Diffraction of light

              Diffraction of Light

When the light rays meet some obstacle in their path they will bend and travel. This process is called Diffraction of Light. Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665.

Wonders of Diffraction of light

1.        Solar glory at the steam from hot springs. A glory is an optical phenomenon produced by light backscattered (a combination of diffraction, reflection and refraction) towards its source by a cloud of uniformly-sized water droplets.

2.        Colors seen in a spider web are partially due to diffraction, according to some analyses.

3.        Different colors appear on C.D surfaces due to diffraction of light.

4.   When seen from far different colors appear near holes due to diffraction.

5.        Streaks appear behind hills during sunrise and sunset due to diffraction.

6.        When we see a lamp with half open eyes different colors appear due to diffraction of light.

Interference of light.

                   Interference of light

When two or more light rays overlap, their wavelengths and phases are changed.This is called Interference of Light. This is discovered by Thomas young

In physics, interference is the addition of two or more waves that result in a new wave pattern. Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency.

The term interference has a different meaning in radio communications.

Wonders of interference of light

1. Different colors appear when oil falls on the water.

2. Soap bubbles exhibit different colours

Scattering of light

                                   Scattering of light

When light rays fall on tiny dust particles present in the atmosphere they completely absorb the light energy and disperse the light in all directions. The velocity of light does not change during this process.Also refer  Lord Rayleigh scattering

 

Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of reflected radiation from the angle predicted by the law of reflection. Reflections that undergo scattering are often called diffuse reflections and unscattered reflections are called specular (mirror-like) reflections.

Wonders of Scattering of light

1. The sky appears blue to us due to scattering of light.

2. The sky appears black to astronauts as there is no scattering of light.

3. The objects are not seen clearly during winter due to more scattering of light.

4. Glass pieces appear as silver when seen from a far place.

Discovery of spectrum of light

Discovery of spectrum of light

 

From the earliest times, people have wondered about the nature of light.

By 300 BC, Greek scholars had begun to study and contemplate optical phenomena generating theories to explain vision, color, light, and astronomical phenomena. Many of those theories turned out to be wrong, but they did serve to inaugurate the science of optics.

During the second century AD, Ptolemy, a Greek astronomer based in Alexandria, Egypt, studied and wrote about many topics in science. He published five books about optics, but only one book has survived to the modern era. This series of works was dedicated to the study of color, reflection, refraction, and mirrors of various shapes.

Few other advances were made in optics until after 1000 AD. The Arab scholar Alhazan, a.k.a. Abu Ali Hasan Ibn al-Haitham, conducted the first serious study of lenses in Basra (Iraq). He studied refraction in lenses, and also carried out research on reflections from spherical and parabolic mirrors. His writings were the first to explain vision correctly, as a phenomenon of light coming into the eye, rather than the eye emitting light rays.

Roger Bacon, an English philosopher from the 13th century, postulated, but could not demonstrate, that the colors of a rainbow are due to the reflection and refraction of sunlight through individual raindrops.

NOTE: The term “light” is often extended to adjacent wavelength ranges that the eye cannot detect - to infrared radiation, which has a frequency less than that of visible light

and to ultraviolet radiation,which have a frequency

 greater than that of visible light.

 green light. If it was the prism that was

coloring the light, the green should come

out a different colour. The pure green light

remained green, unaffected by the

second prism.

             Newtons expirement on prism 




                                     William Herschel
                 The Discovery of Infrared Light

 Even before Newton’s famous experiments  (1665) with light people were using prisms to experiment with colour, and thought that somehow the prism colored the light. Newton obtained a prism, and set up his so that a spot of sunlight fell onto it. Usually, in such experiments a screen was put close to the other side of the prism and the spot of light came out as a mixture of colour. Newton realised that to get a proper spectrum you needed to move the screen a lot further away.

After moving the screen and achieving a beautiful spectrum he did his crucial experiment to prove that the prism was not colouring the light. He put a screen in the way of his spectrum, and this screen had a slit cut in it, and only let the green light go through.

