# Nature of Light

About the nature of light there are two/three theories have been recognized.

## Particle Theory

1st theory is the particle theory, which is presented to explain. The phenomena that were observed concerning light.

According to particle theory, light is a particle with mass, traveling at very high speeds. Light particle can bounce off shiny materials producing reflected beams. For light sources it was believed that light sources actually generated large quantities of these light particles.

Main drawback of this theory was that many phenomena of light could not be explained by this particle theory such as refraction of light when it passes through optically transparent materials.

## Quantum Theory

The quantum theory of light is the theory relating to light propagation and generation. It is also called photon theory. This theory view that light is being made up of packets of energy called photons. The energy contained in each photon is related to the frequency of light by equation.

Ep = h x f

Note that this theory will be used in analyzing & enplaning light generation and detection. It is also helpful in describing the transformation of light into electron current and vice versa.

## Dual Nature of Light Theory

Another theory says that light is a wave traveling with characteristics similar to those of water waves. This theory also fails because many but not all phenomena of light can be explained using this theory. Therefore a dual nature of light theory was proposed up to now it is considered to be true. This theory suggests that light travels in small packets of energy called photons. Even though photons are bubbles of wave energy, they have momentum. Like particle of mass. Thus light is wave energy traveling with some of the characteristics of a moving particle. The total energy transmitted as light is the sum of the energies of all the individual photons emitted. When light travels away from a source then its acts similar to the waves that move across a quiet pond after a small stone is dropped into the water. The waves from a pattern of concentric circles that gets progressively larger as they travel away from the source.

The concept of light wave traveling away from a point source (say candle) can be explained by diagram.

## Characteristics of Light Waves

This light wave defines some very important characteristics that are:

1. ### Amplitude:

The light from the center line to the peak in either the positive or negative direction is called amplitude of the wave.We can also say that amplitude is the strength of light or the amplitude is related to the strength or intensity of light. The greater the amplitude the greater will be the intensity/strength of the wave. The amplitude or intensity of wave is usually measured in units of volts per meter.

2. ### Frequency:

The number of oscillations or cycles per second is referred to as frequency of the light denoted by ‘f’. The unit of frequency is Hertz (Hz). Remember that color of light or portion of the electromagnetic spectrum (to which light belongs) depends on the frequency or rate at which the light oscillates. Light at any particular place in the spectrum oscillates at a fixed but rapid rate.

3. ### Period:

The amount of time required for the one complete oscillation or cycle of the specific light wave is called period or time period of the wave, denoted by T. The unit used for period of the wave is second.
Relationship between Period and Frequency is that:
Period is the reciprocal of frequency i.e. T = 1/ f
Frequency is the reciprocal of the period i.e. f = 1/T

4. ### Wave length:

The distance that light travels during the one period of the light wave is called its wave length, denoted by λ. usually wave length is measured in meters or its subunits.
Note that all electromagnetic wave travels at the same fixed velocity approximately
C = 186000 miles per second

Since 1 mile = 1609 meters
So
C = 186000 X 1609 = 299274000 = 2.993 X 108 = 3 X 108 meter/secRelation between Velocity, Frequency and wave length is

C = fλ

Or

f = C/λ

Since C is constant therefore this equation shows that the frequency of a wave is inversely proportional to the wave length. That is the higher frequency waves should have shorter wavelengths and vice versa.

5. ### Diffraction:

When light passes through a narrow opening then it tends to spread out and bends in different directions. Now the bending of wave as it passes through a narrow opening or along the edge of the barrier is called diffraction.

By figure the diffraction can be shown as:

6. ### Interference:

When two or more waves meet at the same place and time. Their amplitudes add/subtract to each other s then it is called interference. Interference can be either constructive or destructive. When a crest of one wave meets a crest of another wave then their amplitudes added to give large positive amplitude. Similarly if trough meets with trough, then it results in large negative amplitude. According to these situations when light waves meets, then bright areas of light are produced. This is called the constructive interference as shown in diagram below.

The crest of one wave meets the trough of another wave than amplitudes tends to cancel each other with the resultant wave being the difference between the two original waves. According to this situation when light waves meets than dark areas are produced. This is called destructive interference as shown in below diagram.

## Measurement of Light

For the measurement of light there are two primary systems uses, which are

1. ### Photometry or Photometric Measurement System

This is the measurement of visible light as it appears to a human being with normal vision. It is the oldest method for measuring light. The photometric measurement system is based on vision characteristics of human eye. This system includes units such as candles and lumens and so therefore primarily used for illumination measurements.

The eye in the system responds differently to each wavelength that enters to it. The normal eye response can be shown as:

clearly greater to response is approximately 0 to 555 nm and steadily decreases in either side. The human eye cannot see colors or detects wave length less than 400 nm or greater than 700 nm so this 400 to 700 nm is called range of visible light spectrum.

This is the measurement of electromagnetic energy (radiant energy) emitted by a source or the energy falling on a detector. It is comparatively new method for measuring light than photometry. It is based on responses from detectors and so therefore in-sensitive to wavelength.

Intensity of wavelength in light or infrared ranges can be measured with ease and accuracy. Thus 1 watt of light power would be measured as watt regardless of its wavelength.

## YONG Two SLIT EXPERIMENT

The young two slight experiments is a classical experiment that demonstrate the interference of light waves, sometimes it is also called young double slit experiment.

In his experiment the young used a single light source with a small aperture to allow the light to exit and highly filtered light to minimize the number of wavelengths emitted, because common light sources have light with number of wavelengths.

The filtered light was then used to illuminate two narrow slights which were placed together. The waves of light heaving the slits spread out and over lapped, causing constructive and destructive interference. Young observed the bright and dark regions on sensitive photographic film.

Experimental setup of young experiment can be shown by diagram as

If Mr. Young used laser, then his experiment would be much easier and best explanatory of interference because laser produces a narrow beam of light at a specific wave length, which is main requirement for Young 2-slit experiment.

## Electromagnetic Spectrum

From the spectrum it is clear that color is actually the result of wave length and our eyes responds to a narrow range of wave lengths.

The entire electromagnetic spectrum runs from very long waves to extremely shot waves. Waves that are longer than visible light include alternating current waves at 60Hz, radio waves, television waves, micro waves and infrared waves. On the other hand waves shorter than visible light are ultraviolet rays, gamma rays and cosmic rays.

## Important to Note:-

• Shape of waves leaving point source (it is called point source because of its small size)
• Shape of waves leaving extended source ( it is called the extended source because its small size is elongated is straight.
• Blue light has shorter wave length than red lights.
• Angstrom = 10-10 meter is the unit of wave length.
• Speed of light is 3 X 108 m/s = 186000 miles / sec
• T = 1 / f  or f = 1 / T
• Visible laser operate at wave lengths of between 400 nm and 700 nm.
• 1 mile = 1609 meters.