Category: sunlight

Why are any materials transparent? — MZ, Peligna, Italy

Because light is an electromagnetic wave, it is emitted and absorbed by electric charges. For an electric charge to emit light it must move—in fact, the charge must accelerate. For an electric charge to absorb light it must also move—it must also accelerate. However, there are many materials that do not have mobile electric charges. For example, while all electric insulators have electric charges in them, those electric charges can’t move long distances. The electric charges in many electric insulators can’t even move enough to absorb light and the light simply passes right through them. They are transparent.

Could you please give me a precise explanation of light scattering in relation to blue moons and red sunsets. Do dust particles, or whatever, facilitate the transmission of some wavelengths and not others? — DW

While the expression “blue moon” usually refers to the infrequent occurrence of second full moon in a calendar month, there have been rare occasions when the moon truly appeared blue. In those cases, an unusual fire or volcanic eruption filled the air with tiny clear particles that had just the right sizes to resonantly scatter away the red portion of the visible light spectrum so that only bluish light from the moon was able to pass directly to the viewer’s eyes. The moon thus appeared blue.

Red sunsets are much more common and they are caused by Rayleigh scattering—the non-resonant scattering of light by particles that are much smaller than the light’s wavelength. While Rayleigh scattering is rather weak, it’s weaker for long wavelength light (red light) than it is for short wavelength light (violet light). As a result, blue and violet lights are scattered more than red light; making the sky appear blue and the sun and moon appear red, particularly when they are low on the horizon and most of their blue light is scattered away before it reaches your eyes. When there is extra dust in the air, such as after a volcanic eruption, Rayleigh scattering is enhanced and the red sunsets are particularly intense.

How does a foghorn turn on and off? — M, Brant Rock, MA

Although I am not certain, I would guess that most automatic foghorns detect the fog optically. They either send light from a source to a detector and turn on the foghorn when the detector fails to see the light or they send light into their surroundings and turn the foghorn on when they see excessive reflection of that light.

How is sunlight both harmful and beneficial? – CP

Sunlight provides virtually all the energy in our world. Without it, plants wouldn’t grow and we wouldn’t have food or daylight. We wouldn’t even have fossil fuels such as coal and petroleum because those were formed from vegetation that itself derived energy from the sun. However, sunlight also contains ultraviolet light, which can damage chemicals in biological tissue. Long exposure to ultraviolet light can age your skin or cause cancer.

How can I make 1000 nanometer light waves visible to the human eye? — DMB, Broken Aarow, OK

Although our eyes are insensitive to 1000 nanometer infrared light, there are two ways to detect it effectively. The easiest is to use an inexpensive black-and-white surveillance video camera. Many of these cameras are sensitive to a broader spectrum of light than are our eyes and they can see 1000 nanometer light. If you check around, you should be able to find one that sees the light you’re interested in. The other technique is to use a phosphorescent or “glow in the dark” material. When exposed to visible light, the atoms in such a material become trapped in electronic states that can emit visible light only after a very long random wait. But exposing a phosphorescent material to infrared light can shift the states of the atoms in the material to new states that can emit light immediately. Thus exposing some phosphorescent materials to infrared light causes them to emit light promptly. You can then see these materials glow particularly brightly after storing visible light energy in them and then exposing them to infrared light. However, they’ll only glow briefly before you have to “recharge” them by exposing them to more visible light.

How does ultrasound detect cracks or imperfections in metal? Is this to do with density or is it just reflecting off surfaces? — PA, Essex, UK

Like all waves, ultrasound reflects whenever it passes from one material to another and experiences a change in speed (or more accurately, a change in impedance). Any inhomogeneity in a metal is likely to change the speed of sound in that metal and will cause some amount of sound reflection. With the proper instruments emitting sound and detecting the reflected sound, it’s possible to image the imperfections. The same technique is used in medical ultrasound to image organs or fetuses, and even to image the insides of the earth.

