Lesson 2 - EM Waves / EO Intro

What are the two types of waves?

• Mechanical: requires a medium for propagation (ex: sound, water, seismic)

• Electromagnetic: does not require a medium for propagation (light, heat, radio)

What are the two types of motion?

• Longitudinal: motion of medium moves from left to right

• Transverse: motion of medium moves up and down

What are the wave phases?

• Advanced wave: positive phase shift (start before)

• Retarded wave: negative phase shift (starts after)

What is an electromagnetic wave?

Wave of EM energy that is radiated by an accelerating or oscillating electric charge and propagates through a vacuum or a material medium as a periodic disturbance of the EM field at a frequency within the EM spectrum

Relationship between frequency and wavelength

inversely proportional

What is wave propagation and what is it affected by?

• Involves the motion (longitudinal, transverse) but also the shape of the wave front, and how wave interacts w/ surfaces or obstacles it encounters

• Affected by reflection, refraction, diffraction, interference, and attenuation

Wave propagation: reflection

• Media boundaries with dissimilar propagation speeds result in refection

• Specular reflection: surface is smooth

• diffuse reflection: surface is rough

Wave propagation: refraction

• Bending of EM ray when it transmits from one medium to another

• Angle determined using Snell’s Law

• Waves refract towards medium w/ higher refractive index

• Smaller the wavelength, the more it refracts

Wave propagation: diffraction

waves bending around an object

Wave propagation: interference

• When EM waves interact with one another

• Constructive or destructive

What is constructive interference?

Phase shifts between 0 and 120 OR 240 and 360 deg

Wave propagation: attenuation

• Takes the energy out of a wave so it cannot propagate anymore

• 3 parts: spreading, scattering, absorption 

• More attenuation → less distance signal will travel

Attenuation: what is spreading?

Energy distributed over an increasingly larger area

Attenuation: what is scattering?

Energy reflecting off suspended particles within a medium

Attenuation: what is absorption?

Energy dissipated into medium

Relationship between attenuation and frequency

Frequency increase → attenuation increase

Advantages of EO systems

• Passive

• Precision imaging / tracking

• VID possible

Disadvantages of EO systems

• Susceptible to atmospheric conditions

• Scattering, absorption

• Shorter max range

What region of the EM spectrum do EO systems operate?

optical region

What is the unit of measure for EO systems?

• Wavelength 

• Micron = 10^-6 m  

What do visible imaging systems do?

Detect and amplify visible energy reflected (ex: night vision goggles)

What do thermal imaging systems do?

Detect energy radiated from a target (ex: thermal imagers)

What are the two general groups of EO spectrum sources?

• Thermal radiators

• Selective radiators

EO spectrum source: selective radiator

• Output concentrated in narrow wavelength bands

• Discrete wavelengths

EO spectrum source: thermal radiator

• Output over continuous spectrum of wavelengths

• Span of wavelengths