Electromagnetic Waves

Electromagnetic Waves



Electromagnetic waves are formed when an electric field (shown as blue arrows) couples with a magnetic field (shown as red arrows). The combination of oscillating electrical and magnetic fields of the electromagnetic waves are perpendicular to each other and to the direction of the wave. James Clerk Maxwell and Heinrich Hertz are two scientists who studied how electromagnetic waves are formed and how fast they travel.This is difficult to visualize, however the waveform has similar characteristics of other types of waves.

Although they seem different, radio waves, microwaves, x-rays, and even visible light are all electromagnetic waves are part of the electromagnetic spectrum, and each has a different range of wavelengths, which cause they waves to affect matter differently.The creation and detection of the wave depend much on the range of wavelengths.

Electromagnetic spectrum

The range of wavelengths for electromagnetic waves--from the very long to the very short--is called the Electromagnetic Spectrum:

* Radio and TV waves are the longest usable waves, having a wavelength of 1 mile
(1.5 kilometer) or more.
* Microwaves are used in telecommunication as well as for cooking food.
* Infrared waves are barely visible. They are the deep red rays you get from a heat lamp.
* Visible light waves are the radiation you can see with your eyes. Their wavelengths are in the range of 1/1000 centimeter.
* Ultraviolet rays are what give you sunburn and are used in "black lights" that make object glow.
* X-rays go through the body and are used for medical purposes.
* Gamma rays are dangerous rays coming from nuclear reactors and atomic bombs. They have the shortest wavelength in the electromagnetic spectrum of about 1/10,000,000 centimeter.






(Click on picture to get full view)

Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom.

Did you know that electromagnetic waves can not only be described by their wavelength, but also by their energy and frequency? All three of these things are related to each other mathematically. This means that it is correct to talk about the energy of an X-ray or the wavelength of a microwave or the frequency of a radio wave. The electromagnetic spectrum includes, from longest wavelength to shortest: radio waves, microwaves, infrared, optical, ultraviolet, X-rays, and gamma-rays.

There are no gaps in between the electromagnetic spectrum.It is possible to find all possible frequencies that exist in this range.There is no boundary between one type of electromagnetic waves and the next.

(To Take Note:)
Gamma Rays - highest frequency, lowest wavelength
Radio waves - lowest frequency, longest wavelength
Visible light is just one of the 7 members of the family of electromagnetic waves
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Look at the picture of the electromagnetic spectrum. See if you can find answers to these questions:

1. What kind of electromagnetic radiation has the shortest wavelength? The longest?

2. What kind of electromagnetic radiation could be used to "see" molecules? A cold virus?

3. Why can't you use visible light to "see" molecules?

4. Some insects, like bees, can see light of shorter wavelengths than humans can see. What kind of radiation do you think a bee sees?
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Properties

1. Electromagnetic Waves are transverse waves.They are magnetic and electric fields of an electromagnetic wave which are perpendicular to each other and to the direction of the wave.



2.They transfer energy from one place to another.

3.They can travel through vacuum.They don't require any medium to travel from one point to another.

4.They can travel at the speed of light, 3.0 x 10*8 m/s but, will slow down when travelling through water or glass.

5.Equation of wave velocity also applies for electromagnetic waves,
Speed = Wavelength • Frequency

6.They obey Snell's laws of Reflection and Refraction.

7.They carry no electric charges as they are neither positively charged nor negatively charged.

8.The frequencies do not change when they travel from one medium to another as their frequency depends only on the source of the wave.Only their speeds and wavelengths change from one medium to another.

Wave Production and the ripple tank

The Ripple Tank



The ripple tank is a container that when filled with water permits the study of water waves.A concentrated light source positioned above the tank forms images of the waves on a screen beneath the tank.Wave crests and troughs project light and dark lines in the screen.




The crests act as converging lenses that focus light,producing the bright lines..The troughs act as diverging lenses that scatter light, producing the dark lines.
The depth at which the dipper is placed affects the amplitude of the waves, while the frequency of waves is determined by frequency of vibration of the dipper.

Refraction of waves and the depth of ripple tank

Refraction of waves involves a change in the direction of waves as they pass from one medium to another. Refraction is the bending of the path of the waves.It is accompanied by a change in speed and wavelength of the waves. It was mentioned that the speed of a wave is dependent upon the properties of the medium through which the waves travel. So if the medium (and its properties) are changed, the speed of the waves are changed.

The most significant property of water which would affect the speed of waves traveling on its surface is the depth of the water.

This boundary behavior of water waves can be observed in a ripple tank if the tank is partitioned into a deep and a shallow section. If a pane of glass is placed in the bottom of the tank, one part of the tank will be deep and the other part of the tank will be shallow. Waves traveling from the deep end to the shallow end can be seen to refract (i.e., bend), decrease wavelength (the wave fronts get closer together), and slow down (they take a longer time to travel the same distance). When traveling from deep water to shallow water, the waves are seen to bend in such a manner that they seem to be traveling more perpendicular to the surface.If traveling from shallow water to deep water, the waves bend in the opposite direction.



