Turning points - Light Flashcards
What was Newtons corpuscular theory of light?
Newton hypothesised that light was made up of tiny, weightless particles, that travel in straight lines. He called this particles corpuscules.
How did Newton use his corpuscular theory to suggest why shadows occur?
Newton suggested that shadows are formed because when the corpuscles from the light source hit an opaque object, the corpuscles were stopped, and a shadow is formed. The greater the amount of corpuscles blocked, the darker the shadow.
Huygen could not explain why shadows are formed usig his wave theory.
What did Newton suggest about the colour of light?
Newton suggested that the corpuscles of light were coloured, and the different coloures travalled at different speeds due to different particle sizes.
Red travells slower, due due the corpuscles that make up red light having a larger size, compared the blue or violet who have smaller corpuscles and therefore travel slower.
What did Newton suggest about reflection and refraction using his corpuscular theory of light?
During reflection, we know that the angles of incidence and reflection do not change. Newton said that during reflection, a corpuscles velocity component parallel to the mirror is unchanged, and the velocity component perpendicular to the mirror is reversed.
During refraction, the angles of incidence and refraction are different, so Newton realised that a velocity change must occur.
Newton suggested that when light moves from a less dense medium to a more dense medium, like from air to water, the corpusculars velocity increases. Newton said that the for a more dense medium, there is a force of attraction between the light corpuscle and the denser material, so the perpendicular component of the velocity increases while the parallel component of the velocity remains the same, and therefore the speed of light increases in the denser material.
While Newton’s corpuscular theory focuses on particle behavior and attraction/repulsion by the medium, Huygens’ wave theory describes light as a continuous wave process, explaining both refraction and reflection through the bending of wavefronts.
Why was Newtsons theory more widely accepted than Huygens?
- Due to Newtons higher reputation and scientific autority.
- Newton’s theory aligned more with the prevailing scientific thinking of the time, which often leaned toward particle-based explanations in nature.
- Newton’s corpuscular theory offered a simple, intuitive explanation for both reflection and refraction.
While Huygens’ wave theory could explain these phenomena too, it required more complex thinking about wave behavior, and the concept of waves was not as easily accepted at the time.
What was the significance of Youngs double slit experiment on Newtons theory of light?
Young found that light waves interfere, constructively and also destructively, causing bright and also dark fringes.
Newton predicted that there would be no pattern, the corpuscles would travel in straight lines, and there would be two bright regions directly behind the slits. Newton predicted there would be no diffraction, but indeed there was.
The double slit experiment strongly showed evidence for Huygens theory of light however.
What was Fizeaus set up?
Fizeau shone a light beam at a partially reflecting mirror, and this mirror directed the beam between the teeth of a rotating toothed wheel, towards a distance mirror several kilometres away. The mirror directed the beam back through the teeth of the wheel and towards the observer.
What did Fizeau notice when the toothed wheel was stationary?
Fizeau saw that the light beam passed through the gaps in the wheels teeth, was incident on the distant mirror, and then reflected back, passing through THE SAME gap in the toothed, enabling Fizeau to observe continuos light.
What happened as Fizeau rotated the wheel, and how did he measure the speed of the light?
As he rotated the wheel, Fizeau saw that the teeth broke up the light beam into pulses, and flashes of light were seen.
As Fizeau increased the frequency of rotation, a speed was reached where light passing through one gap, returned to the wheel at the instant the next tooth blocked the light from passing through.
Fizeau adjusted the speed of rotation, using a mechanical gear system to reach this speed at which the ligth was blocked by the next tooth, he was able to determine the time it took for the light to travel to the mirror and back, which was a certain distance.
What equation did Fizeau use to determine the speed of light?
The distance between the mirror and tooth was D. So, the distance for the light to travel there and back was given by 2D.
Assume that the wheel has N number of teeth all equally spaced, with an equal gap, so the distance around the tooth is 2N. The time for the wheel to turn a distance equal to the width of one tooth is given by T/2N where T is the time for one complete rotation.
We know T = 1/f. So the time can be written as 1/ 2Nf.
The speed is given by 2D / t.
c = 2D / 1/2Nf = 2D2NF
which is
= 4DNf
What was Maxwells prediction?
Maxwell predicted that electromagnetic waves exist when alternating magnetic fields can create an alternating electric field which could produce an alternating magnetic field and so on. He explained that EM waves could travel through a vaccum and and they are transverse waves, with the electric and magnetic field components perpendicular to each other and the direction of travel.
What was the equation Maxwell came up with?
He calculated the speed of EM waved and it came to be 3x10^8 which is very close to previous calculated values for the speed of light, and so Maxwell concluded that visible light is an EM wave.
