both examples of blackbody radiation. The fluorescent bulbs, on the other hand, were only made up of a few colors of the rainbow, through when combined, the light still looks white. Finally, we looked into how the LED nightlight itself works and what colors of light is puts out.

So where does all this leave us? Let’s look at a summary of the spectra, or color outputs, of the different lights in my house:

The nightlight must have a photodetector – this is usually a semiconductor material that detects light. Just like the materials that emit light of only certain colors, this photodetector can be designed to only detect light of certain colors. My theory (I don’t know for sure) is that the detector was picked so that it would not detect light from the LED nightlight. It would then only be affected by outside light.

We checked this – our external bright LED light did not seem to affect the nightlight (turn it off) unless the nightlight was very close to the LED light bulb. The same is true for the fluorescent bulbs. However, a small amount of incandescent light or sunlight will turn the nightlight off.

Looking at the spectra above, it’s difficult to see why this is. I thought originally that the dip in light from the LED – the 500 nm blue region must be the light that the detector is sensitive to. However, the fluorescent bulb has light there. Perhaps the detector is sensitive to light in the bluish-green – that narrow area where neither the LED or fluorescent bulb emit light? I would need more information on the detector or some more sophisticated equipment than I have in my house to be sure.

The detector does seem to be wavelength dependent, though. It seems to be a poor design to me that it does not turn off in the presence of fluorescent light bulbs since many homes are switching over to fluorescent bulbs as a more efficient alternative to incandescent bulbs. In fact, there are new standards in the US designed to help phase out the inefficient high wattage incandescent bulbs. You can read more here. Since many of the more efficient light bulbs are compact fluorescents or LED light bulbs, my daughter’s nightlight will not be very useful in most houses – it will always remain on. Of course, if I open the shades, the sunlight will turn it off. I suppose the most efficient lighting will always be sunlight anyway, especially in very sunny Colorado.

What does this mean for you and your house lights? You will need to make choices between brightness, color, efficiency and cost. Sunlight and incandescent bulbs are probably easiest on your eyes, but new, better compact fluorescent bulbs and LEDs have better spectra and are much more efficient. Here are a couple of things to keep in mind:

  • BRIGHTNESS: We are used to thinking of brightness in Watts, but really that has nothing to do with brightness. The Watts are just a measure of how much power is required to run a traditional incandescent bulb. The unit of measurement ‘lumens’ is a more appropriate measurement of brightness. The Department of Energy provides the following graph to help you convert between the two:

  • COLOR: The new lighting label on the left above also includes a section entitled “Light Appearance.” This compares the light output to the light output of a blackbody of a certain temperature. If you remember from our discussion of blackbody radiation, a higher temperature means light that is more blue and less red. From the PhET simulation website on blackbody radiation, we see that the temperature of a typical incandescent is about 3000 K, and sunlight is approximately 5700 K. This can help you find a bulb that is more red (warm according to the label) or more blue (cool on the label) depending on what you prefer.

Of course, as we can see with our little handheld spectrometer, none of the lights are quite as good as just opening the shades on a sunny day – that has the best color and uses the least energy, but it is not always an option (especially at night!).

  • EFFICIENCY & COST: The new lighting label also includes an estimated yearly energy cost and lifetime. With that and the cost of the light bulb, you can figure out which light bulbs are most cost efficient. Of course, if saving energy is more important to you than saving money, you may just go with the most efficient option.

Hopefully you enjoyed our light filled April and now feel confident in your knowledge of light bulbs. I at least better understand my daughter’s nightlight and realize that if I want it to turn it off with the room lights on, I had better get an incandescent light bulb, open the shades or possibly explore different compact fluorescent and LED light bulbs until I find ones with the appropriate color spectrum.

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The Great Nightlight Mystery, Part IV: Light Emitting Diodes (LEDs)

Two weeks ago, we embarked on the Great Nightlight Mystery: Why does my daughter’s nightlight turn off in sunlight, but not when I turn on the overhead light?

