Tag: sunlight

When you hold a flashlight to your hand, some of the light comes through. What light frequencies shine through people? Is it possible to see inside people? — PC

Biological tissues themselves are relatively transparent. They’re not good conductors of electricity and electric insulators are typically transparent (quartz, diamond, sapphire, salt, sugar). But we also contain some pigment molecules that are highly absorbing of certain wavelengths of light. For example, the hemoglobin molecules in blood absorb green and blue light quite strongly, so that they appear red. When you look at a flashlight through your hand, the light appears red because of this absorption of green and blue light by hemoglobin. If you use a bright enough red light source and are willing to look very carefully, probably with sophisticated light sensing devices, you can probably see a little light coming through a person’s body. But that light will probably have bounced several times during its passage, so that you won’t be able to learn anything about what the person’s internal organs look like. To get a better view of what a person’s insides look like, you need light that penetrates more effectively and that doesn’t bounce very often. Moreover, you must employ techniques to that block this bouncing light as much as possible so that you only see light that travels straight through the person. The light that does this isn’t visible light—it’s X-rays. X-rays are very high frequency, very short wavelength “light” (or rather electromagnetic waves). Tissue doesn’t absorb these X-rays much at all and they can go through people to form images.

When you spray water from a garden hose into the air, with the sun behind you, you see a rainbow which appears to stretch right across the sky, in the same way that rainbows form by normal rain appear. In the garden hose case, the water droplets are only a few feet in front of the observer. Is the image of a normal rainbow also only a few feet away or is it formed by droplets within the total volume of the rain shower? If this latter case is true, does the rainbow in fact form a complete circle that is cut off by the horizon? — RP, Solihull, England

A rainbow isn’t an image that originates at a specific distance away from your eyes. It consists of rays of colored light that travel at particular angles away from the water droplets that produce them. You see red light coming toward you from a certain angle because at that angle, the water droplets are all sending red light toward you. In the garden hose case, the water droplets are so densely arranged that they are able to create a brilliant rainbow in only a few meters of thickness. In a typical rainstorm, sunlight must travel through hundreds or thousands of meters of raindrops to produce an intense rainbow. When you look up toward the red arc of the normal rainbow, you are seeing light directed toward your eyes by millions of water droplets, some close and others distant, that are all sending a part of the red portion of the sunlight striking them toward you and the other wavelengths of sunlight elsewhere.

You are correct that a normal rainbow is cut off abruptly by the horizon and that it would continue down below to form a full circle if the ground weren’t in the way. People in airplanes sometimes see full 360° rainbows.

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.