Unit of Light

Introduction

Everything around us is visible to our eyes because of light only. Our bodies are completely useless in the absence of light called darkness. Sunlight also acts as a renewable source of energy to generate electricity using solar cells.

But, the main question, which was a matter of discussion among various physicists was – “What this light is made up of or what does it constitute?”. Many theories were given by scientists to explain the nature of light but failed. However, at last, the Quantum theory of light was proposed which superseded the drawbacks of previous theories. Let’s discuss this more in detail.

What is Light?

Many theories were given by physicists to describe the nature of light. Let’s discuss them one by one -

The Corpuscular Theory

The Corpuscular theory was given by Sir Issac Newton. Newton proposed that very tiny particles called corpuscles constitute light. When the light propagates these tiny particles travel with very high velocity. The velocity of light, according to the corpuscular theory, varies with changes in the medium’s density. Newton argued that the color of the light depends upon the individual corpuscle size.

However, Newton’s corpuscular theory faced some serious drawbacks. It was not able to explain the interference, diffraction, and polarization of light. Moreover, Newton mentioned in his theory that the velocity of corpuscles increases with an increase in the temperature of the source but no such outcomes were reported.

Huygen’s Theory

Huygen’s proposed that the light waves are both mechanical and transverse.

Huygen assumed that the complete space is filled with a low-density homogeneous and isotropic material called Ether, which acts as a medium for light waves to travel. Huygen’s theory explained interference and diffraction but failed to explain phenomena such as Compton and the photoelectric effect. Later, it was found out that no such medium as ether exists.

Maxwell’s Electromagnetic Theory

According to Maxwell, light is a propagating transverse wave consisting of Electric and Magnetic fields. Both, the Electric field and the Magnetic field oscillate in a plane perpendicular to each other and both of these planes are perpendicular to the plane in which the light propagates. Maxwell’s electromagnetic explanation faced a serious drawback when it was not able to explain the blackbody radiation.

Max Planck's Quantum theory of Light (Correct theory)

The Quantum theory by Max Planck is a modified version of Maxwell’s electromagnetic theory. Planck proposed that light consists of very small discrete particles of energy called quanta or photons. These quanta or photons travel in the form of waves of a specific frequency (say ν). The energy of each quanta is given by -

$$\mathrm{E=h
u}$$

where - h is Planck’s Constant and ν is frequency. The electromagnetic theory by Maxwell suggested that the energy absorbed by the body or emitted from the source of light is continuous. However, Planck argued that the absorption and emission are discontinuous (in the form of packets). Quantum theory was able to explain the blackbody radiation, photoelectric effect, and Compton effect. Planck’s theory was able to give a correct explanation of the phenomena.

After discussing so many theories we can finally give one proper definition of light - Light is electromagnetic radiation that can be perceived by the human eye. This light is also called visible light (400 nm<λ<700 nm).

Properties of Light

The properties of light are discussed below -

• Medium of Propagation: Light does not require any medium to travel. It travels in a straight line.

• Fastest known Speed: The speed of light is $\mathrm{c=3\times 10^8 \:m/s^2}$. No object can move faster than the speed of light i.e. the speed of light is the maximum speed an object can move with.

• Incapability of Vector addition Laws: The laws of vector addition do not apply to the speed of light. One might think that we can go further beyond the speed of light by the method of vector addition. However, the resultant velocity will always be equal to $\mathrm{3\times 10^8 \:m/s^2}$.

• Reflection Property: A ray of light when falls on the reflecting surface reflect obeying the laws of reflection.

• Refraction Property: A ray of light bends when it travels from one medium to another. It also obeys snell’s law.

• Dispersive Property: A ray of light passing through the triangle-shaped prism made up of glass, splits into its constituent color wavelengths. This phenomenon is called dispersion.

Figure 1. Dispersion of Light.

• Polarizing Property - The electric field vectors in a ray of sunlight vibrate continuously in all the directions perpendicular to the propagation of light. If this light passes through a polarizer, we can sustain these electric field vibrations in one plane only. This process is called the polarization of light.

Figure 2. Polarization of Light.

• Diffractive Property - The bending of light around the corners or through a slit (provided that the size of slit >= lights wavelength) such that the light spreads into the region of shadow formation and illuminates it. This phenomenon is called diffraction.

Units of Light

Light is not a physical quantity that can be measured. However, some of its parameters can be measured which can tell us about the nature of light. Let’s discuss them -

• Candela (cd): The SI unit of luminous intensity is candela. Luminous intensity is the measurement of luminous power emitted by the source per unit solid angle.

• Lumen (lm): It is the SI unit of luminous flux. Luminous flux is the rate at which the light is emitted from the source. On candela holds a total luminous flux of 12.57 lumen.

• Lux (lx): It is the SI unit of illuminance. Illuminance is the measure of flux per unit area. one lux equals one lumen per square meter.

$$\mathrm{1\:lx=1\:lm/m^2=1cd\:sr/m^2}$$

Property	Unit
Luminous intensity	Candela (cd)
Luminous flux	Lumen (lm)
Illuminance	Lux (lx)

Conclusion

Light is an electromagnetic wave that possesses both wave and particle nature. The Quantum theory of light was given by Max Planck. The speed of light is the fastest speed with which an object can move. Reflection, refraction, diffraction, and polarization are some of the phenomena associated with light. Candela, Lumen, and lux are some of the units used for the measurement of the properties associated with light.

FAQs

Q1. Name the phenomenon involved in the formation of the Rainbow.

Ans. Total Internal Reflection of the light.

Q2. What is the rest mass of the photon?

Ans. Zero.

Q3. What is Light year?

Ans. The distance traveled by the light in one year is called a light year.

$$\mathrm{1\:light\:year = 5.88\:trillion \:miles}$$

Q4. What is Tachyon?

Ans. A Tachyon is a hypothetical particle that travels faster than the speed of light. However, no such particle was found in experiments (although scientists predict that it exists).

Q5. Explain the formation of Shadow.

Ans. The light then strikes the human body and gets reflected away from it leaving a dark shadow (depending on the shape of the body) behind it.