First, we looked into how a spectroscope works so that we could investigate the different spectra of the lights in my house. We discovered that incandescent light bulbs contained all of the colors of the rainbow, much like the sunlight that comes through my windows. This is because sunlight and incandescent bulbs are both examples of blackbody radiation. The fluorescent bulbs were only made up of a few colors of the rainbow, though when combined, the light still looked white.

This week, we are going to look into how Light Emitting Diodes (LEDs) are different (and similar) to the other types of bulbs in my house. With this information, we should be able to not only solve the Great Nightlight Mystery, but also understand why our eyes and our wallets prefer certain types of light bulbs.

If you recall from last week, fluorescent bulbs contain a gas of molecules that glow at very specific colors due to the molecule’s changing energy levels after a high voltage is applied. LEDs work on a very similar concept. They are solids, though, so the molecules work together instead of independently like they do in a gas. This causes some differences in the output light.

The atoms in semiconductors (the materials out of which LEDs are made) are in very close proximity and work together to form bands of energy. LEDs are formed by putting two semiconductors in close proximity, one that has an excess of electrons (negatively charged particles) in a higher energy band, and one that has an excess of positive charge, or holes where the electrons can fall into, in a lower energy band. Putting these materials together and applying a current causes the electrons in the higher energy band to fall into the lower energy band. Just like in the fluorescent bulbs, the excess energy is emitted in the form of light. The color, or frequency, f, of this light is directly proportional to the energy difference between the higher and lower energy bands:

The higher the energy difference, the higher the frequency, or the more blue the light that is emitted looks. The energy bands are wider in solids than in atoms, so the light emitted has a larger bandwidth – this means that a wider range of colors is emitted, but they are still very narrow band compared to sunlight.

You have probably seen quite a few LEDs as lights on electronics. They are most commonly green, blue, or red. But the LED nightlight in my house appears to emit white light. If an LED can only emit a single band of color, how do they make white LEDs?

I read an article recently in Optics and Photonics News that discusses the various ways of making good white LEDs and talks about whether or not LED lights are ready to take over the light bulb industry (Jeff Hecht, “Changing the Lights: Are LEDs Ready to Become the Market Standard?”, Optics and Photonics News, 23 (March 2012).).

From this article, I learned that one way to make a white LED is to combine three colored LED’s – red, green and blue – so that the result looks white. It is generally less expensive and more efficient, though, to use a single blue LED and a phosphor. The LED emits light at 460 nm (blue light). This light then hits a phosphor. The energy from the light excites the phosphor to a higher energy level (yes, just like the atoms in fluorescent bulbs and the materials in the LEDs) and then atoms then fall to a lower energy level and emit light. The phosphors used in white LEDs generally emit light over a fairly broad range, from about 500 to 700 nm. The white LEDs use the very energy efficient LEDs in the blue to provide the energy (and blue light) to then get the longer wavelength light from the phosphor. The resulting spectrum looks something like this (from Wikimedia Commons):


This graph shows a big peak of intensity at 460 nm due to the blue LED (made of gallium nitride (GaN) or indium gallium nitride (InGaN) in this case – those are the semiconductor materials that make up the LED). There is also a smaller, wider peak of light covering the other visible wavelengths from the cerium doped yttrium aluminum garnet phosphor (Ce:YAG).

There are a couple of interesting qualities of LEDs that we can figure out from this graph. First, they emit a lot of blue light, so they look pretty blue. There is also a dip of little light in the blue-green (around 500 nm) so they do not emit much light there.

Using different phosphors, or multiple phosphors can change the output light to make it look more like incandescent bulbs, which emit more red light and less blue light. Bulbs with better color outputs can be less efficient, though (information from Wikipedia).

