2 Youngs Double Slit Experiment And EM Waves Flashcards
What was the Young double slit experiment
Thomas Young in 1803 showed that light passed through double slits produces an interference pattern. Since interference is a wave property, Young’s experiment challenged the accepted theory that light consists of corpuscles.
Explanation of Youngs experiment
bright fringe formed where light waves arrive in phase so they constructive interference
-dark fringe is formed where light waves from each slit arrive 180° out of phase
•larger the slit spacing : the smaller the fringe spacing is as less diffraction.
-Therefore, more fringes are observed
-Newton’s corpuscular theory of light predicted that corpuscles would pass through each slit so two bright fringes would be seen so wave theory could explain interference
What are EM waves formed of?
-alternating magnetic and electric fields travelling IN PHASE and 90* to each other
-Direction of wave travel is PERPENDICULAR to the oscillation of electric and magnetic fields
speed of light formula in a vacuum and who made it
-Maxwell
C = 1/root(permeability of free space x permittivity of free space)
Mew : the magnetic flux density produced by the wire to the current in the wire
E0 : electric field strength to the charge on the object , which formed the field
What did Hertz discover
-Discovered radio waves when sparks jump across a gap of air. They are detected with a dipole detector or a wire loop with a gap in
What’s a dipole detector
-pair of aligned metal rods placed at the centre of a concave reflector
How did Hertz carry out the experi
Air gap between wires and put a large potential difference across this gap, so high voltage sparks bridged the gap
-These sparks generated radio waves, so this was a radio wave transmitter
What equipment did Hertz use
-A circular wire with a small break in the circuit produced sparks across the break when held near the source of radio waves : detects waves alternating magnetic field as field enter the loop causes a change in flux inducing EMF
-Dipole receiver : A concave metal sheet with two parallel metal rods at the centre which had oscillating potential difference induced across them by the radio waves’ alternating electric field
How did Hertz show the wave can be reflected?
He placed a metal screen behind the source and measured a stronger signal with the detector
This showed some radio waves were reflected off the screen and back towards the detector
How did Hertz show the waves can
penetrate insulators
-When an insulator was placed between the transmitter and detector, there was no difference in the signal detected
How did Hertz show the waves can be polarised
-When the detector was rotated 90° perpendicular to the path of the radio waves, sparks stopped being produced in the detector
This showed the electric fields of the radio waves were only oscillating in a single plane
The cause for this was that electrons were only being accelerated in one direction so the radio waves were all polarised in the same plane
How did Hertz measure the speed of radio waves
-He reflected radio waves from the transmitter off of a flat metal sheet
This produced a standing wave
When passing a detector across the region containing the standing wave, a large signal was detected at antinodes, and no signal was detected at nodes
This allowed Hertz to find the wavelength and he knew the frequency by using the properties of the transmitter circuit
He used the wave equation (v = fλ) to determine the speed of the waves
This speed was very close to the value calculated by Maxwell, showing that radio waves are electromagnetic waves
Fizeau measure of speed of light
- Fizeau shone a beam of light at a mirror several kilometres away
In the path of the light, he placed a “toothed wheel” which was spinning at a very high speed
The toothed wheel was positioned so the teeth of the wheel and the gaps between them periodically passed over the beam of light
This created regular pulses of light travelling towards the distant mirror
What was the path of the light passing through the gap of the toothed wheel
t = 2D/C
Why does the person see a minimum
-from when the light has just moved through the gap and towards the reflector and then back.. the wheel has moved exactly 1 tooth distance .. so it was a gap and it’s moved to a tooth so it’s now a tooth and blocked all light
