Category: sunlight

At what angles do light rays reflect out of a prism? — BC, Farmersville, TX

It depends on the shape of the prism and the angle at which the light arrived at the prism. Whenever light’s speed changes as it passes through a surface at an angle, the light bends. Since light travels faster in air than in glass (or plastic), it bends when it goes from air to glass or from glass to air. When light enters glass, it slows down and it bends toward the normal to the surface (toward the line that’s at right angles to the surface). When light leaves glass, it speeds up and it bends away from the normal to the surface. To know exactly how far the light bends, you need to know how much the glass slows light (the glass’s refractive index) and the angle at which the light encountered the glass surface (the angle of incidence). You can then use one of the basic laws of optics, Snell’s law, to determine the angle at which the light continues through the glass. You can then do the same for the light’s emergence from the glass and determine the angle at which it leaves.

What does the SPF on sun screens mean? – RC

Sunscreens contain pigments that absorb invisible ultraviolet radiation. While they appear clear and transmit visible light so that you can’t see them when they’re on your skin, sunscreens are almost opaque to ultraviolet light. A sunscreen’s SPF is related to the fraction of ultraviolet light that it absorbs. An SPF of 15 means that a normal layer of this sunscreen on your skin transmits only 1 part in 15 of the ultraviolet light that reaches it from the sun. An SPF of 40 means that a layer of this sunscreen transmits only 1 part in 40 of the ultraviolet light. The true transmission of the sunscreen depends somewhat on how you apply it and how much you apply, so these SPF ratings are only approximate. A sunscreen contains a mixture of dye molecules that transmit visible light but absorb ultraviolet light (and convert its the light’s energy into thermal energy). Most if not all of these dye molecules are artificial organic compounds that have been carefully selected to be non-toxic and non-irritating. The first popular sunscreen contained a compound called PABA that caused skin reactions in many people, but more recent dye choices are less likely to cause skin trouble.

Why do the earth’s oceans appear blue to an observer on the moon?

The earth’s oceans and sky both appear blue to everyone who observes them. They do this because water absorbs blue light less strongly than it absorbs other colors. When ocean water is exposed to sunlight (white light), it absorbs most of the red light quickly and a good fraction of the green light. But the blue light penetrates to considerable depth in the water and there is a reasonable chance that this light will be scattered back upward to an observer on the shore, in the air, or even on the moon.

Does light have mass? If so, then how can it travel at the speed of light? Doesn’t the mass of an object (particle) approach infinity as its velocity approaches the speed of light?

Light has precisely zero mass and that makes all the difference. You’re right that taking a massive particle up to the speed of light is impossible because doing so would, in a certain sense, give the particle an infinite mass. But the more important issue here is that doing so would require an infinite amount of energy and momentum.

Most physicists use the word mass to mean a particle’s mass at rest—its rest mass—and as you bring the particle to higher and higher speeds, its rest mass doesn’t change. However, the relationship between the particle’s energy and its momentum does change with speed and the particle’s momentum begins to increase more rapidly than it should according to the older, pre-relativistic mechanical theories. In an effort to explain this anomalous increase in momentum while retaining the old Newtonian laws of motion, people sometimes assign a fictitious “mass” to the particle; one that equals the rest mass when the particle is stationary but that increases as the particle’s speed increases. As a particle approaches the speed of light, its momentum increases without limit and so does its “mass.” Not surprisingly, the limitless rises in energy, momentum, and “mass” prevent the massive particle from ever reaching the speed of light.

As for light, it really does have zero mass and therefore can’t be described by the Newtonian laws of motion. All light has is its momentum and its energy. In fact, light can’t travel slower than the speed of light because that would require it to have a mass! So the world of particles is divided into two groups: massless particles that must travel at the speed of light and massive particles that can never travel at the speed of light.

During a total solar eclipse, does the moon make first contact with the sun on the eastern limb or the western limb? Can you explain this to me?

The moon orbits the earth from west to east. By that, I mean that if the earth were to stop turning, the moon would then rise in the west and set in the east. During a total solar eclipse, the moon is drifting directly in front of the sun. Since the moon moves from west to east, it will first block the western edge of the sun, the western limb. In contrast, during a total lunar eclipse, the moon is drifting into the earth’s shadow. Since it is moving from west to east, its eastern edge will enter the shadow first.

Is it possible to create a “fog” in a small enclosed area without using dry ice or ultrasound?

The two techniques you mention, dry ice and ultrasound, are both intended to make tiny droplets of water in the air, effectively producing an artificial cloud. While I can’t think of any better ways to make such water droplets, I can think of ways to make fogs of other materials. Tiny particles of any clear material will work because what you are seeing is the random scattering of light as it’s partially reflected from the front and back surfaces of clear particles. I’d suggest a chemical process that produces tiny clear particles. The easiest one I can think of is to place a dish of household ammonia (ammonium hydroxide—ammonia gas dissolved in water) and a dish of hydrochloric acid (hydrogen chloride gas dissolved in water, sold as muriatic acid by hardware stores) in your enclosed area. The two gases will diffuse throughout your enclosure and react to form tiny clear particles of ammonium chloride. The enclosure will fill with a dense white fog. The particles are so small, that they will remain in the air for a very long time, but they will eventually settle on surfaces and leave a white powdery residue. So, unlike a water fog, this chemical fog is a little messy. You shouldn’t breathe the fog, either.

Is refraction the idea behind eyeglasses? If so, how?

Yes, refraction is used in eye glasses. By carefully sculpting the front and back surfaces of a sheet of glass or plastic, the light passing through that sheet can be bent in remarkable ways. We will look at image formation in the section on Cameras.

Why do you think you see water on a road ahead of you when it’s not really there?

On a sunny day, heat from the pavement can create a layer of very hot air at the surface of the road. Since hot air is less dense than cold air, its index of refraction is slightly less than that of cold air, too. As light from the sky enters this layer of low-index air, that light is bent. Light from the sky far out in front of you is turned upward so that you see the sky “reflected” from the road’s surface (actually bent upward by the air above the road’s surface). You interpret this sky light as coming from a pool of water on the road. But as you approach the road and look down at it, you see that the road is dry and black.

As long as the sun is to our back, shouldn’t the rainbow stay visible; instead of disappearing when we approach it?

If the sky were uniformly filled with water droplets and uniformly illuminated with sunlight, then you would always see the rainbow, no matter where you moved. However it would always appear out in the distance. The light that reaches your eyes as the rainbow comes from a broad range of distances, but it appears to come from pretty far away. As you walked toward this perceived rainbow, you would begin to see light from other raindrops, still farther away. You could never actually “reach” the rainbow. It would just move about with you; always appearing to be in the distance.

What causes a magnifying glass within a ray of sun to burn such a small, specific spot? Is it the shape of the glass?

The magnifying glass is a lens, a carefully shaped piece of glass that can refract sunlight to create an image. When you burn wood with a magnifying glass, you are creating an image of the sun on the wood. This tiny image, a circle that looks just like the sun itself, only much smaller, is so bright and contains so much thermal radiation that it overheats the wood that it strikes and causes that wood to burn.