Why do colors fade in the sun? – RD

While light travels as electromagnetic waves, it’s emitted and absorbed as particles called “photons.” Each photon carries with it a tiny bit of energy. The amount of energy in a photon depends on the wavelength of the light associated with it. While a photon of red light contains too little energy to cause chemical processes to occur in most molecules, a particle of violet or ultraviolet light contains enough energy to cause significant chemical damage to a typical molecule. Since sunlight contains a substantial amount of violet and ultraviolet lights, it can cause a fair amount of chemistry to occur in the molecules that absorb it. That’s why colors often fade in sunlight. Many colored molecules are relatively fragile and are damaged by photons of ultraviolet light. The portion of a dye molecule that gives it its color is called a “chromophore” and is usually the most fragile part of the molecule. Destroying its chromophore will often leave a dye molecule colorless. Exposure to sunlight was the traditional way to bleach fabrics and make them white.

How do the 2″ diagonal color LCD screens used in some of the new digital video cameras work? — M, Waynesboro, MS

Like most liquid crystal displays (LCD), these devices use liquid crystals to alter the polarization of light and determine how much of that light will emerge from each point on the display. Liquid crystals are large molecules that orient themselves spontaneously within a liquid—much the way toothpicks tend to orient themselves parallel to one another when you pour them into box. The liquid crystals used in an LCD display are sensitive to electric fields so that their orientations and their optical properties can be affected electronically. The liquid crystals in the display occupy a thin layer between transparent electrodes and two polarizing plastic sheets. Light from a fluorescent lamp passes through a polarizing sheet, an electrode, the liquid crystal layer, another electrode, and another polarizing sheet. The orientation of the liquid crystal determines whether light from the first polarizing sheet will be able to pass through the second polarizing sheet. When electric charges are placed on the two electrodes, the liquid crystal’s orientation changes and so does light’s ability to pass through the pair of polarizing sheets.

To create a full color image, the display has many rows of electrodes on each side of the liquid crystals and a pattern of colored filters added to the sandwich. In “active” displays, there are also thin-film transistors that aid in the placement of charges on the electrodes. Overall, the display is able to select the electric charges on each side of every spot or “pixel” on the screen and can thus control the brightness of every pixel.

What would things look like if I could see wavelengths of the spectrum other than just visible light (e.g., X-rays, radio waves, ultraviolet, infrared, gamma rays, etc.)? — SH, Hurricane, UT

As you looked around, you would see a general glow of radio waves, microwaves, and infrared light coming from every surface. That’s because objects near room temperature emit thermal energy as these long-wavelength forms of light. While we don’t normally see such thermal radiation unless an object is hot enough for some of it to be in the visible range, your new vision would allow you to see everything glow. The warmer an object is, the brighter its emission and the shorter the wavelengths of that emission. People would glow particularly brightly because of their warm skin.

You would also see special sources of radio waves, microwaves, and infrared light. Radio antennas, cellular telephones, and microwave communication dishes would be dazzlingly bright and infrared remote controls would light up when you pressed their buttons.

You would see ultraviolet light in sunlight and from the black lights in dance halls. But there wouldn’t be much other ultraviolet light around to see, particularly indoors. X-rays and gamma rays would be rare and you might only see them if you walked into a hospital or a dentist’s office. Gamma rays would be even rarer, visible mostly in hospitals.

On really cold winter days at temperatures well below zero, I’ve noticed that sunlight is brighter and whiter than on days that are a little below freezing. Why does this happen? — CP, Madison, WI

The colder the air is, the less humidity it can hold. That’s because at low temperature, water molecules in the air are much more likely to land on a surface and stick than they are to break free from a surface and enter the air. Thus cold air is relatively free of water molecules. Water molecules in the air tend to bind together briefly and form tiny particles that scatter light. The sky is blue because of such scattering from tiny particles. With less water in the air, there is less scattering of sunlight. As a result, the sky is a darker blue, almost black, and the sunlight that reaches you directly from the sun retains a larger fraction of its blue light. The sun appears less red and more blue-white than on a warmer, more humid day.