Then he put a second prism in the way of the green light. If it was the prism that was colouring the light, the green should come out a different colour. The pure green light remained green, unaffected by the second prism

William Herschel (1738 - 1822) was one of the most important astronomers that ever lived. In 1800 he performed a famous experiment where he tried to measure the temperature of different colors of the spectrum by placing a thermometer on each colour. He found to his amazement that the hottest part of the spectrum was in a place where there was no colour at all. It was a spot beyond the red end of the spectrum. For the first time it was possible to talk about invisible light. This hot light became known as Infrared (below the red) because it was shown to have longer wavelength than visible light. Apart from its wavelength, Infrared has all the other properties of light.

Johann Ritter
The Discovery of Ultraviolet Light

After learning about William Herschel's discovery of infrared light, which he found beyond the visible red portion of the spectrum in 1800, Johann Ritter began to conduct experiments to see if he could detect invisible light beyond the violet portion of the spectrum as well. In 1801, he was experimenting with silver chloride, which turned black when exposed to light. He had heard that blue light caused a greater reaction in silver chloride than red light did. Ritter decided to measure the rate at which silver chloride reacted to the different colors of light. He directed sunlight through a glass prism to create a spectrum. He then placed silver chloride in each color of the spectrum and found that it showed little change in the red part of the spectrum, but darkened toward the violet end of the spectrum. Johann Ritter then decided to place silver chloride in the area just beyond the violet end of the spectrum, in a region where no sunlight was visible. To his amazement, this region showed the most intense reaction of all. This showed for the first time that an invisible form of light existed beyond the violet end of the visible spectrum. This new type of light, which Ritter called Chemical Rays, later became known as ultraviolet light or ultraviolet radiation (the word ultra means beyond).

Huygens wave theory of Light

     Huygens Wave theory of light

Huygens is remembered especially for his wave theory of light, expounded in his Treatise on light, 1678  The later theory of light by Isaac Newton in his Opticks proposed a different explanation for reflection, refraction and interference of light assuming the existence of light particles. The interference experiments of Thomas Young vindicated Huygens' wave theory in 1801, as the results could no longer be explained with light particles

The earliest comprehensive theory of light was advanced by Christiaan Huygens, who proposed a wave theory of light, and in particular demonstrated how waves might interfere to form a wavefront, propagating in a straight line. However, the theory had difficulties in other matters, and was soon overshadowed by Isaac Newton's corpuscular theory of light. That is, Newton proposed that light consisted of small particles, with which he could easily explain the phenomenon of reflection. With considerably more difficulty, he could also explain refraction through a lens, and the splitting of sunlight into a rainbow by a prism. Newton's particle viewpoint went essentially unchallenged for over a century

The Huygens–Fresnel principle  (named for Dutch physicist Christiaan Huygens and French physicist Augustin-Jean Fresnel) is a method of analysis applied to problems of wave propagation (both in the far field limit and in near field diffraction). It states that each point of a medium (disturbed by passing wave) becomes source of disturbance which propagates from this point in all directions indiscriminately. (Indeed, when a uniform medium is disturbed at some point then due to directional symmetry this disturbance propagates in all directions equally and without any path/direction discrimination). The interference (=addition) of all disturbances then results in a certain amplitude of detected wave (say in certain location at a screen). This simple yet very fundamental principle elegantly explains each and all wave phenomena such as diffraction, interference, etc

 

Newtons's Corpuscular theory of Light

                 Newton's Corpuscular Theory                  
                                   

In optics, the corpuscular theory of light, set forward by Sir Isaac Newton, states that light is made up of small discrete particles called "corpuscles" (little particles) which travel in straight line with a finite velocity and possess kinetic energy. Newton's corpuscular theory rules out the presence of any medium for propagation of light. In its contemporary incarnation, the theory of photons, this idea explains many properties of light, in particular the photoelectric effect. However, it fails to explain other effects, such as interference and diffraction. It was therefore superseded by the wave theory of light, later understood as part of electromagnetism, and eventually supplanted by modern quantum mechanics and the wave–particle duality.

Newton's theory remained in force for more than 100 years and took precedence over Huygens' wave front theory, partly because of Newton’s great prestige. However when the corpuscular theory failed to adequately explain the diffraction, interference and polarization of light it was abandoned in favour of Huygen's wave theory.Newton's

corpuscular theory was an elaboration of his view of reality

as interactions of material points through forces. Note Albert

Einstein' description of Newton's conception of physical reality

[Newton's] physical reality is characterised by concepts of space, time, the material point and force (interaction between material points). Physical events are to be thought of as movements according to law of material points in space. The material point is the only representative of reality in so far as it is subject to change. The concept of the material point is obviously due to observable bodies; one conceived of the material point on the analogy of movable bodies by omitting characteristics of extension, form, spatial locality, and all their 'inner' qualities, retaining only inertia, translation, and the additional concept of force.