What was the setup of Hertz experiment
Hertz used an induction coil and a capacitor to generate alternating high-voltage sparks, creating electromagnetic waves (radio waves).
A spark gap detector with a wire loop and two brass spheres was used. The electromagnetic waves induced an electric current in the wire loop, causing sparks to form between the spheres, which confirmed the presence of radio waves.
What did Hertz find which confirmed Maxwells theory?
Hertz found that electromagnetic waves could induce a current in a wire loop, causing sparks in the gap between the spheres (confirming the presence of waves).
He showed that radio waves could be reflected by a metal sheet, focused by a concave reflector, and transmitted through some materials (especially insulators).
By measuring the nodes and antinodes of stationary waves, Hertz calculated the wavelength of radio waves and used the formula
c=f×λ to calculate their speed, which matched Maxwell’s predicted speed of light.
What is a black body?
Kirchoff proposed the idea that a black body is the perfect absorber and emitter of thermal radiation, and a black body does not absorb, nor does it reflect or allow any radiation to be transmitted through.
How did Wien measure black body radiation and what was his displacement law?
He punched a hole in the side of a microwave and measured the radiation that came out of it at different temperatures. This led to Wiens displacement law which stated that ‘ The peak wavelength is inversely proportional to the Kelvin temp’
What was the Rayleigh prediction and the ultraviolet catastrophe?
An equation was derived and it correctly predicted the shape of the curve at higher wavelengths, but the ultraviolet catastrophe was a problem. It was predicted that at short wavelengths, the radiation emitted became infinite, and so even at ordinary temperatures, objects would emit UV radiation, which is not true.
How was the photoelectric effect first observed ?
It was first observed by Hertz, who saw that the sparks created in the gap between the electrodes of his spark gap detector were much stronger when UV radiation was directed at the electrodes
What was Huygens wave theory unable to explain about photoelectricity?
Huygen proposed that the energy of a wave depends on its amplitude. When light is shone on a metal surface, each free electron in the metal would get a small amount of energy from each wave front in the light, and after some time, the electron would have enough energy to escape.
However, it was seen that when light above a certain frequency is shone upon a metal surface , electrons are emitted IMMEDIATELY so Huygen was wrong.
Huygen could also not prove the concept of threshold frequency. He proposed that even low frequency light can emit electrons, it would just take a longer time for the electrons to gain enough enough energy and escape.
How was De Broglies wavelength equation confirmed using electron diffraction?
Physicists Davisson and Gammer, directed a beam of electrons (generated by an electron gun) and accelerated these electrons through an anode voltage, through a vacuum tube. They directed the electrons towards a target material consisting of Nickel. The electrons were scattered by the nickel target and a diffraction pattern was found on a fluorescent screen.
The pattern found was similar to the interference pattern created by X-rays passing through crystals, and therefore this proved that electrons exhibit wave properties.
What was the equation that resulted from De broglies equation, and was used in the electron diffraction experiment?
1/2mv^2 = eV
mv^2 = 2eV
Multiply everything by m
M^2v^2 = 2meV
Mv = root (2meV)
De broglie wavelength - h/mv
Sub mv as root (2meV)
Wavelength = h / root (2meV)
Increasing the anode voltage, causes the electrons towards accelerate more and have a greater kinetic energy and therefore a greater velocity and this reduces the wavelength.
Explain the concentric ring pattern on the fluorescent screen by electron diffraction
The bright concentric rings are areas of constructive interference. As the wavelength increases, this gives a larger angle of diffraction and therefore this produces more widely spaced concentric rings. The wavelength can be increased by reducing the anode voltage so that the electrons do not accelerate much so they have lower kinetic energy and lower velocity.
A smaller wavelength means a smaller angle of diffraction and therefore this gives more tighter concentric rings.
How does a TEM work?
A beam of electrons is accelerated by an electron gun, and then passed through the condenser lens, where the electron beam is focused by magnetic fields, to form a wide parallel beam of electrons. When this beam reaches the sample, the electron beam can either pass through, be absorbed or be transmitted. The electrons that pass through are focused on a fluorescent screen on the projector lens, and light is given off. lens An imagine is formed. The dark areas of the imagine represent areas oh high density where the electrons are absorbed and the white areas of the image represent areas of low density where the electrons are transmitted.
The electrons hit the objective lens where an image is formed and then the projector lens casts a second image on a fluorescent screen.
What are the drawbacks of TEM?
In the TEM the electrons must pass through the sample
This reduces the speed of electrons, increasing wavelength and reducing resolving power so electron waves are unable to resolve as much detail as their short wavelength would allow
