Tag: sunlight

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.

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.

Why do we see colors when light strikes atoms? — GN, Marine City, MI

When white light strikes a molecule, that molecule may absorb some of the light. Light interacts with molecules as particles called “photons” and whether a particular photon is absorbed depends on the structure of the molecule and the color of the photon. Each molecule has the ability to absorb only certain colors of light. For example, a particular molecule may absorb only red photons. As a result, your eye will see only green and blue light photons coming from that molecule when it’s exposed to white light and you will perceive that molecule as having a blue-green color known as cyan. In general, the colors that you see coming from molecules that are illuminated by white light are the colors of light that the molecules don’t absorb.

How do scientists measure the speed of light? — DZ, Illinois

There are many possible methods for measuring the speed of light, but the classic technique is easiest to describe. In this method, a rapidly spinning mirror is used to direct a beam of light down a long pipe toward a stationary mirror at the end of that pipe. The first mirror is spinning in such a way that the beam it reflects sweeps across the pipe and can only strike the second mirror during that brief moment when the first mirror is perfectly aligned to direct the light down the pipe. A scientist then looks into the spinning mirror to observe the flash of light that returns from the second mirror. Because it takes a small but finite amount of time for the light to travel back and forth through the pipe, the spinning mirror will have turned a little between the moment when it sent the beam of light toward the far mirror and the moment when that beam of light returns to the spinning mirror. By studying the angle at which the reflected beam leaves the spinning mirror and by knowing how quickly the mirror is spinning, the scientist can determine the speed of light.

However, something has changed since those sorts of measurements were done: the speed of light is now a defined constant. It isn’t measured any more—it’s simply defined to be 299,792,458 meters per second. The second is defined in a similar manner—as 9,192,631,770 periods of a particular microwave emission from the cesium-133 atom. Because of these two definitions, an experiment that “measures the speed of light” is now used to determine the length of the meter.

What is light? — KB, Winnipeg, MB

Light consists of electromagnetic waves. An electromagnetic wave is a self-sustaining disturbance in the electric and magnetic fields that can exist even in empty space. You have probably seen two electrically charged objects push or pull on one another, such as when a sock clings to a shirt as you pull the two from the clothes dryer. You have probably also seen two magnetically poled objects push or pull on one another, such as when a magnet pulls itself toward a refrigerator door. These electric and magnetic forces are mediated by electric and magnetic fields respectively and, while those fields certainly exist in the space between the sock and shirt or between the magnet and refrigerator, they can also exist all by themselves. In an electromagnetic wave, the electric field creates the magnetic field and the magnetic field creates the electric field so that these two fields go on creating one another indefinitely as the wave travels through space at an enormous speed—the speed of light. Electromagnetic waves are distinguished by their frequencies or wavelengths, characteristics that are familiar to anyone who has watched water waves approaching the beach. But only a certain group of electromagnetic waves are visible to our eyes—those with frequencies between about 4.0*10¹⁴ cycles per second and 7.5*10¹⁴ cycles per second (wavelengths between about 750 nanometers and 400 nanometers). Outside of this range are infrared light at the low frequency end and ultraviolet light at the high frequency end.

What is the scientific explanation of a rainbow? — RS, Salinas, CA

A rainbow is caused by three important optical effects: reflection, refraction, and dispersion, all working together. The rainbow forms when sunlight passes over your head and illuminates falling raindrops in the sky in front of you. This sunlight enters each spherical raindrop, partially reflects from the back surfaces of the raindrop, and then leaves the raindrop and heads toward you. The raindrop helps some of the sunlight make a near U-turn. But the path that the light follows after it enters the raindrop depends on its color. Light bends or “refracts” as it changes speed upon entering water from air and the amount it bends depends on how much its speed changes. Since violet light slows more than red light, a phenomenon called “dispersion,” the violet light bends more than the red light and the two colors begin to follow different paths through the drop. All the other colors are spread out between these two extremes.

The colored rays of light then partially reflect from the back surface of the raindrop because any change in light’s speed also causes partial reflection. Now the various colors are on their way back toward you and the sun. The light bends again as it emerges from the raindrop and the various colors leave it traveling in different directions. Only one color of light will be aimed properly to reach your eyes. But there are other raindrops above and below it that will also send light backward and some of that light will also reach your eyes. But this light will be a different color. What you see when you observe the rainbow is the lights that many different raindrops send back toward your eyes. The upper raindrops send their red light toward your eyes while the lower raindrops send their violet light toward your eyes. You see a series of colored bows from these different raindrops.

