## What is the particle theory of light?

The particle theory of light was the proposal made by Newton in 1704 in his treatise Opticks. This is the most basic light theory, in which light is thought to be made up of microscopic particles known as "corpuscles" and that's why this particle theory of light is also named as Corpuscular theory of light.

The most famous proponent of the particle theory of light was Isaac Newton. During his careful investigation into the properties of light in the 1660s, Newton found that white light is made up of a range of colors.

## Postulates of Newton’s Corpuscular theory of light

• Light is made up of little particles or corpuscles.
• As other masses like baseballs and planets, they obey the same laws of physics.
• These particles of light are  tiny such that any two intersecting beams do not scatter off each other.
• The speed of light is so great that we see the beam of light as a straight line.

## Failures of Newton's corpuscular theory

• The idea that light travels quicker via a denser medium than through a rarer medium was disproved.
• There is no reason to believe that the varied color of light is caused by variations in corpuscle size.
• Diffraction, interference, and polarization of light were not explained by corpuscular theory.

## Polarization and particle theory

It was Newton who first explained the fact that light could be polarized. For that, he used his particle theory. A mathematical particle theory of polarization was also developed by Étienne-Louis Malus in 1810. In 1812, Jean-Baptiste Biot also tried to prove that all known phenomena of light polarization can be explained by the same theory.  Polarization was considered as the proof of the particle theory during that time.

## Colour and particle theory

According to Newton, a beam of white light disperse into the colors of the rainbow where red light refracts the least, and violet light the most. According to him, the mass of the light particle varied with color. Since red light particles have more mass than violet they will be deflected lesser. When light particles cross any interface, they experience some force that is inversely proportional to their inertia. Since red light particles have more inertia, they will be deflected less by the same force than violet particles with less inertia.

## Different theories on the light before Newton's

Light, according to the quantum physics notion of wave-particle duality, will have a particle and will also have a wave nature depending on the conditions. Diffraction, polarization, and interference may all be explained if the light is assumed of as a wave. If the light is formed up of particles called photons, the photoelectric effect may be explained.

Light, according to Einstein, is a photon, and the movement of these photons is a wave. Louis de-Broglie proposed a groundbreaking theory in 1924 based on the notion that radiation might be considered to have a dual nature. If radiation behaves as a wave in some tests but as a particle in others, Louis de-Broglie proposed that particles like electrons should display wave nature in certain cases.

Since the waves do not travel in straight lines, Isaac Newton proposed that the geometric nature of light reflection and refraction could only be explained if light were made up of corpuscles. Newton was attempting to refute Huygens' idea that light was made up of waves.

### Wave theory of light

A Dutch physicist named Christiaan Huygens believed that light was made up of waves that oscillated up and down perpendicular to the direction of wave propagation, therefore he created a way to visualize wave propagation. The 'Huygens Principle' was born as a result of this.

As per the wave theory of light, a source of light throws out disruption in all directions. When these energy-carrying waves reach the eye, they stimulate the optic nerves, resulting in the feeling of vision.

There is a proposal made by Huygens - that every point where a disturbance meets becomes a spherical wave source. The sum of the secondary waves that arise as a result of the disturbance determines the form of the new wave. This light theory is known as the 'Huygens Principle'.

Huygens was successful in obtaining the rules of light reflection and refraction using the above-mentioned concept. Using wave theory, he was also able to describe how light travels in both linear and spherical directions. However, he was unable to describe the consequences of light diffraction.

The photon, Einstein's light particle, is named after him. The quantum theory of light is founded on the concept that light's energy is proportional to its frequency of oscillation (known as the frequency in the case of radio waves). The wavelength of light divided by the speed of light equals the oscillation frequency.

#### Reflection theory

The repulsion between the corpuscles and the reflecting surface causes the angle of incidence to equal the angle of reflection when the corpuscles impact the reflecting surface.

