Nature of light

Tactile Theory

The earliest views on the nature of light came to us from the Greeks. Plato thought that light consisted of filaments (think tentacles), emitted by the eye. When these filaments came into contact with an object, it was possible to see the object. We refer to this as the tactile theory of light.

Theory by Plato

emission theory

Light is emitted through our eyes it reflects off an abject and returns to our eyes allowing us to see the object
The Pythagoreans believed that light traveled as a stream of fast-moving particles. According to this emission theory, objects sent out beams of light that would ricochet off objects and enter the eye.

Newtons Belief

Newton was the principle advocate of the particle, or corpuscular theory. Newton's theory stated that light consisted of particles that traveled in straight lines.

Christian Huggens belief

The wave theory was supported principally by Christiaan Huygens of Holland. Huygens' wave theory stated that light consisted of waves and exhibits wave-like properties in its behavior.

electromagnetic theory

At the end of the 19th century, James Clerk Maxwell combined electricity, magnetism, and light into one theory. According to Maxwell's electromagnetic theory, light was an electromagnetic wave with the same properties as other electromagnetic waves. Maxwell's theory was flawed, however. It was unable to explain all of the properties of light, including one called the photoelectric effect.

quantum hypothesis

In 1900, Max Planck proposed the quantum hypothesis, suggesting that light was transmitted and absorbed in small bundles of energy called quanta. Albert Einstein agreed with Planck's theory and used it to explain the photoelectric effect using the particle model of light.

quantum mechanics

The theory of quantum mechanics, developed over several years in the early 1900s, combines the two major theories of light by suggesting that light sometimes behaves as a particle and sometimes behaves as a wave.

the details of Newton's particle model of light

Rectilinear Propagation
light travels in straight lines.
Newton argued that, since the path of light has no noticeable curve, the speed of light must be extremely high. He also argued that since light does not exert any noticeable pressure, the mass of the particles must be extremely small.
Newton also noticed that two beams of light pass through each other without reflecting to the side. This indicated to Newton that the particles must also be extremely small in size.
Newton argued that a wave should spread (diffract) out a great deal as it passes through an opening, filling almost the whole region beyond the opening. He felt, therefore, that waves could not possibly produce a narrow beam. He also argued that waves would diffract around objects to fill the area behind them, resulting in there being no shadows.
We know today that the reason light doesn't diffract noticeably has to do with its wavelength. Waves with small wavelengths diffract less - light has a very small wavelength, and so it diffracts very little.
Reflection
When light falls on the smooth surface of a mirror, it reflects in such a way that the angle of incidence is equal to the angle of reflection
Refraction
When light passes from air into water, it bends (refracts) towards the normal. Rays of light will always bend towards the normal when they pass from a less dense to a more dense material.
Newton predicted that the speed of light in water must be greater than the speed of light in air
Dispersion
When a beam of light passes through a prism, the light can change from white to a range of colors that is most often referred to as "the colors of the rainbow." This is called dispersion.
Newton hypothesized that the particles of light are not all the same mass: different colors could be explained as differences in the mass of the light particles

Newtons Corpuscular Model

Newton's corpuscular theory of light provided a satisfactory explanation for four properties of light:
• rectilinear propagation
• reflection
• refraction
• dispersion It was weak in its explanation of other effects:
• diffraction
• partial reflection and partial refraction
• speed of light
Diffraction
Newton believed that light travels in straight lines and, therefore, does not diffract around corners. It had been observed, however, that light passing through successive narrow slits produced on a screen a band of light slightly larger than the width of the slits. It was Newton's position that this effect resulted from collisions between the particles of light and the edges of the slit.
\Partial Reflection and Partial Refraction
When light refracts, some of the light is reflected. Newton's explanation of this behavior was the so-called "theory of fits." He said that particles of light arrived at the surface sometimes in a "fit" of easy reflection and sometimes in a "fit" of easy refraction. This was obviously a weak explanation, which Newton himself acknowledged.
Speed of Light
Newton predicted that the speed of light would be faster in water than in air. We know today that this is, in fact, not the case.

