Umakant Jaiswar

Introduction to Diffuse Reflection Every time I go to the beach, I am fascinated by all of the colored pebbles I find. Tiny stones of pink, yellow, blue, and violet flash under the shallow waves. I can't resist the urge to collect them, and I usually go home with a pocketful of my favorite pebbles. But once I get them washed and dried, I am often disappointed with how dull they all appear. Where did their vibrant colors go? What happened to their glossy sheens? I should have known from the beginning that the dry pebbles would look different because of the effects of diffuse reflection. Specular vs. Diffuse Reflection We've already learned that the reflection of waves involves a change in the direction of waves when they strike a surface. When talking about reflection, we think of waves as straight-line rays. The incident ray is the ray that initially strikes a surface. The reflected ray, obviously, is the one that reflects off a surface. The law of reflection tells us that the angle of reflection is equal to the angle of incidence. In other words, the angle at which the incident ray strikes the surface is going to be the same as the angle at which it reflects. Rays and angles are convenient ways to talk about single waves. But sources of sound and light typically send out many waves at once. How do we talk about the law of reflection when more than one wave is involved? All we have to do is imagine a whole bunch of rays parallel to one another. This picture here could represent a concentrated beam of light waves, say, from a flashlight. Since each ray is hitting the surface at the same angle - the angle of incidence - then each ray reflects at the same angle - the angle of reflection. A person standing in the path of the reflected rays would see a perfect image of the flashlight's beam. It wouldn't be much different from staring into the flashlight itself! This type of reflection, in which a smooth surface causes reflected rays to travel in the same direction, is called specular reflection.

A shooting star is another name for a meteoroid that burns up as it passes through the Earth’s atmosphere. So, a shooting star isn’t a star at all. Most of the shooting stars that we can see are known as meteoroids. These are objects as small as a piece of sand, and as large as a boulder. Smaller than a piece of sand, and astronomers call them interplanetary dust. If they’re larger than a boulder, astronomers call them asteroids. A meteoroid becomes a meteor when it strikes the atmosphere and leaves a bright tail behind it. The bright line that we see in the sky is caused by the ram pressure of the meteoroid. It’s not actually caused by friction, as most people think. When a meteoroid is larger, the streak in the sky is called a fireball or bolide. These can be bright, and leave a streak in the sky that can last for more than a minute. Some are so large they even make crackling noises as they pass through the atmosphere. If any portion of the meteoroid actually survives its passage through the atmosphere, astronomers call them meteorites. Some of the brightest and most popular meteor showers are the Leonids, the Geminids, and the Perseids. With some of these showers, you can see more than one meteor (or shooting star) each minute.

Pure substances are defined as substances that are made of only one type of atom or only one type of molecule (a group of atoms bonded together). The measure of whether a substance is pure is known as purity. For example, pure iron would only contain iron atoms, and, as in the sugar cube mentioned above, pure sugar would only contain molecules of the substance called sucrose. Elements An element is a pure substance which cannot be broken down by further chemical techniques. These include heating, cooling, electrolysis and reacting with other chemicals. (By the way, it is correct that an atom can be destroyed, but NOT by chemical means. You must use a more powerful reaction, called a nuclear reaction, to destroy or change atoms. That is a topic for a lesson in a different unit.) Compounds A compound is a pure substance composed of two or more different atoms chemically bonded to one another. A compound can be destroyed by chemical means. It might be broken down into simpler compounds, into its elements or a combination of the two. The key distinction is that compounds break down whereas the SAME techniques do not cause an element to break down.