Introduction to Properties of Waves

Introduction to Properties of Waves Waves travel through materials as vibrations and transmit energy. Though nearly all waves travel through matter, they never transmit matter. Waves are created when a source material sets up wave-like disturbances that spread away from the source of the disturbance. This means that every wave starts somewhere. Where is the source of the wave? Can you explain why the rope is forming a wave? Waves are grouped by how they behave. All waves have repeating patterns that give them each a shape and length, which we use to describe wave behavior and, therefore, categorize waves according to these descriptions. Waves change their behavior as they travel through different types of matter. To apply these wave properties, first we must understand how each wave is measured. Do you see any characteristics in the waving rope above that might help us describe a wave? Wave behavior is measured by the distance between peaks (wavelength), the size of the peak (amplitude), and the speed of the peaks (frequency). Sound and earthquake tremors are examples of how waves appear and are measured differently. These and other waves move at different speeds in different materials. All waves have the following: frequency: the rate at which a vibration occurs that constitutes a wave amplitude: the size of the peak of a wave wavelength: the distance between the peaks of a wave 1

Introduction to Properties of Waves amplitude: the height of a wave wavelength: the distance between adjacent crests trough: the lowest point of a wave crest: the highest point of a wave Take a Closer Look Can you use the wave vocabulary to label the parts of the transverse wave? F. G. H. J. 2

Introduction to Properties of Waves Low Amplitude High Amplitude Low Frequency High Wavelength Low Wavelength High Frequency Note: As frequency goes up, wavelength goes down. 3

Introduction to Properties of Waves This activity requires a rope or string, and a stationary object like a tree or doorknob. Tie the string or rope to the stationary object and move it up and down to create a wave. Your task is to vary the movement of your arm to create different waves of amplitude, wavelength, and frequency. 1. What did you have to do to change the amplitude? 2. What did you have to do to change the wavelength and frequency? 4

Introduction to Properties of Waves Wave Properties Slinky Applications Slinkys are one of the coolest toys ever created, and they have been around for decades. The science applications for Slinkys are perfect for an introduction to the properties of waves. Play, plan, demonstrate! Materials: Slinky Mission: Create a wave using the Slinky and your partner. Try to keep the wave going for as long as possible (at least twenty to thirty seconds). Change the distance between you and your partner and repeat the wave. Observations of original wave: Observations of second wave: Sketch and label your first wave here: Sketch and label your second wave here: What did you have to do to create the wave motion with the Slinky? What did you have to do to keep the wave moving? What happened to the amplitude of the waves as you changed the distance? The frequency? The wavelength? 1

A rainbow is a perfect example that shows how visible light is composed of a spectrum of colors. To see a rainbow, your back must be to the Sun as you look up at about a 40 angle to where the air has suspended droplets of water. Each droplet of water acts as a tiny prism that disperses, refracts, and reflects it back to your eye. Electromagnetic Waves: Visible light is one example of electromagnetic radiation, which is energy that travels as particles called photons. Each photon is actually a packet of electric and magnetic waves. Think of water waves. The distance from one crest to the next is called the wavelength of the waves. Each photon is composed of waves with only one wavelength. Different photons can have different wavelengths associated with them. Photons span the range of the electromagnetic spectrum. In the order of decreasing wavelengths, they are called radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. Photons with shorter wavelengths carry more energy than do photons with longer wavelengths. Gamma rays are the highest energy photons, while radio waves are the lowest energy photons. Everyday Life: Uses of the Electromagnetic Spectrum You need visible light to see, but did you know you use other parts of the electromagnetic spectrum in your everyday life? Every time you use a cell phone or listen to the radio, you are using radio waves. Radio waves transmit music, a conversation, or a television image, among other things. A microwave oven uses microwave radiation to energize the atoms that make up your food, which heats up cold food. Doctors and dentists use X-rays to examine your bones and teeth. When you get a sunburn, it is because of the ultraviolet (UV) radiation that the Sun gives off. The UV rays can destroy skin cells and make blisters, or, in some cases, cause cancer. Regardless of wavelength, all types of electromagnetic radiation travel at about the same speed of 300,000,000 kilometers/second, which is 186,000 miles/second. In addition to wave speed and wavelength, electromagnetic waves can be measured by frequency. Frequency refers to the number of waves that pass a given point in a given period of time (usually one second). Electromagnetic waves with longer wavelengths have lower frequencies; in other words, fewer waves pass a given point each second. Electromagnetic waves with shorter wavelengths have higher frequencies, which means more waves pass a given point each second. 1

