Electromagnetic Waves Chapter 24 Electromagnetic Waves

The Nature of Electromagnetic Waves Benefits of Electromagnetic Waves Visibility (Every thing we see can be attributed to Electromagnetic Waves) Satellite Communication Telecommunication Remote Control Microwave Oven X-Rays Radio Transmission Laser Surgery Aircraft Navigation.

The Nature of Electromagnetic Waves Electromagnetic Wave is a coupled time varying electric and magnetic fields that propagate in space. Two straight wires connected to the terminals of an AC generator can create an electromagnetic wave. Only the electric wave traveling to the right is shown here.

24.1 The Nature of Electromagnetic Waves Faraday – Lenz stated that Time-Varying Magnetic Field generates an Electric Field. Changing Magnetic Flux induces an EMF which in turn produces a Current, hence an Electric Field. Maxwell stated that if a time varying magnetic field generates an electric field, then a time varying electric field should generate a magnetic field. The current used to generate the electric wave creates a magnetic field.

The Nature of Electromagnetic Waves This picture shows the wave of the radiation field far from the antenna. (Similar to oscillating pendulum. Acceleration of charges Produces a changing electric field) Frequency of oscillating charge is frequency of electromagnetic wave. The speed of an electromagnetic wave in a vacuum is:

The Energy Carried by Electromagnetic Waves The total energy density carried by an electromagnetic wave Energy in an electromagnetic wave is partly carried by the electric field and partly by the magnetic field. Represents permittivity of free space. Represents permeability of free.

The Energy Carried by Electromagnetic Waves Properties of Electromagnetic Waves 1. Velocity of Electromagnetic Wave in free space or vacuum is a constant. 2. No material or medium is necessary for propagation. 3. Propagates in the presence of material medium. 4. Electromagnetic waves carry energy and momentum. 5. Electromagnetic waves exert pressure known as Radiation Pressure.

The Electromagnetic Spectrum Like all waves, electromagnetic waves have a wavelength and frequency, related by:

The Electromagnetic Spectrum Radio Micro Infra Red Visible Light Ultraviolet X-Rays Gamma Ray Like all waves, electromagnetic waves have a wavelength and frequency, related by:

The Electromagnetic Spectrum Radio Micro Infra Red Visible Light Ultraviolet X-Rays Gamma Ray Minimum Frequency Maximum Wavelength Maximum Frequency Minimum Wavelength

The Nature of Electromagnetic Waves Application of Electromagnetic Waves Radiowaves (produced by accelerated motion of charges in conducting wires) Used in radio and television communication. A radio wave can be detected with a receiving antenna wire that is parallel to the electric field.

The Nature of Electromagnetic Waves With a receiving antenna in the form of a loop, the magnetic field of a radio wave can be detected.

The Nature of Electromagnetic Waves Bands of Radio Waves AM (Amplitude Modulated): 550 kHz to 1710 kHz SW (Short Wave): Up to 54 MHz TV Wave: 54 MHz to 890 Mhz FM (Frequency Modulated): 88 MHz to 108 MHz UHF (Ultra High Frequency)

The Energy Carried by Electromagnetic Waves 2. Microwaves: Short Wavelength Radio Waves Produced by Special Vacuum Tubes Used in Microwave Ovens and RADAR Systems Microwave is absorbed by water fats and sugars. RADAR Systems: Air Traffic Control, Speed Detection, etc… Electromagnetic waves, like water waves, carry energy.

The Energy Carried by Electromagnetic Waves 3. Infra Red Waves (Heat Waves): Infra Red Lamps, Infra Red Camera, LED Lamps (Remote Control) Green House Gas Effect : Sun Rays bounce off earth and get trapped by CO2, CH4 leading to global warming) 4. Light Waves (Red, Orange, Yellow, Green, Blue, Indigo, Violet) 5. Ultraviolet Waves (Produced by Very Hot Bodies, example Sun). Ozone Layer protects us from UV Waves. 6. X-Ray (Produced by bombarding metal target by high energy electrons). Penetrates Flesh Low density but cannot penetrate bone (high density) 7. Gamma Rays (Produced by Nuclear Reactions and emitted by Radioactive materials). Used in cancer treatment.

POLARIZED ELECTROMAGNETIC WAVES Polarization POLARIZED ELECTROMAGNETIC WAVES Polarization is the process of transforming unpolarized light into polarized light Polarization is observed only in transverse waves Unpolarized Wave: Wave vibrates in multiple planes. The wave vibration keeps changing but it is always perpendicular to the direction of wave travel. Polarized wave: Wave vibration is confined to a specific plane. Polarization is done by scattering, reflection and refraction and also by use of polaroids. Polaroids are materials consisting of long chain molecules alighed in a particular direction. Intensity of light is lower for polarized light.

POLARIZED ELECTROMAGNETIC WAVES Polarization POLARIZED ELECTROMAGNETIC WAVES Linearly polarized wave on a rope.

In polarized light, the electric field Polarization In polarized light, the electric field fluctuates along a single direction.

Polarized light may be produced from unpolarized light with Polarization Polarized light may be produced from unpolarized light with the aid of polarizing material.

MALUS’ LAW intensity before intensity after analyzer analyzer Polarization MALUS’ LAW intensity before analyzer intensity after analyzer

Example 7 Using Polarizers and Analyzers Polarization Example 7 Using Polarizers and Analyzers What value of θ should be used so the average intensity of the polarized light reaching the photocell is one-tenth the average intensity of the unpolarized light?

Polarization

When Polaroid sunglasses are crossed, the intensity of the Polarization When Polaroid sunglasses are crossed, the intensity of the transmitted light is reduced to zero.