UNIT 7 Listening Introduction to Electromagnetic Waves

Both infrared light and visible light are examples of electromagnetic radiation. Electromagnetic radiation is the transfer of energy by waves traveling through matter or across empty space. The waves that transfer this energy are called electromagnetic waves. In this lesson, you’ll learn how electromagnetic waves differ from mechanical waves such as ocean waves and sound waves. Watch the video below. What Are Electromagnetic Waves? An electromagnetic wave is a wave that consists of vibrating electric and magnetic fields. A familiar example will help you understand the fields that make up an electromagnetic wave. Think about a common bar magnet. It exerts magnetic force in an area surrounding it, called the magnetic field. You can see the magnetic field of a bar magnet in Figure below. Because of this force field, a magnet can exert force on objects without touching them. They just have to be in its magnetic field. An electric field is similar to a magnetic field (see Figure below). An electric field is an area of electrical force surrounding a charged particle. Like a magnetic field, an electric field can exert force on objects over a distance without actually touching them. How Electromagnetic Waves Begin An electromagnetic wave begins when an electrically charged particle vibrates. This is illustrated in Figure below. When a charged particle vibrates, it causes the electric field surrounding it to vibrate as well. A vibrating electric field, in turn, creates a vibrating magnetic field (you can learn how this happens in the chapter "Electromagnetism"). The two types of vibrating fields combine to create an electromagnetic wave. You can see an animation of an electromagnetic wave in the video below. An electromagnetic wave starts with a vibrating charged particle. How Electromagnetic Waves Travel As you can see in Figure above, the electric and magnetic fields that make up an electromagnetic wave occur are at right angles to each other. Both fields are also at right angles to the direction that the wave travels. Therefore, an electromagnetic wave is a transverse wave. No Medium Required Unlike a mechanical transverse wave, which requires a medium, an electromagnetic transverse wave can travel through space without a medium. Waves traveling through a medium lose some energy to the medium. However, when an electromagnetic wave travels through space, no energy is lost, so the wave doesn’t get weaker as it travels. However, the energy is "diluted" as it spreads out over an ever-larger area as it travels away from the source. This is similar to the way a sound wave spreads out and becomes less intense farther from the sound source. Wave Interactions Electromagnetic waves can travel through matter as well as across space. When they strike matter, they interact with it in the same ways that mechanical waves interact with matter. They may reflect (bounce back), refract (bend when traveling through different materials), or diffract (bend around objects). They may also be converted to other forms of energy. Microwaves are a familiar example. They are a type of electromagnetic wave that you can read about later on in this chapter, in the lesson "The Electromagnetic Spectrum." When microwaves strike food in a microwave oven, they are converted to thermal energy, which heats the food. Wave or Particle? Electromagnetic radiation behaves like waves of energy most of the time, but sometimes it behaves like particles. As evidence accumulated for this dual nature of electromagnetic radiation, the famous physicist Albert Einstein developed a new theory about electromagnetic radiation, called the wave-particle theory. This theory explains how electromagnetic radiation can behave as both a wave and a particle. In brief, when an electron returns to a lower energy level, it is thought to give off a tiny "packet" of energy called a photon (see Figure below). The amount of energy in a photon may vary. It depends on the frequency of electromagnetic radiation. The higher the frequency is, the more energy a photon has. A photon of light energy is given off when an electron returns to a lower energy level. Sources of Electromagnetic Radiation The most important source of electromagnetic radiation on Earth is the sun. Electromagnetic waves travel from the sun to Earth across space and provide virtually all the energy that supports life on our planet. Many other sources of electromagnetic waves that people use depend on technology. Radio waves, microwaves, and X rays are examples. We use these electromagnetic waves for communications, cooking, medicine, and many other purposes. You’ll learn about all these types of electromagnetic waves in this chapter’s lesson on "The Electromagnetic Spectrum." Lesson Summary •An electromagnetic wave consists of vibrating electric and magnetic fields.•An electromagnetic wave begins when an electrically charged particle vibrates.•Electromagnetic waves are transverse waves that can travel across space without a medium. When the waves strike matter, they may reflect, refract, or diffract, or they may be converted to other forms of energy.•Electromagnetic radiation behaves like particles as well as waves. This prompted Albert Einstein to develop his wave-particle theory.•The most important source of electromagnetic waves on Earth is the sun, which provides virtually all the energy that supports life on Earth. Other sources of electromagnetic radiation depend on technology and are used for communications, cooking, and other purposes. Determine if the following statements are true or false. True When a charged particle vibrates, it causes the electric field around it to vibrate. The two fields of an electromagnetic wave occur at right angles to each other. Electromagnetic waves may spread out and travel around obstacles. When electrons return to lower energy levels, they give off particles of matter. Electromagnetic waves are used for communications, cooking, and medicine. Einstein explained how light can behave both as a wave and as a particle. False Both fields of an electromagnetic wave vibrate in the same direction that the wave travels. Electromagnetic waves cannot travel through matter. The human eye can detect all frequencies of electromagnetic waves. All of the sun’s electromagnetic radiation travels to Earth. Examples of electromagnetic waves include Radio Waves X rays light all of the above A vibrating electric field creates a mechanical wave charged particle magnetic field photon As an electromagnetic wave travels through space, it becomes stronger keeps changing direction loses energy to the medium spreads out over a larger area When electromagnetic waves strike matter, they may Reflect Diffract refract all of the above Which of the following statements about electromagnetic radiation is false? It provides virtually all the energy for life on Earth It behaves like a wave most of the time Sometimes it behaves like a particle All of its wavelengths are harmful What do radio waves and sound waves have in common? Both waves are transverse waves Both waves are mechanical waves Both waves transfer energy Both waves need a medium An electromagnetic wave consists of a vibrating magnetic field electric field particle of matter two of the above Match each definition with the correct term electromagnetic radiation transfer of energy by waves such as radio waves and light Wave-particle theory explanation for how light can behave as both a wave and a particle electric field invisible area of force surrounding a charged particle transverse wave wave in which vibrations occur at right angles to the direction the wave travels photon packet of electromagnetic energy electromagnetic wave wave that consists of vibrating electric and magnetic fields magnetic field invisible area of force surrounding a magnet Fill in the blank with the appropriate term. Waves in a rope and electromagnetic waves are both transversewaves.Unlike mechanical waves, electromagnetic waves can travel across space.An electromagnetic wave begins when a(n) electricallycharged particle vibrates.When microwaves strike food in a microwave oven, the waves are converted to thermalenergy.The wave-particle theory of light was developed by Albert Einstein.The amount of energy in a photon depends on the frequencyof electromagnetic radiation.The most important source of electromagnetic radiation on Earth is the sun.