Light

Introduction

Most of us are familiar with waves, whether they are waves of water in the ocean, waves made by wiggling the end of a rope, or waves made when a guitar string is plucked. Light, also called electromagnetic radiation, is a special type of a wave. In order to understand how Rutherfod's model of the atom evolved to the current atomic model, we need to understand some basic properties of light.

The Wave Form of Energy

Before we talk about the different forms of light or electromagnetic radiation (EMR), it is important to understand some of the general characteristics that waves share.

The high point of a wave is called the crest. The low point is called the trough. The distance from one crest to the next crest is called the wavelength of the wave. You could also determine the wavelength by measuring the distance from one trough to the next or, in fact, between any two identical positions on successive waves. The symbol used for wavelength is the Greek letter lambda,

. The distance from the crest (or the trough) to the undisturbed position is called the amplitude of the wave.

Another important characteristic of waves is called frequency. The frequency of a wave is the number of cycles that pass a given point per unit of time. If we choose an exact position along the path of the wave and count how many crests pass the position each second, we would get a value for frequency. Frequency has the units of cycles/sec or waves/sec, but scientists usually just use units of 1/sec or Hertz (Hz).

All types of light (EMR) travel at the same speed. Because of this, as the wavelength increases (the waves get longer), the frequency decreases (fewer waves pass). Conversely, as the wavelength decreases (the waves get shorter), the frequency increases (more waves pass).

Electromagnetic Radiation

Electromagnetic waves (light waves) have an extremely wide range of wavelengths, frequencies, and energies. The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The highest energy form of electromagnetic waves is gamma rays and the lowest energy form (that we have named) is radio waves. As the wavelength of the wave decreases (or the frequency increases), the energy of the wave increases.

In the following figure, the electromagnetic waves on the far left have the highest energy. These waves are called gamma rays and can cause significant damage to living systems. The next lowest energy form of electromagnetic waves is called x-rays. Most of you are familiar with the penetration abilities of these waves. Although they can be helpful in imaging bones, they can also be quite dangerous to humans. Humans are advised to try to limit as much as possible the number of medical x-rays they have per year. After x-rays, ultraviolet rays are the next lowest in energy. These rays are a part of sunlight, and rays on the upper end of the ultraviolet range can cause sunburn and eventually skin cancer. The next tiny section in the spectrum is the visible range of light. The band referred to as visible light has been expanded and extended below the full spectrum. These are the frequencies (energies) of the electromagnetic spectrum to which the human eye responds. Lower in the spectrum are infrared rays and radio waves.

The electromagnetic waves that are in the visible range cause the human eye to respond when they enter the eye. The eye sends signals to the brain and the individual “sees” various colors. The waves in the visible region with the highest energy are interpreted by the brain as violet, and as the energy of the waves decreases, the colors change to blue, green, yellow, orange, and red. When the energy of the wave is above or below the visible range, the eye does not respond to them. When the eye receives several different frequencies at the same time, the colors are blended by the brain. If all frequencies of visible light enter the eye together, the brain sees white, and if no visible light enters the eye, the brain sees black.

All the objects that you see around you are light absorbers – that is, the chemicals on the surface of the objects absorb certain frequencies and not others. Your eyes detect the frequencies that strike your eye. Therefore, if your friend is wearing a red shirt, it means that the dye in that shirt absorbs every frequency except red and the red is reflected. When the red frequency from the shirt arrives at your eye, your visual system sees red. If your only light source was one exact frequency of blue light and you shined it on a shirt that absorbed every frequency of light except for one exact frequency of red, then the shirt would look black to you because no light would be reflected to your eye. In any light source that emits many frequencies, the shirt would appear red but if no frequencies are reflected, it would be black. The light from many fluorescent light bulbs do not contain all the frequencies of sunlight, so clothes inside a store may appear to be a slightly different color than when you look at them under the sun.

Lesson Summary

• • Wave form energy is characterized by velocity, wavelength, and frequency.

  • As the wavelength of a wave increases, its frequency decreases. Longer waves with lower frequencies have lower energy. Shorter waves with higher frequencies have higher energy.

  • Electromagnetic radiation has a wide spectrum, including low energy radio waves and very high energy gamma rays.

  • The different colors of light differ in their frequencies (or wavelengths).

Vocabulary

Wavelength

The distance between a point on one wave to the same point on the next wave (usually from crest to crest or trough to trough).

Amplitude

The height of a wave, or the distance from the crest (or trough) to the undisturbed portion of the wave.

Frequency of a wave

The number of waves passing a specific point each second.

Electromagnetic spectrum

A list of all the possible types of light in order of decreasing frequency, or increasing wavelength, or decreasing energy. The electromagnetic spectrum includes gamma rays, x-rays, ultraviolet rays, visible light, infrared rays, and radio waves.

http://www.ck12.org/book/Introductory-Chemistry/r1/section/2.4/The-Nature-of-Light-%253A%253Aof%253A%253A-The-Structure-of-the-Atom/