Light: Particle or Wave?Sir Isaac Newton advocated the particle theorythat light consists of tiny particles of matteremitted by a source and spreads outwards instraight lines called rays.Christian Huygens, Dutch mathematician andscientist, theorized that light consists of aseries of waves with their wave fronts at rightangles to the path of the rays
Five properties of light weredeveloped: a. rectilinear propagation of light b. Reflection of waves c. refraction of light d. interference e. diffraction
Rectilinear propagation inside an unchangingmedium.
Reflection of waves from the surface of the medium
Refraction, thebending of waveswhen a change ofspeed takes place
Interference, when two waves of oppositephases are superimposed
Diffraction, the bending of waves aroundcorners
During the time of Newton andHuygens, only the first threewere recognized and observed. a) Rectilinear b) Reflection c) Refraction
Spite of scientific discussions in defense of thetheories, the problems were unresolved formore than a hundred years.In the early 19th century, the last twoproperties of light, interference anddiffraction, were observed for the first timeSince these two unusual occurrences could notbe clarified and explained by the particletheory, the wave theory become moreacceptable.
The Photoelectric EffectAt the turn of the 20th century, it was observedthat electrons were emitted by some metalswhenever light with certain frequency wasmade to shine on their surface. It was also observed that the greater the intensity of the light that shone on the metals, the greater the rate of emission.
The photoelectric effect weakened the wave theory.An increase in light intensity is a result in theincrease in the velocity of the photoelectrons,according to Huygenss theory.This did not happen. Instead an increase in intensityincreased the number of electron emitted and theirvelocities are the same as those emitted at lower lightintensity.Even Newton’s theory could not explain whyvelocities of the electrons remained constant in spiteof an increase in intensity of the light.Photoelectric effect did not help in solving thedilemma of which theory was correct.
In spite of these apparent contradicting theories oflight, technological advances have been made.Florescent lamp have been invented making use ofthe photon theories of Max Planck and AlbertEinstein. The photon theory explains the activation of particles by light energy into higher energy level and the emission of the same amount of energy in the form of light as the particle fall back to their natural level
The laser, proposed in 1958 by A. L. Schawlow andC. H. Townes, consists of an active material which isexcited by an external source to disturb the normaldistribution of electrons in the material and movethem to higher energy levels.As the electrons fall back to their lower levels, theygive off excess energy in the form of radiation.When the energy is lost by absorption ortransmission, the laser beam oscillates.
But this beam differs from an ordinary light beam.It does not diffuse and the energy waves emitted by thestimulated atoms travels in the same direction, at thesame frequency, and in perfect step with thestimulating radiation, which results in greatlymagnified intensity
The Quantum TheoryMax Planck presented the theory that electronsabsorbed energy only in discrete quantitiesproportional to their frequencies.Albert Einstein adopted Plancks concept andformulated the quantum theory which helped NielsBohr build a workable and acceptable model of theatomic structure.Bohr’s concept of discrete energy levels provided theFrench physicist Louis de Broglie with the theory ofthe dual nature of light.
De Broglie suggested that light had bothparticles and wavelike properties.This theory of De Broglie is known aswave mechanics.Wave Mechanics accepted Einstein’s ideaof the interchangeability of mass andenergy (E=mc2 )Einstein said that in every mechanicalsystem, waves are associated with massparticles.
Speed of LightThe first attempt to measure thespeed of light was made byGalileo Galilei.Galileo and his assistant, eachcarrying a lamp, positionedthemselves separately on twohilltops.
They tried to measure the time it takes the light fromone lamp to travel between the two hilltops.The assistant was to flash his lantern as soon as hesaw Galileo flash his. Galileo tried to measure thetime between the flash of his lantern and the timewhen he received the light from his assistant’slantern.The experiment was not successful because lighttraveled so fast that for distances on the earth’ssurface within one’s visual range, the time involved inits transmission was so short as to be negligible andnot noticeable.
It was the Danish astronomer Olaus Roemer who was first to compute the velocity of light by observing the eclipses of one of the moons of Jupiter.His calculation determined the speed of light at277,000 km/s.
Professor Albert A. Michaelsonrefined the figure proposed byRoemer by publishing a reporton his Mt. Wilson and Mt. SanAntonio in California.His computations showed the speed of light in air as299, 790 km/s.
In a vacuum the speed of light is slightly faster at299,792.8 km/sThe common value of 300,000 km/s or 3.0 x 108 m/smay be used in ordinary computations with very littleerror.At this velocity, the earth’scircumference (40,000 km)could be transverse sevenand one-half times in onesecond.
The light reflected from the moon 384 000 km away from us would reach us in 1.3 seconds.The light from the sunwould take roughly eightminutes to travel to theearth.
The distance traveled by light in one year is called a Light-year.This is the unit ofdistance used by astronomers to measure distances in the universe.
Light :An Electromagnetic WaveElectromagnetic wave tells usthat electric and magnetic fieldsare present in light.In 1856, James Clerk Maxwellwas doing a theoretical study ofelectromagnetic waves.He realized that a changing electric field gives rise toa changing magnetic field which in turn creates achanging field, and so on.
