What Is Light?

Light is a phenomenon that allows living things to see as a result of a beam hitting objects or reflecting directly after it leaves the light source.

Objects that emit light on their own are called light sources.

Light refers to visible light with its widespread use.

Electromagnetic waves that can be perceived by the human eye, that is, visible to humans, are called visible light.

The wavelength of visible light is between 380 nm and 760 nm. In the electromagnetic spectrum, this area lies between infrared and ultraviolet.

The wavelength of visible light is shorter than infrared and longer than ultraviolet.

Whether light is a wave or a particle throughout history has been the subject of much debate.

Newton has several theories that light is a particle and Huygens has developed the claim that light is a wave.

In modern physics, while light has the property of wave, it is also known to have particle-like properties.

Wave Theory

As shown in Figure 1, as the light moves through a vacuum, it consists of an electric field and a magnetic field that intersect each other at right angles. The distance between successive peaks of the electric field or magnetic field is the wavelength.

Figure 1 – Electric and Magnetic Field Formation in Light

The experiment that Young concluded that light is in the form of a wave is well known. As shown in Figure 2, the monochromatic light emitted from a light source (L) passes through a single slit and then through two slits, S1 and S2. As a result, varying patterns of bright and dark stripes appear on the screen.

Figure 2 – Young Experiment

This can be explained by considering S1 and S2 as light sources in phase with each other. It reinforces each other at the points where the waves are in phase, and cancels each other at points outside the phases of the waves.

It can be said that if this paper surface are considered as the surface of a body of water, and the holes are considered as water-permeable sections, the waves moving from left to right will behave in the same way.

Electromagnetic waves are referred to under different names in accordance with their wavelengths, as shown in Figure 3.

Figure 3 – Classification of Light by Wavelength

Light usually refers to electromagnetic waves in the range, which includes infrared radiation and ultraviolet radiation, but in some cases it only points to visible light.

A light with a wavelength ranging from about 400 nm to 800 nm is called visible light and is the light that people can see with the naked eye. Visible light can be defined as the type of light people are familiar with, due to the ability of people to see.

For example, light at a wavelength of 470 nm is blue, light at a wavelength of 540 nm is green, and light at 650 nm is red.

Particle Theory

Let’s look at the particle-like behavior of light. The developments that helped define this behavior were made by a series of experiments on the photoelectric effect realized at the end of the 19th century and the beginning of the 20th century.

The results of these experiments revealed that light cannot be explained by thinking as a wave, but can only be explained by thinking it as a particle.

Figure 4 shows the basic concept of photoelectric effect. The figure shows how electrons spread after light hits a metal surface. The emitted electrons are called photoelectrons. If the wavelength of light decreases, the number of emitted electrons does not change, but the energy of the electrons increases.

Figure 4 – Concept of Photoelectric Effect

Particle-like properties were accepted by the results of the experiments on the Compton effect and its combination with other experiments.

We looked at the dual nature of light with a mixture of wave and particle-like properties. It may seem strange that light has these two opposing features, but this is the way light is defined in modern physics.