Is Light a Wave or a Particle?

Is light a wave or a collection of particles?

The answer is both, Light can be modelled as an electromagnetic wave or a stream of photons

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Light is an electromagnetic wave as it travels through a vacuum of outer space to transfer its energy from one location to another. ( The physics classroom, 1996-2014)

Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of 3.00 x 108 m/s (a speed value commonly represented by the symbol c).

The Wave theory of was originally developed by Huygens

His theory said that light travelled through space by travelling through a medium known as the ether, a mystical weightless substance, which exists as an invisible entity throughout air and space. Huygens believed that ether vibrated in the same direction as light, and formed a wave itself as it carried the light waves. Huygens’ Principle described how each point on a wave could produce its own wavelets, which then added together to form a wavefront.

Light also displays certain properties of waves such as reflection, refraction and diffraction. These occur when a wave reaches the end of the medium.

Reflection is when the light bounces off an obstacle. The most common example is the reflection of light waves off mirrored surface results in the formation of an image. Another characteristic of wave reflection is that the angle at which the wave approaches a flat reflecting surface is equal to the angle at which the wave leaves the surface. Reflection is observed in water and sound waves and is also observed in light. (The physics classroom, 1996-2014)

Refraction is when a wave passes from one medium to another medium. When the wave crosses the boundary between the two mediums the direction of the wave changes and the path of the wave is essentially bent. The direction of the bend depends on the speed at which the wave is moving through the mediums, if it is moving from a fast medium to a slow medium it will bend one way and going from a slow medium to a fast medium it will bend the opposite way. The angle of the bend will depend on the actual speeds of the two mediums. (The physics classroom, 1996-2014) Refraction occurs in sound and water waves. It can be seen in light in the refraction of light through a glass or a mirage is an optical illusion caused when light waves moving from the sky toward the ground are bent by the heated air

Diffraction involves a change in direction of waves as they pass through an opening or around an obstacle in their path. Water and sound waves have the ability to travel around corners, around obstacles and through openings.

When light encounters an obstacle in its path, the obstacle blocks the light and tends to cause the formation of a shadow in the region behind the obstacle. Light does not exhibit a very noticeable ability to bend around the obstacle and fill in the region behind it with light. Nonetheless, light does diffract around obstacles. In fact, if you observe a shadow carefully, you will notice that its edges are extremely fuzzy. Interference effects occur due to the diffraction of light around different sides of the object, causing the shadow of the object to be fuzzy. This is often demonstrated with a laser light and penny demonstration. Light diffracting around the right edge of a penny can constructively and destructively interfere with light diffracting around the left edge of the penny. The result is that an interference pattern is created; the pattern consists of alternating rings of light and darkness. As can be seen in this photo.

How can we test if light is a wave?

An experiment called the double slit experiment was designed by the scientist Thomas Young. It required a light source, a thin card with two holes cut side by side and a screen.

To run the experiment, Young allowed a beam of light to pass through a pinhole and strike the card. If light contained particles or simple straight-line rays, he reasoned, light not blocked by the opaque card would pass through the slits and travel in a straight line to the screen, where it would form two bright spots. This isn’t what Young observed. Instead, he saw a bar code pattern of alternating light and dark bands on the screen. To explain this unexpected pattern, he imagined light traveling through space like a water wave, with crests and troughs. Thinking this way, he concluded that light waves traveled through each of the slits, creating two separate wave fronts. As these wave fronts arrived at the screen, they interfered with each other. Bright bands formed where two wave crests overlapped and added together. Dark bands formed where crests and troughs lined up and canceled each other out completely. This proves the theory that light is a wave.

The particle theory of light

This theory was developed by Sir Isaac Newton and Albert Eistein and said that the energy emitted by light travelled as small minimum quantities or packets of electromagnetic energy called photons

According to the photon theory of light, photons . . .

move at a constant velocity,c= 3 x 108m/s (i.e. “the speed of light”), in free space
have zero mass
carry energy and momentum, and The energy of each photon is directly proportional to the frequency of the radiation.
can be destroyed/created when radiation is absorbed/emitted.
can have particle-like interactions (i.e. collisions) with electrons and other particles.

(Jones, 2014)

The photoelectric effect supports the particle theory of light

The photoelectric effect is the phenomena where a metal releases electrons when exposed to light or electromagnetic radiation of a particular frequency. These emitted electrons are called photoelectrons.

So basically this is how it works

Every metal has a threshold frequency this is the minimum frequency of light or electromagnetic radiation that will causes the release of electron from the surface of a metal. This is because this frequency will supply the minimum amount of energy needed to overcome the force of attraction between the metal and the electron this is known as the work function of the metal

What happens is when a light photon with energy equal to hf (h being planks constant and f being greater than or equal to the threshold frequency) strikes a metal surface all the energy of the photon is transferred to the electron. (Louw, 2014)

Another way to prove the particle theory is the Atomic Spectra

Emission spectra are produced when light from a light source such as a filament or a gas discharge tube is observed through a diffraction grating or a prism

Continous spectra are produced when light from a glowing solid state material such as a filament passes through a triangular prism.

This is because Energy transitions take place this means that in an atom electrons have specific energy levels as you can see

An electron is excited to an unstable higher energy level and then falls back to a lower level. Photons with a specific energy and frequency are emitted.

In such dense substances, the atoms are so close together that many different transitions of electrons can take place and light photons of any wave length can be radiated. This results in the different colours being emitted.

The production of the line emission spectra

A glowing gas is obtained by heating it or passing an electric current through it

The glowing gas in the discharge tube is then observed through a diffraction grating ( a diffraction creating is a transparent plate on which parallel lines are ruled very closely i‚±600 per mm

The electrons in an atom have only specific energy levels

When an atom is in a gaseous state, electrons can be excited to a higher energy level through heat or electricity

The electrons absorb a discrete amount of energy for a specific jump for example between energy level 3 and 1

The excited state is unstable so the electron falls back almost immediately to a lower level and emits light energy in the form of photons

The energy of an emitted photon equals the energy difference between the higher and lower energy levels. The energy is emitted as light with a definite frequency and specific colour. Each element will produce a different line spectra due to the differing energy levels

Atomic spectras that are observed can only be explained if light was a stream of particles.

It can therefore be stated that light as dual particle-wave nature


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