Discovery of Photon

                                   
                     

       Discovery of photon

  

The photon is known as the quantum of electromagnetic radiation. In physics, a quantum is a basic indivisible unit or state that may be present or absent but never stronger or weaker.

In 1905, Albert Einstein published a paper describing his discovery of the photoelectric effect where a photon acts like a particle.

Einstein proposed that for some purposes light can be regarded as made up of photon particles.

In 1905, Einstein was the first to propose that energy quantization was a property of electromagnetic radiation itself. Although he accepted the validity of Maxwell's theory, Einstein pointed out that many anomalous experiments could be explained if the energy of a Maxwellian light wave were localized into point-like quanta that move independently of one another, even if the wave itself is spread continuously over space.

In 1909 and 1916, Einstein showed that, if Planck's law of black-body radiation is accepted, the energy quanta must also carry momentum , making them full-fledged particles.

The 1921 physics Nobel prize was awarded to Einstein in most famous for his theory of relativity, but it is his discovery of photons that is mentioned by the Swedish Academy.

This photon momentum was observed experimentally by Arthur Holly Compton, for which he received the Nobel Prize in 1927

Discovery of Quantum particles

In 1900 Max Planck made a profound discovery in modern physics / Quantum Theory. He showed (from purely formal / mathematical foundations) that light must be emitted and absorbed in discrete amounts if it was to correctly describe observed phenomena (i.e. Blackbody radiation).
Prior to then light had been considered as a continuous electromagnetic wave, thus the discrete nature of light was completely unexpected, as Albert Einstein explains;

About fifteen years ago [1899] nobody had yet doubted that a correct account of the electrical, optical, and thermal properties of matter was possible on the basis of Galileo-Newtonian mechanics applied to molecular motion and of Maxwell's theory of the electromagnetic field. (Albert Einstein, 1915)

Then Planck showed that in order to establish a law of heat radiation (Infra red light waves) consonant with experience, it was necessary to employ a method of calculation whose incompatibility with the principles of classical physics became clearer and clearer. For with this method of calculation, Planck introduced into physics the quantum hypothesis, which has since received brilliant confirmation. (Albert Einstein, on Quantum Theory, 1914)

Discovery of Speed ofLight

1.Speed of light
The speed of light(actual value) is 299,792,458 metres per second
By 1665 another famous scientist, Robert Hooke, noted that Descartes' and Galileo's experiments failed to prove the instant propagation of light.

What they did prove, Hooke felt, was, that if light took time to travel from one point to another, it did so "exceeding quick." If, for instance, light took two minutes instead of two hours to go past Earth to the Moon and back to Earth, the deviation from straight line of Sun, Earth, and Moon during an eclipse would be too small to be detected. A similar argument could be raised against Galileo. If light took a very small fraction of a second to travel 10 miles, its motion would appear instantaneous. In other words, Galileo's and Descartes' observations were inconsistent with a comparatively slow speed of light, but they still left open the question whether light took unimaginably short time to travel great distances or whether it took no time at all to travel any distance.

By 1676, the question whether light takes time to travel 

from one point to another, or whether it takes no time

at all had been resolved by the Danish astronomer,

Ole Roemer. Roemer provided the first clearcut proof

that light takes time to move from one point to another,

and the first reasonable estimate of its actual speed.

X rays

First, the discovery of X-rays

In late 1895, a German physicist, W. C. Roentgen was working with a cathode ray tube in his laboratory. He was working with tubes similar to our fluorescent light bulbs. He evacuated the tube of all air, filled it with a special gas, and passed a high electric voltage through it. When he did this, the tube would produce a fluorescent glow. Roentgen shielded the tube with heavy black paper, and found that a green colored fluorescent light could be seen coming from a screen setting a few feet away from the tube. He realized that he had produced a previously unknown "invisible light," or ray, that was being emitted from the tube; a ray that was capable of passing through the heavy paper covering the tube. Through additional experiments, he also found that the new ray would pass through most substances casting shadows of solid objects on pieces of film. He named the new ray X-ray, because in mathematics "X" is used to indicated the unknown quantity