The Fascinating Physics of Light

When the night comes we usually flick on a switch and our rooms are bathed in light. When driving and it gets dark we put on the headlights to improve vision. The light is seen in both displays, signboards, TVs, lasers, and virtually in a lot of applications. Just pause for one second and imagine the world without light. Imagine the light-based technologies all gone. You would agree with me that it would make for a boring lifestyle.

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Light is an intriguing phenomenon, whether the source is from the Solar System's stars, the sun or a planet from galaxies thousands of light years from us. Or the neon signs from the friendly neighbour's shop, the light forms our everyday part of life. We have it as a communication tool (fibre-optics), it is a great navigation element, we learn with light.

The importance of light and light-based technologies made the United Nations (UN) declare 2015 the International Year of Light (IYL 2015). The observance planned to raise awareness on light science plus its usefulness in today's society.

The light that we see (visible light spectrum) is just a fraction of the wavelengths that we have.

The human brain can only "decode" or see about 0.0035% of the entire electromagnetic waves comprising of radio waves to gamma rays.

That is not bad. Imagine if the human eye can see the radio waves the same way it sees the light. Any time you look at any technology, it will look like light up like a Christmas tree. We would be able to see the blackholes, the Sun, Jupiter and Earth. But the Mars would be invisible; the Red Planet lacks magnetic field. So, it is not so bad the eye could only manage light in the range of wavelength of between 400 to 700 nanometers.

The light exists as an electromagnetic (EM) spectrum. The EM spectrum encompasses all frequencies or spectrum of the different range of electromagnetic radiation of lights that exist; including their various photon energies and wavelengths.

The Duality of Light, An Old Tale of the Elephant and Blind Men

Light is a fascinating subject of study. It behaves (possesses) the qualities of both a particle and a wave.

It reminds one of the old Indian tale of an elephant and the blind men. They all touched a different part of the elephant. The one that felt the trunk said the elephant looks like a snake. The one that fell against the broad side said the elephant is built like a wall. By the end of the day, they have different conception to what they thought of the elephant depending on the part touched.

The light behaves similarly, depending on the scale. It could be both a wave and a particle.

The photons, which is the smallest unit of light energy, could when travelling depict the actions of both a wave and a particle.

The inherent trait of interference of light shows it is a wave. Light when travelling, can through interference, build up or down.

A coherent light source such as that produced by a laser diode is passed through two parallel slits. A screen is placed behind the slit. The wave nature of light will cause interference that will produce bright and dark shades on the screen.

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The light could also be seen as a particle. We could see this when we observe the photoelectric effect. The photoelectric effect is the ejection of some electrons, from specific metals, when they are exposed to light.

The photons are quantised. The quanta of energy travel as discrete units behaving the same way as a particle.

Planck, the German scientist, was the first to propose that energy is not continuous ( as erroneously stated by the classical physics wave theory) but quantized.

Einstein pushed Planck's observation further by noting that light must be made up of quanta.
That is, they are broken into smallest multiple units of energy referred to as photons.

Scale is King

Here the next thing to ponder on is the model (wave or particle) to use in describing an object. The answer depends on the scale of measurement.

The wave model is often the choice to use while measuring an object with similar wavelength to that of the light. Or we can use the particle model if the object's energy is close to that of a photon.

The Einstein postulated that the energy of a photon, E= hv.

where h= Planck's Constant
v= frequency of light

This implies that low frequency photons, like the radio waves, emits lower energies when compared with photons that have higher frequencies like the x-rays.

Checking Presented Evidences

Checking on the two models which could be used to describe light we have the following results:

1. Reflection
   Do they bounce off an object?
   Particle Model: Yes
   Wave Model: Yes
2. Refraction
   Do they bend through mediums as they travel?
   Particle Model: Yes
   Wave Model: Yes
3. Interference
   Do they interference occur when they move?
   Particle Model: No
   Wave Model: Yes
4. Diffraction
   Do they bend through a medium?
   Particle Model: No
   Wave Model: Yes
5. Polarisation
   Can the amount of them be reduced as they travel, say, through a slit in a medium?
   Particle Model: No
   Wave Model: Yes
6. Photoelectric Effect
   This is the time the wave model failed, but that of particle worked.
   Particle Model: No
   Wave Model: Yes

Conclusion

So the duality of light is born. The model to use is dependent on scale. If the object is comparable to the wavelength, the wave model is employed. But if the energy is comparable to the size, the particle model is used.

The appropriate model for spatial interaction with matter is a function of scale.

References

1. What Would the Night Sky Look Like If We Could See in All the Wavelengths at Once?

2. The photoelectric effect suggest that light has particle type properties

3. The failure of classical physics and the advent of quantum mechanics