Electromagnetic radiation is made of particles called photons. It is important to remember that photons are a special type of particle. Unlike protons and electrons, photons have no mass. How the energy of the electromagnetic spectrum behaves simultaneously as both particle and wave is one of the great mysteries of science. Visible Light and Color Visible light is the only part of the electromagnetic spectrum human eyes can detect. Visible light human eyes can detect is made up of photons of different wavelengths. Visible light ranges in wavelength from about 400 to 700 nanometers (nm). A nanometer is a billionth of a meter. Each of these different wavelengths corresponds to a different color. We see the longest wavelengths of visible light as red light. We see the shortest wavelengths as violet light. White light can be separated into different colors using a special lens called a prism. When sunlight shines through raindrops, the raindrops can act as a prism and create a rainbow. This model of the electromagnetic spectrum shows the different colors of visible light. Red light waves, which have the longest wavelengths and lowest frequencies, are on the left. Violet light waves, which have the shortest wavelengths and highest frequencies, are on the right. Wave Behavior When electromagnetic waves encounter surfaces, they behave differently in the form of reflection (bouncing), scattering (incoming photons going out in all directions), refraction (bending), diffraction (spreading or bending around the edges of an obstacle), or absorption (light energy transfers to another medium). A prism separates white light into different colors. 2

Reflection The most commonly observed wave behavior is reflection. You observe this phenomenon whenever you see your image in a mirror. Reflection occurs whenever a light wave strikes a surface and then bounces back. The angle of incidence equals the angle of reflection. Scattering is different from reflection in that scattered light goes back out in all directions. White surfaces, like many walls, scatter light in all the colors that strike them. Not all light is reflected or scattered. Black absorbs all light. Colored surfaces absorb all wavelengths except for the color reflected. Absorption occurs when some or all of the light energy from light waves is transferred from one medium to another. If a star is emitting light waves at a lower frequency than expected that is, if a star is exhibiting redshift it must be moving away from observers on Earth. How can this effect, called Doppler Effect, be used to show that the universe is expanding? Astronomers gather the light from the stars in distant galaxies, and use a spectroscope to separate it into its spectral wavelengths to study if the lines have shifted to the red (means the object is moving away) or to the blue (the object is moving closer). By measuring the intensity of a star s redshift, scientists can determine the speed at which a star or galaxy is moving and use this to calculate its distance from Earth. For example, light reflecting off of a liquid surface can cause a bright glare on a lake on a sunny day. It is also the reason we can observe images of objects in water. 3

Law of Reflection The reflection of waves from a boundary is similar to the way a billiard ball strikes and bounces away from a wall. If a ball strikes a wall head on (meaning it is traveling perpendicular to the wall), the ball will bounce back in exactly the same direction from which it traveled. However, if a ball strikes a wall at an angle to the perpendicular (called the angle of incidence), it will bounce away from the wall at the same angle to the perpendicular (the angle of reflection). Waves behave in the same way, as shown below. In fact, this is called the Law of Reflection. While a wave s direction changes during reflection, its speed does not. The reflection of hot air balloons is seen on the surface of a still lake. Some of the light that has been reflected from the hot air balloon travels through the air to the lake. When this light strikes this boundary, some of it reflects back into the air. This causes an image of the hot air balloon to appear on the lake s surface. A wave (red arrow) strikes a barrier (blue line) at an angle θ i to the perpendicular line. This is the angle of incidence. The wave bounces away at an angle θ r. This is the angle of reflection. The Law of Reflection states that the angle of incidence is equal to the angle of reflection. Light waves reflect from surfaces in different ways. For example, mirrors reflect all the light that strikes them. However, light behaves differently on other surfaces. For example, opaque surfaces do not allow light to pass through them. Light striking an opaque surface is either absorbed by the surface or reflected from it. Some objects absorb certain frequencies (colors) of light and reflect other frequencies. The particular frequencies of light that an object reflects determine the color of the object. For example, grass absorbs all frequencies of light except for green. That is, grass scatters green light. This is why grass appears green. White objects scatter most of the light striking them, while black objects absorb most of the light. frequency: the number of times a wave cycles (or vibrates) in a given unit of time 4

