For years there have been debates between the luminaries in science over the nature of light. Some say that light acts as a wave, while some say that light behaves as a particle. However, if we observe deeply, then light has dual nature of radiation. Sometimes the light portrays a wave nature, while light displays a particle nature in some conditions.

The dual nature of radiation was established by performing various experiments. During these experiments, it was observed that light behaves as a wave during the phenomenon of diffraction, interference and reflection. While during the photoelectric effect, light behaves as a particle. An application of the particle nature of light can be seen in the solar panels, which work on the principle of the photoelectric effect.

Wave nature of light

Radiation is a type of energy that can be transferred from one point to another in space. A light wave consists of two components which are crests and troughs. Some points to remember about waves are as follows: 

  • The distance between two consecutive troughs or crests is called wavelength. The wavelength is defined using the SI unit angstrom.
  • The number of waves passing through any point in unit time is known as frequency. The frequency of waves is defined by using cycles per second or hertz.

 The distance travelled by the wave in one second is called the velocity of the wave and is denoted by ‘c’. The velocity of the light wave is 300,000 kilometres per second. 

 The different phenomena display the wave nature of light.

There are primarily three phenomena that display the wave nature of light are as follows:

  • Reflection of light 

One of the phenomena contributes towards the dual nature of radiation and in turn, the wave nature of light. All waves in the universe undergo reflection. The reflection of light waves of a mirror surface results in the formation of the image. One of the common properties during reflection is that the angle at which the wave approaches a flat surface is equal to the angle at which the wave leaves the flat surface. This characteristic is observed for water waves and sound waves as well.

  • Refraction of light

Like reflection, every wave in the universe undergoes refraction when they pass from one medium to another. Refraction is a phenomenon where the direction of a wave changes suddenly when it passes from one medium to another. The speed at which a wave will bend depends on the relative speed of the media. A wave will move in a certain way when it passes from slow media to fast media. In contrast, the wave will turn in the opposite direction when it enters from fast media to slow media. 

  • Diffraction of light

Till now, we have gained knowledge about two phenomena that contribute to the dual nature of radiation. The third phenomenon which contributes to the dual nature is diffraction. Diffraction is the phenomenon where the light waves move along the surface of the obstacle in their path. Similar to light, water waves and sound waves have the capability to move around the corners of an obstacle. An example of diffraction is shadow formation, where the light is blocked by the object and the rays then move along the corners of the object to form a shadow.

Particle Nature of Light

German physicist Heinrich Rudolf Hertz performed an experiment in the year 1887, which was named the photoelectric effect. This experiment result shows that light has a particle nature. 

Photoelectric effect

  • In the phenomenon of the photoelectric effect, when the light strikes the surface of a metal, electrons get ejected from the surface of the metal. The photoelectric effect is also known as the photoemission effect. The electrons ejected during the photoelectric effect are called photoelectrons. 
  • The reason behind the emission of electrons from the surface of the metal was explained by 19th-century scientists. According to them, the oscillating electric field of the light was heating the electrons on the metal surface and was causing them to vibrate. The vibration of electrons eventually led them to get free from the metal surface. 

 

  • During the photoelectric effect, the kinetic energy of the photoelectrons increases with the amplitude of light.

 

  • The rate at which an electron emits from the surface is directly proportional to the light frequency. Thus as the frequency of the light increases, the rate at which electrons  emitted from the surface of the metal also increases.

 

  • Moreover, the kinetic energy of the photoelectrons increases with the frequency of light. 

 

  • With the increment in light frequency, there is no change in the electric current. Thus the electric current remains constant.

 

  • The increment in electric current doesn’t depend on the frequency of light. Rather, it depends on the amplitude of light. Hence, as the amplitude of light increases, the electric current produced also increases.

 

  • The kinetic energy of the photoelectrons remains constant with the increase in the amplitude of light.

 Conclusion

The dual nature of radiation has been an important discovery in mankind. Today, suppose the photoelectric effect wasn’t discovered, then in that case, the world wouldn’t have thought about using sunlight for generating electricity. Thus the idea of solar energy hadn’t been born. Therefore, having in-depth knowledge about the dual nature of radiation can prove to be fruitful in designing different sustainable means for producing energy.