Water waves travel fastest when the medium is the deepest. Thus, if water waves are passing from deep water into shallow water, they will slow down and also the wavelength of the plane waves shorten.The frequency remains the same as it is determined by the dipper.Using the equation, v:f x L,the speed of the waves is therefore slower at the shallow water.



Refraction of waves can be demonstrated by placing the plastic sheet at an angle to the incoming waves .As observed earlier, the differene in the depth of water causes a change in speed of waves.Similar to light, when waves enter a region of shallow water at an angle, the waves refract.



Reflection of waves can be demonstrated by placing a straight barrier upright in the water causing the incoming incident waves to be reflected.The law of reflection is obeyed and the Angle of incidence is equal to the angle of reflection.



Reflection, refraction and diffraction are all boundary behaviors of waves associated with the bending of the path of a wave. The bending of the path is an observable behavior when the medium is a two- or three-dimensional medium. Reflection occurs when there is a bouncing off of a barrier. Reflection of waves off straight barriers follows the law of reflection. Reflection of waves off parabolic barriers results in the convergence of the waves at a focal point. Refraction is the change in direction of waves which occurs when waves travel from one medium to another. Refraction is always accompanied by a wavelength and speed change. Diffraction is the bending of waves around obstacles and openings. The amount of diffraction increases with increasing wavelength.

Properties of Wave motion

Describing Waves





(click picture to get full view)

Crest or peak: The highest point reached by a wave.

Trough: The lowest point reached by a wave.

Phase : Any Two sources of wave motion are said to be in phase if, at any moment, they have the same fractional displacement from the rest position and are moving in the same direction. If both conditions are not met the sources are out of phase.
Any two crests or troughs are always in phase

Wavelength : The shortest distance between any two points in a wave that are in phase.In a transverse wave, the points are two successive crests or troughs.In longitudinal wave, it is the distance between two successive compressions or rarefactions.S.I. Unit: Metre (m)

Amplitude : The maximum displacement (difference between an original position and a later position) of the material that is vibrating. Amplitude can be thought of visually as the highest and lowest points of a wave.It is the height of a crest or the depth of the trough measured from rest position.S.I. Unit: Metre(m)

In a longitudinal Wave,



(click picture to get full view)

Compression : A point on a medium through which a longitudinal wave is traveling which has the maximum density.It is a region where the coils are pressed together in a small amount of space.

Rarefaction : a point on a medium through which a longitudinal wave is traveling which has the minimum density.It is a region where the coils are spread apart, thus maximizing the distance between coils.



The wavelength of a wave is the length of one complete cycle of a wave. For a transverse wave, the wavelength is determined by measuring from crest to crest. A longitudinal wave does not have crest; so how can its wavelength be determined? The wavelength can always be determined by measuring the distance between any two corresponding points on adjacent waves. In the case of a longitudinal wave, a wavelength measurement is made by measuring the distance from a compression to the next compression or from a rarefaction to the next rarefaction

Questions:

Consider the diagram below in order to answer questions #1-2.



1. The wavelength of the wave in the diagram above is given by letter ______.

ANS

2. The amplitude of the wave in the diagram above is given by letter _____.

ANS

3. Indicate the interval which represents one full wavelength.



a. A to C

b. B to D

c. A to G

d. C to G

ANS

Displacement-Distance Graph

A wave:



Any points on graph above the rest position show positive displacements
and any points on graph below the rest position show negative displacement.

(click picture to get full view)

Displacement-Time Graph



If we freeze the wave motion at various times, we can observe the up-and-down movements of points on transverse wave.If we trace the movement of wave over one second ,we can obtain the displacement-time graph of wave.

Period (T): It is the time for a particle on a medium to make one complete vibrational cycle. Period, being a time, is measured in units of time such as seconds, hours, days or years.

Frequency(f) : It refers to how often the particles of the medium vibrate when a wave passes through the medium. S.I. unit: Hertz (Hz)where 1 Hz is equivalent to 1 cycle/second.

Frequency = Number of cycles/Time Interval

(The period (T) is the time required to complete one full cycle.)

Period and frequency exhibit a reciprocal relationship.



Since the symbol f is used for frequency and the symbol T is used for period, these equations are also expressed as:



Higher the frequency, more the no. of waves produced in one second,T is shorter.

Wave Speed (v) : In a time of one period, a crest on a transverse wave will have moved a distance of one wavelength.

Hence,

Equation for the Speed of a wave :

v : L/T

where ,
v- wave speed
L- wavelength
T-Period

Since f:1/T,

v:f x L



S.I. unit : m/s

Wave front: It is the imaginary line on a wave that joins all points that are in the same phase.It is usually drawn by joining all the wave crests.Depending on the formation of waves, wavefront can be concentric circles,plane straight lines or any shape.



Questions:

1. As the frequency of a wave increases, the period of the wave ___________.

a. decreases

b. increases

c. remains the same

ANSWER

2.The period of the sound wave produced by a 440 Hertz tuning fork is ___________.


Answer

3.A child in a swing makes one complete back and forth motion in 3.2 seconds. This statement provides information about the child's

a. speed

b. frequency

c. period

ANSWER

4. A period of 5.0 seconds corresponds to a frequency of ________ Hertz.

a. 0.2

b. 0.5

c. 0.02

d. 0.05

e. 0.002

Answer