Refraction A wave can also be transmitted through a boundary, meaning that it passes through the boundary from one medium to another. In fact, at most boundaries, part of the wave is reflected and part of it is transmitted. Most waves move at different speeds through different media. For example, light moves quickly through air, but more slowly through water. Waves change speeds as they transmit from one medium to another. This change in speed usually causes a wave to refract, or bend, as it passes through the boundary. Refraction can be thought of as black dots in this diagram, which represent a group of students marching across a room. The students represent a light wave. The left side of the room represents air (a relatively fast medium), and the right side represents water (a relatively slow medium). When the students pass from left to right, one student crosses the boundary first. This student begins moving slowly, but the rest of the group still travels quickly, causing the students to change direction, or bend, as they move into the new medium. This is known as refraction. Refraction causes light from the Sun to spread out into different colors as it passes from air into new media, such as water or glass. For example, the different wavelengths of sunlight all refract at different angles when they pass through a raindrop. This causes sunlight to spread out into a rainbow as it refracts through raindrops after a storm. Refraction through concave or convex lenses allows correction to poor vision or to magnify objects. Refraction through water bends the image. Refracting telescopes use lenses to gather light from distant sources such as planets, stars, or galaxies, and focus that light through an eyepiece. 5

Diffraction Have you ever watched water waves pass through a narrow slit in a barrier? When the waves pass through, they spread out radially. This is known as diffraction. For example, when a light bulb is turned on in a dark room, light waves spread out radially from the light bulb. However, once the waves have traveled a certain distance, they can be thought of as plane waves that move in parallel sheets, like in the image on the right. When a plane wave passes through a narrow slit or a barrier, the wave will act as though it is originating from a point source again. Thus, the wave spreads out, or diffracts. Plane waves (left) spread out radially (right) when they pass through a slit in a barrier. This can also happen when a wave approaches a solid barrier. In these cases, when the wave passes along any edge of the barrier, it will spread out radially around the barrier. In this way, waves will appear to bend around barriers. This is the reason you can hear sounds that are emitted from behind a wall or large building. Everyday Life: Uses of Diffraction Astronomers attach special diffraction gratings inside spectroscopes to their telescopes to separate by wavelengths the light coming from distant objects. This process produces a spectrum that gives information about celestial objects. Diffraction is also used in producing special holographic images. Special photographic techniques can diffract light and record it in 3-D. These diffraction images are used as holograms on credit cards or other identification cards as a security measure, providing an image that can be read by an optical scanner. Supermarket checkout scanners use holographic optical elements (HOEs) that can read a universal product code (UPC) from any angle. 6

What do you know? 1. In this exercise, you will show what you know about reflection and refraction. In the diagram on the right, the double lines represent a pane of glass and the single line represents a mirror, both shown edgewise. The arrow is a ray of light. Draw the path of the light ray until it reaches point X. Show how the change in the speed of the light ray affects its direction as it passes from one medium to another. Remember, light travels more slowly in glass than it does in air. HINT: The light will bend both when it enters and exits the glass! 2. Explain how a rainbow is produced. 3. Why do you see something as white, black, or another specific color? 4. What are electromagnetic waves and how can they be seen? 7

Building a Periscope A periscope is a tool in which several mirrors at opposite ends of a long tube allow people to see around objects. Designing and building a periscope is an excellent way for your child to learn about the fundamental laws of reflection. Remember that light travels in straight lines, and the angle of incidence (where it hits) equals the angle of reflection (how it bounces off). In other words, the angle at which a light ray approaches a mirror is the same as the angle the light ray bounces off the mirror. Plans and instructional videos for building a periscope can easily be found on the Internet. Use search terms such as periscope plans. For most designs, you will need these items: A long, square box or enough cardboard to make such a box Two small pocket mirrors Protractor Sharp knife Duct tape As you and your child position the mirrors at either end of the box, explain the significance of the angles at which the mirrors are set. (The first mirror must reflect light entering the periscope toward the mirror at the other end of the periscope. This mirror must then reflect light toward the eyepiece of the periscope.) You may also watch videos of how a periscope is used onboard a submarine. Encourage your child to find other uses for the periscope. For example, a periscope can allow someone to look around a corner or above a couch. The other end of this periscope is inside a submarine underwater, where someone is observing events above the surface. 8

Electromagnetic Radiation Visible Light Key Word Act Write your knowledge of these words and share it with your partner. Visible Light Frequency Reflection Refraction Diffraction Wavelength Photon 1

Electromagnetic Radiation Categorize It! Sort the words at the top into the bottom three categories showing the sensory input type. spear fishing rainbow pattern on CD back glasses echos hologram looking in the mirror spoon in a glass of water ocean wave hits a wall hearing music come from a room camera lens colors in soap bubble water wave goes around a boat Reflection Refraction Diffraction 1