I used my spectrometer to look at my LED nightlight and another LED light bulb that we bought because we were curious. Here is the picture of the spectrum (note the blue is on the right in this picture unlike the graph above):

We can see the bright blue on the right from the LED, then the dip in light between the blue and green, then the broad green-yellow-orange-red light from the fluorescing phosphor. Just like we expected. Cool!

LEDs are very energy efficient – more so than both incandescent bulbs and fluorescent bulbs, but they are also still fairly expensive to buy in the store, so it will cost quite a bit to replace all the bulbs in the house.

Now that we understand all the light sources in the house, we can figure out why my daughter’s nightlight turns off in sunlight but not in room lights. Coming up next…

The Great Nightlight Mystery, Part III: Fluorescent Bulbs

Two weeks ago, we started on the Great Nightlight Mystery: Why does my daughter’s nightlight turn off in sunlight, but not when I turn on the overhead light?

The first week, we looked into how a spectroscope works – how I measure the colors that makes up the various lights around my house. Then, last week, we looked into why sunlight is made up of all the colors of the rainbow and how incandescent light bulbs are similar (and different) from sunlight.

But we still have to tackle the mystery of my kitchen light, which showed a very different spectrum:

It contains only a few discrete colors of the rainbow, not all of them. Why?

My kitchen light is a fluorescent light bulb. Instead of having a thin metal filament that is heated until it glows, fluorescent bulbs consist of tubes that are filled with gas of a very particular type of molecule. A large voltage is applied across the gas tube and this causes the tube to glow. Why does it glow and why only at certain wavelengths, or colors?

Let’s take a little side trip into what atoms are made of and how they work. We’ll pretend there is Hydrogen in the tube since it is the simplest atom to think about. Physicists always like to simplify a problem as much as possible and then extend the results to other situations. It’s really because we’re lazy, but Shhhh! Don’t tell anyone. Fluorescent bulbs are not filled with Hydrogen, but different types of atoms and molecules act much the same way, at least in this situation.

Atoms are generally made up of a nucleus, which contains protons and neutrons. Just one proton for Hydrogen. Around this nucleus are electrons that orbit the nucleus. It’s more complicated than that, but this is a good approximation. Again, Hydrogen only has one electron. You can see why I chose this one? Lazy, lazy, lazy…

So here’s the really strange part: Electrons can only orbit the nucleus at certain distances with certain energies. This is really weird. We can drive our cars at any speed, right? It would be weird if we could only drive at 5 mph, 10 mph, 15 mph, etc., right? Or if I could only heat my coffee up to 150°F or 250°F and nothing in between? (This would be especially tragic since the National Coffee Association states that coffee should be brewed at about 200°F.)

When we start looking at things on a smaller and smaller scale, like electrons and atoms, there are rules that apply to the energies that particles can have. Here’s a rough picture of that means for an atom:

The electron can orbit the atom on any of the green lines, but cannot ever be anywhere. This may bother some of you – how can it get from one orbit to another without ever being between them? Quantum mechanics can sometimes make you dizzy. It is best not to think too much about it.

So there are a lot of these little atoms floating around in a glass tube in my fluorescent light bulb. I apply a high voltage across the gas. The electrons are attracted to the positive side of the voltage and this gives them energy to move. They tend to move away from their nucleus into a higher orbit. But electrons, like most of us, don’t like to spin around at high speeds and prefer to relax after a while, and so they drop back down to a lower orbit closer to their nucleus. Now they have less energy, and that extra energy that they gave up had to go somewhere. It can go into a photon – a bundle of light that leaves the atom.

The energy in a photon is directly related to its frequency, or color. The higher the frequency, the higher the energy. You may remember that blue light has a higher frequency than red light, so blue photons have more energy than red photons.

When an electron moves from one orbit to another, it changes energy by a very specific amount and so it gives off light of a very specific color. Since electrons can only exist and move between specific orbits, when they move around, they only give off very specific wavelengths of light.

