Evans SPH 4U1

Physics Grade 12

Unit 5: Waves

Note 8: Electromagnetic Waves and Light


Reference: Chapter 10.8

Electromagnetic Waves:

Electromagnetic Spectrum:

Text Questions

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Evans SPH 4U1

Physics Grade 12

Unit 5: Waves

Note 9: Applications of Electromagnetic Waves


Reference Chapter 10.9


Radio Waves (see figure 1 and 2 in text):

Sound waves are converted into an audio signal. This audio signal is amplified (by an amplifier) and then passed into a modulator. The modulator is where the signal is either amplitude modulated (AM see below) or frequency modulated (FM see below) onto a carrier with the help of an RF oscillator. This modulated signal is amplified and supplied to a transmitting antenna.  The transmitter converts the electrical signal into an electromagnetic wave.

At the far end there is a receiving antenna that detects the electromagnetic waves.  The incoming oscillating electromagnetic fields cause electrons in the conducting material of the antenna to move, creating a current in the antenna.  The antenna takes the electromagnetic wave and converts it to an electrical signal. The antenna will obviously have waves from many different radio stations. The job of the RF tuner is to select the specific carrier frequency for the selected radio station.  This selected RF signal is amplified and sent to a demodulator which separates the audio signal from the carrier signal. This separated audio signal is then amplified and sent to the speaker to be converted into sound waves.

AM : Amplitude Modulation (Variation)


If we change the amplitude, or strength, of the signal in a way corresponding to the information we are trying to send, we are using amplitude modulation, or AM. The earliest means of radio communications was by Morse code, and the code key would turn the transmitter on and off. The amplitude went from nothing to full power whenever the key was pressed, a basic form of AM.

Modern AM transmitters vary the signal level smoothly in direct proportion to the sound they are transmitting. Positive peaks of the sound produce maximum radio energy, and negative peaks of the sound produce minimum energy.

The main disadvantage of AM is that most natural and man made radio noise is AM in nature, and AM receivers have no means of rejecting that noise. Also, weak signals are (because of their lower amplitude) quieter than strong ones, which requires the receiver to have circuits to compensate for the signal level differences.

FM: Frequency Modulation (Variation)

In an attempt to overcome amplitude noise, frequency modulation was invented. Instead of modulating the strength (or amplitude) of the transmitted signal, or carrier, the frequency is modulated (varied).

In a frequency modulated system, the frequency of the carrier is varied according to the modulating signal. For example, positive peaks would produce a higher frequency, while negative peaks would produce a lower frequency.

Since the recovered audio is dependent only on the frequency, and not the strength, no compensation for different signal levels is required, as is the case with AM receivers.


NOTE: AM waves have longer wavelengths (lower frequencies) than FM waves. (Recall radio stations 680 kHz AM, 102.1 MHz FM) QUESTION: Which waves would bend around a corner more? (remember longer wavelengths diffraction more)



Infrared Waves: originate from hot objects that radiate higher frequency light as the objects' temperature rises.

            ~ infrared images are performed on humans to reveal infection and/or tumours.

            ~ infrared photos can reveal areas of heat loss in buildings.



Ultraviolet Waves:

            ~ small amounts assist in producing vitamin D in the body (sunshine vitamin)

            ~ large amounts can cause sunburn, skin cancer, cataracts.




Incoherent Light Source:

Most light sources (not lasers) produce numerous frequencies and since they produce numerous frequencies they are called incoherent.

A light bulb is a common source that works by exciting atoms in the filament to a higher energy level by a high temperature.  Once excited they emit light as they return to a lower energy level, over a wide set of frequencies.


Coherent Light Source:

A Laser is a coherent light source where the emitted waves are all in phase and are all of the same wavelength (i.e. one colour - monochromatic).

Lasers do not emit at random like other common sources do.

Since lasers are in phase they produce an interference pattern and they can combine their energies to produce constructive interference for a powerful, intense laser light.

Laser light is emitted primarily in one direction.

As a result, lasers are used for surveying, medical operations, fibre optics.


Ionizing Radiation:

Radiation with frequencies higher than ultraviolet is ionizing. The high frequency causes so much energy that the atoms become ionized and expel electrons.

Ionization can break chemical bonds.

Ionizing radiation can alter cell make-up, causing illness and cancer.


Discuss controversy of EMF exposure and cell phone use - some argue that since they are non-ionizing frequencies they are not harmful. However, some studies have shown otherwise.


Text Questions

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