#### Refraction theory

When corpuscles get close to the reflecting surface, they are drawn to it. When they transit from a denser to a rarer medium, their speed rises and they shift direction.

Some of the characteristics of light are as follows:

• When dealing with light waves, we deal with the sine waveform. One entire 360-degree sweep will be the length of the waveform.
• Light waves have two basic properties: wavelength and frequency.
• Light waves have wavelengths that are measured in nanometers.
• The frequency will be termed as the number of waves traveling through the given place in a second.
• The following is the equation that describes the bond between wavelength and frequency:

$f=\frac{1}{T}$

Here, f is the frequency, T is the time period.

• Photons, as per the great scientist Einstein, are tiny packets of energy that make up light. The formula given by the Planck's is mainly used to calculate the energy of the photon and shows that it is proportional to the frequency of light.

Here 'h' will be the Planck's constant is equal to 6.62607004 × 10-34 m2 kg / s

### Photoelectric effect

Electrons are released from a metal's surface when the light of a specific frequency strikes it. This phenomenon is known as the photoelectric effect. Photoelectrons are released electrons.

The photoelectric effect is governed by three laws:

1) After a specific frequency, known as the threshold frequency, the emission of electrons from the surface ceases.

2) The intensity of incoming light is exactly proportional to the number of electrons released from the surface.

3) The kinetic energy of the released electrons is determined by the incident light's frequency and is unaffected by its intensity.

### Result of photoelectric effect experiment

The photoelectric effect experiment suggested that light possesses energy as packets and other forms of electromagnetic energy comprised of the quanta of energy, the photon is nothing but constituents of energy. As a result, it aids their understanding of light's particle nature.

The speed of light is equal to  and it always moves in a straight path, creating shadows in the process.

## Context and Applications

A particle model may readily explain many known characteristics of light. It is studied under various courses such as

• Bachelors in Science in Physics
• Masters in Science in Physics
• Bachelors in Technology
• Masters in Technology

## Practice Problem

1. Which of the following theories accounts for the photoelectric effect?

a) Electromagnetic theory
b) Magnetic theory
c) Electric theory
d) Wave theory

Explanation- According to electromagnetic theory, the photoelectric effect is caused by the transfer of energy from light to an electron. Variations in the kinetic energy of the electrons released from the metal will be induced by changes in the intensity of light, according to this theory.

2. What causes light to move in waves?

a) It is made up of tiny packets called photons.

b) It is made up of an electric field.

c) It is not made up of photons.

d) Because of its electromagnetic properties

Explanation- Light moves as a wave because of its electromagnetic nature. Maxwell's equations, which indicate that changing magnetic fields give an upsurge to electric fields and changing electric fields give rise to magnetic fields, make this even easier.

3. What principle states that the amplitude of light is determined by its intensity?

a) Huygens principle
b) Classic wave theory
c) Louis de – Broglie hypothesis
d) Einstein’s particle theory

Explanation- The basic premise is the wave theory. As per the classical wave theory, the amplitude of the wave is dogged by the strength of light. As a result, a higher light intensity causes the electrons in the metal to vibrate more violently and expel out with more kinetic energy.

4. In which of the following four spectrum areas does the photon have the most energy?

a) Visible rays
b) Violet
c) Black
d) White

Explanation- As per the equation given by Planck’s:

$E=\frac{hc}{\lambda }$

The wavelength is inversely proportional to the energy. Because a photon in the violet range has the shortest wavelength, it has the most energy.

5. What will be the energy of a photon whose wavelength is $\lambda$?

a) $hc\lambda$

b) $hc$

c) $\lambda$

d) $\frac{hc}{\lambda }$

Explanation- Since the formula for the energy of a photon is given by the equation $E=\mathrm{hv}$.

As $v=\frac{c}{\lambda }\phantom{\rule{0ex}{0ex}}$

Therefore,

$E=\frac{hc}{\lambda }$

• Refraction by particle theory of light
• Reflection by particle theory of light
• Wave theory
• Crompton effect

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