Huygens' Principle and Rectilinear Propagation

Huygens' Principle states that every point on a wave front can be considered as a point source of tiny secondary wavelets that spread out in front of the wave at the same speed as the wave itself.
the wave theory predicts that light will move more slowly in water than in air

index of refraction

The ratio of the speed of light in a vacuum to the speed v in a given material is called the index of refraction, n , of that material

critical angle

When light passes from one material into a second material with a smaller index of refraction (say, from water into air), the light bends away from the normal. At a particular incident angle, the angle of refraction will be 90° , and the light will reflect instead of refracting. The incident angle at which this occurs is called the critical angle, θc , and is given by
This phenomenon is known as total internal reflection.

Young's Experiment

proved that light has wave properties since intense light directed through two openings will produce an interference pattern
Notice that, in the case of light, the nodal lines (areas of destructive interference) are dark, and the antinodal lines (areas of constructive interference) are bright. If a screen was placed some distance away from the two point sources, then we would expect to see a pattern on the screen consisting of alternating bright and dark lines, as shown below

Young's equation

In Young's equation, you can see that the spacing of the nodal lines in a two-slit interference pattern varies with each of the other three quantities - wavelength, distance from slits to screen, and separation of the slits. Specifically: • the longer the wavelength, the farther apart the nodal lines • if the distance to the screen increases, so does the spacing of the nodal lines • if the spacing of the slits is increased, the spacing between the nodal lines decreases

wave model

energy\= amplitude (or brightness)
to eject electrons from the metal they must be hit with sufficient energy
too dim\= no electrons ejected
bright enough\= electrons ejected
the brighter the light the more electrons will be ejected
brighter light should result in faster moving electrons

particle model

energy\= frequency (or colour) ex: E\=wavelength(f) "each colour has their own frequency that means they have their own energies"
red lowest
vi highest
threshold frequency \= frequency needed to eject an electron
colours above the threshold will always eject an electron regardless of brightness
colours below threshold will never eject an electron regardless of brightness
each photon ejects a single electron from the metal
brighter \= more photons \= more electrons rejected
colours with more energy will result in faster moving electrons

photoelectric effect

The photoelectric effect is the ejection of electrons from the surface of a metal by light.

summary

light interacts like a particle moves like a wave

What is the source of electromagnetic waves

vibrating charged particles

How do UV and IR compare

Infrared\=lowest
Ultraviolet\=highest

What is a photon

a particle of light

How long does light take to travel a distance of one light year

One year

Diffraction

The bending of a wave as it moves around an obstacle or passes through a narrow opening

Does diffraction spreading become more pronounced for narrow openings

Become more spread out the smaller gap in is travelling through

Is it possible for a light wave to cancel out another wave

Yes, deconstructive interference

What was Thomas young's discovery

Light and dark fringes form when light waves interfere

What causes light and dark wave fringes

Interference of light waves

Why is diffraction more evident for sound waves then for light?

Sound waves are longer therefore they diffract better

Why do sound waves diffract around buildings when light waves do not

Sound have a longer wavelength than light

Doppler effect

An observed change in the frequency of a wave when the source or observer is moving

Object moving towards has higher frequency

Object moving away has a lower frequency

Does bright light

photoelectric effect

Wave model predicts

Ejection of electrons from the surface of a metal by light, depending on the intensity of the light( frequency has no effect)

Particle model

Predicts that this should be determined by frequency and rather than intensity

Diffraction wave vs particle

Newton beloved light did not diffract because the diffraction was not observable at this time

Later evidence proved that light does diffract provided that it passes through a very very small opening

Wave model better explination

Interference of a wave particle vs wave theory

Refraction of light wave vs particle

Wave model - light faster in air than water
Particles model- light faster in water than air

Wave model is correct

Reflection wave vs particle

Particle theory- newton believed that hard spheres collide with hard surfaces

Does the photoelectric effect support the wave theory of light? The particle theory of light?

particle theory

rectilinear propagation

Light travels in straight lines
Speed of light must be extremely high
Mass of particles must be extremely small

How fast does light in a vacuum move

300, 000, 000m/s