So how does this light bulb work? It applies a big voltage to give electrons energy to move them up to orbits further from their nuclei. They get tired (yes, I am anthropomorphizing here and giving electrons human characteristics they likely do not have) and relax to a lower orbit, giving off a photon of a specific color. If I have lots and lots of the same type of atom, then the atoms will glow only at certain specific colors. What colors these are depends on what type of atoms are in the tube. Different atoms glow at different wavelengths, or colors. Cool, huh?

The fluorescent bulbs that you have in your house contain mostly Mercury, but will also contain some other gases as well. Including more types of gases increases the number of colors represented in the light that comes from your light bulb since different types of molecules emit different colors. This helps the light appear more like white light. Unfortunately, it also decreases the efficiency of the light bulbs.

Incandescent light bulbs have all the colors of the rainbow and therefore look more like white light. This is easier on our eyes since our eyes developed to be accustomed to sunlight. However, they are terribly inefficient. Fluorescent bulbs, or compact fluorescents as we call the smaller bulbs used in our homes, are less like sunlight and therefore harder on our eyes, but require much less energy to produce the same amount of brightness as the incandescents.

Below are pictures of the two types of light bulbs along with their spectra. A compact fluorescent is shown at top left with its spectra at top right. An incandescent bulb is at bottom left with its spectra at bottom right.

Next week we will look at another increasingly common light source in homes: Light Emitting Diodes, or LEDs. Once we have a good understanding of all the lights in the house, we can hopefully solve the mystery of my daughter’s nightlight.

Side Note: For those of you who like to play with simulations, the PhET website has a great one on the models of the atoms. Click here, then run the simulation, select Experiment from the top left and choose Bohr from the types of models. It will show you what happens when you shine light on an atom to give it energy and then watch as it emits photons. If you click Show Spectrometer on the bottom right, you will see what colors Hydrogen emits.

The Great Nightlight Mystery, Part II: White Light

Last week, we started on the Great Nightlight Mystery: Why does my daughter’s nightlight turn off in sunlight, but not when I turn on the overhead light? I decided to investigate the spectrum of the different lights around the house – what colors each light is made up of. Last week, we learned about how a spectroscope works and saw how the sunlight and my kitchen light were different. Before we talk about the bedroom light and how this spectrum might affect my daughter’s nightlight, I want to explore why different lights are made up of different colors. What causes this?

Sunlight is an example of blackbody radiation. When we heat things up, they glow, or emit radiation. As we heat something up very hot, it begins to glow red, then orange, and eventually appears white. Before it begins to glow, when the object is not yet very hot, it still emits radiation in the form of heat. This radiation is in the infrared – at longer wavelengths than visible light. If you would like a reminder of the wavelengths of different colors of light, click here.

The sun is very hot and therefore emits light in the visible. Humans are much less hot than the sun (thank goodness!) and so we emit light in the infrared. Have you ever been to a science museum with an infrared camera? When you stand in front of the camera, your image appears to glow because this special camera can ‘see’ the infrared light that your body emits. If you have not been to a science museum lately, go! They are a lot of fun for adults and children alike.

So let’s look a little more closely at how this radiation works. There is an excellent website at the University of Colorado that has a number of computer simulations to help you learn about math and science: PhET. They have a simulation on Blackbody Radiation that will help us better learn how this works.

Before I go any further, check out the website and play around with the simulation. It shows a graph (the red line) of intensity of light emitted (on the vertical scale) versus wavelength (on the horizontal scale) for different temperatures. You can change the temperature on the right side.  Try setting it to the temperature of the sun. Below is a screenshot of what that would look like:

The graph shows the intensity of the light that the sun emits vs. wavelength. The rainbow is shown for reference. The peak of the graph is in the green. That makes sense since the peak of the sunlight we see is in the green. We can also see that the sun emits light across the entire visible range – all the colors – and also in the ultraviolet (those rays that we worry about in the summer) and a lot of light in the infrared as well. We can only see the light that is visible but there is a lot of other light coming from the sun as well. Not all of that makes it through our atmosphere to our eye, but that’s a topic for another day.