When light hits an object, how do we recognize the color? — CM, Levering, PA

White light is a mixture of various light waves with different wavelengths and thus different colors. When white light hits an object, some of the light waves are absorbed while others are not. The light that isn’t absorbed may pass through the object or it may be reflected in a new direction. The light that you observe coming from the object is this transmitted or reflected light. If the light that you see doesn’t include the same mixture of wavelengths that first hit the object, you won’t see this light as white. Instead, you’ll see it as colored. If the light you see contains mostly long wavelengths of light, you’ll see it as red. If the light contains mostly short wavelengths of light, you’ll see it as blue or violet. The wide range of colors that objects have comes from subtle differences in the wavelengths of light they absorb. However, when an object is illuminated with colored light, the light that it transmits or reflects may be altered. After all, it can’t transmit or reflect a light wave that never hit it in the first place. Even variations in “white” light can affect an object’s color—makeup looks different in incandescent “white” light than it does in fluorescent “white” light because those illuminations contain different mixtures of light waves.

Why is the sky blue? – Z

As it passes through the atmosphere, sunlight can be deflected by a process known as Rayleigh scattering. When sunlight passes through any material, its light waves cause electric charges in the material to jiggle back and forth. That’s because light waves contain electric fields and electric fields exert forces on electric charges. When the charges in a material jiggle back and forth, they may emit light. In this case, the material can absorb the sunlight for an instant and reemit it in a new direction. This process, whereby jiggling electric charges in a material absorb a light wave and reemit it in a new direction, is Rayleigh scattering.

Rayleigh scattering is extremely inefficient in particles that are much smaller than the wavelength of the light, so that visible light can travel through miles of molecules in the atmosphere before it experiences significant Rayleigh scattering. But blue light has a shorter wavelength than red light and thus experiences Rayleigh scattering more often than red light. As a result, the atmosphere tends to send the blue portion of sunlight off in every direction. Thus when you look at the atmosphere, it appears blue.

A reader (TAC) points out that the above explanation would seem to imply that the sky should appear violet, since violet light scatters more strongly than blue light. But the spectrum of sunlight peaks in the green—sunlight contains more green light than blue light and more blue light than violet light. The sky combines these two effects together (more green light but better scattering of violet light) and acquires an overall blue appearance.

What material is used in glass to make it polarize light? — FG, Torrance, CA

Actually, the polarizing material you are referring to is a plastic that has been impregnated with iodine atoms. The plastic, polyvinyl alcohol, is heated and stretched to align its long molecules in a particular direction. This plastic is then exposed to iodine, which binds to the long molecules and forms the equivalent of molecular wires along the direction of the aligned plastic molecules. These molecular wires absorb light that is polarized along them because the light’s electric field points along its polarization direction and pushes electric charges wastefully along the iodine wires. This light is absorbed and its energy is converted to thermal energy, leaving only light with the other polarization.

How does a prism work? — RH, Louisville, KY

When light enters a material such as glass, the light slows down. That’s because the electric charges in the material delay a light wave by interacting with the wave’s electric and magnetic fields. The higher the frequency of the light wave, the more it interacts with the charges in most materials and the more that light wave slows down. Thus high-frequency violet light slows more than low-frequency red light as the two enter a piece of glass.

Because of this slowing effect, light bends when it encounters a glass surface at an angle. The wave has a width and as it encounters the glass surface, one side of the wave reaches the glass before the other side of the wave. Since the side that arrives first also slows first, the whole wave bends so that it travels more directly into the glass. Since violet light slows more than red light, the violet light also bends more than the red light. The two colors thus follow different paths through the glass.

The same bending occurs in reverse when the light leaves the glass. Light speeds up as it leaves glass and again the violet light bends more than the red light. In a prism (or any carefully cut glass, crystal, or plastic), the colors of light bend differently at each surface and follow slightly different paths both in and out of the prism. The light rays then appear separately when they strike a surface outside the prism or when you look at those light rays with your eyes.