Electromagnetic Spectrum, Diffraction and Polarisation Flashcards
EM Wave Properties
Electromagnetic Waves
Travel at c = 3 x10^8 ms^-1 in a vacuum
Progressive Waves: Transfer Energy
Transverse Waves: Reflection, Refraction, Diffraction, Interference, Polarisation
Gamma Rays
x10^-16 < λ < x10^-10
X Rays
x10^-13 < λ < x10^-8
Ultraviolet Rays (UV)
x10^-8 < λ < 4x10^-7
Visible Light Rays
400nm < λ < 700nm
Infrared Rays
7 x10^-7 < λ < x10^-3
Microwave Rays
x10^-3 < λ < x10^-1
Radio Rays
x10^-1 < λ < 10^6
How to find relative frequencies
c = f x λ
c: Speed of light in a vacuum (constant)
λ: Wavelengths of EM wave
Diffraction
When waves pass through an aperture (gap) or around an obstacle and spread out into the space beyond
Place objects in a ripple tank to demonstrate this
Conditions for Maximum Diffraction
The wavelength of the source is equal to the gap through which it is diffracting
If the wavelength and the gap are of the same order of magnitude (x10^n) then they are closely matched
Gap»_space; Wavelength: No diffraction
Gap > Wavelength: Minimal diffraction
Gap = Wavelength: Maximum diffraction
If a gap is too large or small in comparison with the wavelength then the waves continue not diffracted
Wavelengths are unaffected by diffraction
Why is diffraction of sound more noticeable than diffraction of light
The wavelength of light is around 5 x 10^-7 m
The wavelength of sound is around 1m
The aperture needs to be the same size as the wavelength of the wave
Common g[as and obstacles are of the order of 1m, so the diffraction of sound is commonly observed
Light would require a very small aperture
Polarisation
Transverse wave only (e.g. EM waves)
A plane polarised wave has oscillations in one plane only, perpendicular to the direction of energy transfer
Longitudinal waves have no perpendicular oscillation, so cannot be polarised
Light is made up of an electric and magnetic field, both perpendicular to the direction of wave travel
Most light sources e.g. sun are unpolarised
Showing that light is polarised
As the plane polarised light and polaroid are parallel, maximum intensity of light passes through
As the polaroid is rotated, the intensity until it is perpendicular to the plane of polarisation, when zero intensity is experienced
Polarisation of Microwaves
An emitter, transmits polarised microwaves
Rotate the receiver through 90
Observer the change in intensity
Detector must receive in the same plane of polarisation
Metal Grille
When bars are in the same direction as the microwaves, the electric field causes free electrons in the wires to oscillate, which absorb the microwave energy
No signal is detected
At 90, a signal is detected