The temperature is listed in Kelvin, which many of you may not be familiar with. Here is a conversion to Fahrenheit and Celsius:

°C = K – 273°

°F = (K – 273°) * 9/5 + 32°

Okay, so do we understand how the graph works? Try changing the temperature scale. What happens to the graph? You can zoom in using the ‘+’ and ‘-‘ buttons on both scales. Play around a bit and think about some of these questions as you play:

  • What happens to the graph as you decrease the temperature?
  • What happens to the peak wavelength? That’s the wavelength where the object emits the most light.
  • What happens to the height of the curve? This indicates how much energy is being emitted.
  • What temperature is a light bulb? Based on this curve, do light bulbs seem efficient (think about what their main purpose is)?
  • Normal human body temperature is about 310 K. What wavelengths of light does your body emit? Why don’t you appear to be glowing when you look in the mirror (or maybe you do)?

For those of you who like equations as well as graphs…

The peak wavelength, lpeak, emitted by a glowing body of temperature, T, is given by

As you increase the object’s temperature, the wavelength at which it emits the most light decreases. That means as things get hotter, they emit colors that are more blue. And as they get cooler, they emit more red.

Another interesting characteristic of this type of radiation is how much power, P, is emitted.

The power emitted increases like the fourth power of the temperature. From the simulation, it looks like the temperature of a light bulb is about 10x the temperature of a human body. That means that the light bulb emits 10,000x more power than our bodies. As you heat an object up, the power of the light emitted increases very rapidly!

Okay, this is all really cool, of course, but what does this have to do with my daughter’s nightlight? One interesting thing about all the graphs from this simulation is that they are continuous. There are no sharp peaks. There may be more of one wavelength of light represented in each curve, but all the visible wavelengths are emitted by the sun, and even by the light bulb. Here is the screen shot of light emitted from a light bulb:

I changed the vertical scale so that it is easier to see that the peak wavelength is in the infrared – at a longer wavelength than the visible light that we can see. Just for reference here is a screenshot with both the sun and a light bulb represented:

The red line represents the light emitted by the sun and the tiny yellow line represents the light emitted by a light bulb. Not surprisingly, the sun emits a lot more light than a light bulb!

But none of these pictures explain the spectrum I showed you last week of my kitchen light:

This light bulb emits only certain colors of light – not a continuous spectrum of light.

Not all light bulbs are the same. Incandescent light bulbs act like the light bulb in the blackbody radiation simulation. They are very simple – they work by heating up a filament until it glows. They emit a continuous spectrum of light that is more like sunlight. The light in my bathroom is an incandescent bulb, so I can show you a picture of what that spectrum looks like through my spectroscope. Below are the two spectra for the incandescent bulb and for sunlight.

The two spectra look very similar but you may be able to see that the incandescent bulb has a little more light in the red while the sunlight has a little more light in the blue/purple.

Many of us also have fluorescent light bulbs in our homes that emit light like my kitchen light – a set of discrete colors. While incandescent bulbs are more like sunlight and therefore tend to be easier on our eyes, they are extremely inefficient. Most of the light they emit is in the infrared where we can not see it. That is not very useful for lighting a room!

How do my fluorescent light bulbs work? They must be more complicated than just a hot piece of metal or they would look more like the sunlight. Next week, we will discuss these interesting little light bulbs and hopefully get closer to understanding why my daughter’s room light does not affect her nightlight in the same way the sunlight from her window does.

References: All of the screen shots were taken from the PhET website on Blackbody Radiation. My husband took the pictures of the spectra in the spectroscope.

Also, I have taught this topic enough that I know all the equations and temperature conversions off the top of my head, but I am sure I learned them from this book:

Thornton & Rex, Modern Physics 3rd Edition, Belmont, CA: Brooks/Cole (2006).

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.