Turning points 2 Flashcards
(45 cards)
What was Newton’s theory for light
Made up of tiny particles called corpuscles
How did Newton explain why light reflects
- When corpuscles collide with a surface, a repulsive force pushes them back
- Force causes perpendicular component of velocity to change direction but parallel component remains constant
How did Newton explain why light refracts
- When corpuscle enters a denser medium at an angle, short range attractive forces act perpendicular into the surface
- Causes velocity of corpuscle perpendicular to surface to increase while parallel component stays constant
- Causes it to bend towards the normal as the particle is now more perpendicular to the surface than before
What did Newton believe about the speed of light as it passes into a denser medium
Perpendicular component of velocity is greater whilst parallel component is the same so speed increases
What was Huygens principle
- Point source produces waves joined by a wave front
- Every point on the wave front becomes another point source of secondary “wavelets” which form a new wave front (cycle continues)
What did Huygens believe about the speed of light as it enters a denser medium
Slows down (and bend towards the normal)
Give 3 reasons why Newtons theory of light preferred over Huygens
- Newton had a very high reputation at the time
- Diffraction had not been observed
- Speed of light had not been measured
How did Young’s double slit experiment prove Huygens theory of light
- When light was shone through a double slit with each slit acting as a coherent point source, bright and dark fringes formed due to diffraction and interference
- These are wave properties
What would have occurred in Young’s double slit experiment if Newton was right
There would be only 2 bright fringes corresponding to the 2 slits instead of an interference pattern
When did scientists start agreeing with Huygen’s wave principle
- Years after Young’s experiment when the speed of light in water was measured to be slower than in air
What is an EM wave formed of
- Alternating magnetic and electric field travelling in phase and perpendicular to each other and the direction of wave propagation
- The electric field goes up and down and magnetic field is side to side when drawing
What was Maxwell’s formula for the speed of an EM wave in a vacuum
c = 1 / sqrt (μ0ε0)
where μ0 is the permeability of free space which relates the magnetic flux density of a wire to the current flowing through it
and ε0 is the permittivity of free space, relating the electric field strength to the charge of the object producing it
Explain Fizeau’s method for measuring the speed of light
- Light source causes beam of light which is passed through a rotating wheel
- The wheel has teeth and gaps so the light only passes through if it passes through a gap
- There is a mirror about 8km away so the light comes back at the wheel after reflecting off the mirror
- Alter frequency of wheel until the light will pass through the gap to the mirror and get blocked by the next tooth as it comes back
- The distance travelled is about 16 km and time taken can be calculated using the time period of the wheel at the frequency and speed = distance/ time
What calculations did Fizeua do after his experiment to calculate the speed of light
- Distance travelled = 2d as it goes to the mirror and back
- If a wheel has n teeth and n gaps, 1/ 2n of a revolution a tooth will replace a gap so the light is blocked
- Time period = 1/f so a tooth will replace a gap every 1/2nf seconds
- c = 2d / (1/2nf) = 4dnf
Why did people believe light was an EM wave after Fizeau’s experiment
His value for speed of light was very close to Maxwell’s value for speed of EM wave
What was the apparatus used in Hertz’s experiment to find the speed of radio waves
- 2 charged plates called electric dipoles
- Induction coil to produce a high pd (powered by battery)
- Spark gap connected between the plates where the pd is applied
- Loop of wire with gap in it
- Metal plate for radio waves to reflect off
How did Hertz produce radio waves in his experiment to find the speed of them
When the induction coil produced a high pd causing sparks to jump across the gap of air radio waves are produced
How did Hertz detect the magnetic field of the radio waves
- When the radio waves enter the loop of wire, wire experiences a changing magnetic field causing EMF to be induced
- Another spark is produced across the gap in the loop
How did Hertz detect the electric field of the radio waves
Instead of the loop of wire, use another dipole parallel to the first and the radio waves will create an alternating current in the second dipole
Explain how Hertz calculated the speed of radio waves
- Place metal sheet so that radio waves are reflected back producing a stationary wave
- Detect strength of wave at different points by using the loop of wire as a detector i.e. no sparks due to B field at nodes
- Measure the distance between 2 nodes and double for wavelength
c = fλ
How did Hertz’s experiment prove that radio waves were EM waves
They travelled at the same speed that Maxwell predicted for the speed of EM waves
How could Hertz show that radio waves were polarised
Rotate the loop of wire from 0 to 90 degrees and the signal varies from a max to min value
Max value as detector was perpendicular to oscillations and min value when detector was parallel
Define a black body
A body that can absorb all wavelengths of EM radiation and can emit all wavelengths
e.g. the sun is a black body but a mirror is not as it reflects most wavelengths instead of absorbs
What was the predicted graph of intensity of EM radiation emitted against wavelength of EM radiation emitted according to classical wave theory
As wavelength decreases, intensity increases graph is in the form y = k/x