James Clerk Maxwell developed equations showingthe relationship of electricity and magnetism.Based from these equations, he found that the netresult of these interchanging fields was to create atrain of invisible waves of electric and magnetic fieldsthat could propagate through space.These waves are called electromagnetic (EM) waves.Using the equations developed by Maxwell theyfound out that EM waves travels at the speed of 3.0 x108 m/s, the same as the measured speed of light in avacuum.
James Clerk Maxwellthought that there was arelationship betweenelectric and magneticfield.He concluded that lightwaves are only aparticular type of generalcategory calledelectromagnetic waves.
Electromagnetic SpectrumMaxwell’s theory of light was fullyaccepted after EM waves were firstcreated and detected experimentallyby Heinrich Hertz in 1887.Hertz give experimental evidence that light andelectromagnetic waves have the same nature and thatthey travel at the same speed and exhibit the sameproperties such as refraction, reflection, diffraction,and interference.The difference in some properties was found to bedue to their different wavelength.
The electromagnetic waves make up what is calledcollectively as the Electromagnetic Spectrum.
Each wavelength is different from another inits wavelength, but all waves travel in vacuumwith the speed of light.The wavelengths vary from 3 x 107 m at highfrequencies.In Angstrom unit (A0 ), the common unit tomeasure wavelengths is from 3 x 1017 A0 (1 A0= 1.0 X 10-10 m).It is found that the wavelength of visible lightrange from 3.5 x 10-7 m to 7.5 x 10-7 m (3500 A0to 7500 A0 )
The visible spectrum includes all radiation visible tothe human eye and is commonly called Light.All radiation above and below the visible waves havewavelengths that are too short or too long for the eyeto detect.
How much have you learned?1. According to James Clerk Maxwell, what is the relationship between light and EM waves?2. What is the main difference between light waves and EM waves?3. Approximately, what part of the whole EM spectrum does our visible spectrum occupy?
Luminous andIlluminated Objects Luminous Object – any object that emits their own visible light. - They become visible because of the light they emitted. Illuminated Object -object that can be seen because it reflects the light they received.
What happens to light when it falls on the surface of the object?Light from a source travels in all direction.This decreases the amount of light that falls on acertain area.When light strikes the surface of an object, severalthings may happen.If the radiation is turned back by thesurface without entering the object, thelight is said to be REFLECTED.
Most of the objects around us are not emittersof light but instead as reflectors of light.Some of the light that falls on an object may beabsorbed.The radiant energy is transformed intomolecular motion which warms the object.All objects that absorb light experience a risein temperature.
Objects which allow light to pass throughare said to transmit light and aredescribed as TRANSPARENT
Some other substances can transmit lightbut allow the light to scatter or diffuse,making it difficult to distinguish objectsthat are behind them. This substances arecalled TRANSLUCENT.
Substances that do not transmit light atall, like stones and wood are classified asOPAQUE objects.
Reflection of LightReflected light is a light that is bounced backfrom the surface of an object.
Part of the light coming from the source,passing through the air, and touching theboundary of a medium is reflected.The rest of the light enters into the mediumand is partially absorbed and partiallytransmitted.
The amount of reflected light depends on threefactors:1. The kind of medium the object is made.
2. The smoothness of the surface of the medium.
3. The angle at which the light strikes thesurface.
The ratio of the amount of light reflected by asurface to the amount of light falling on it iscalled Reflectance of the surface and is usuallyexpressed as a percentage.Polished metals will have a higher percentage ofreflectance than wood; smooth glass will reflectmore light than rough glass. The more perpendicular the angle of the light,the less the reflectance.
Law of Reflection1. The incident ray, the reflected ray, and thenormal to the reflecting surface lie on the sameplane.
2. The angle of incidence, and the angle ofreflection are equal.
Regular and Diffuses Reflection Regular reflectionWhen the reflectiontakes place from aperfect plane surfaceit is called RegularReflection.
Regular reflectionWhen in incident beam of light falls on anirregular surface, the reflected light willdispersed in all directions.The dispersal of reflected light is calledDiffusion.
1. Mirrors affect light by _____________ a. refracting it b. reflecting it c. changing its speed d. changing its color
2. Rainbow formation is due to __________. a. reflection of sunlight in the sky. b. refraction of sunlight in the sky. c. reflection and refraction of sunlight in the sky. d. reflection and refraction of sunlight in a raindrops.
Refer to the figure:3. The incident ray on the reflecting surface is _____ a. AO b. NO c. OB d. NB
Refer to the figure:4. The reflected ray on the reflecting surface is _____ a. AO b. NO c. OB d. NB
Refer to the figure:5. The normal drawn from the point of incidence is_________ a. AO b. NO c. OB d. NB
Refer to the figure:6. The angle of incidence is_________ a. AON b. AOB c. i d. r
Write A if the object is Luminousand B if the object is Illuminated.7. Planets8. Comet9. Star10. Asteroid11. A meteor
12. He advocated the particle theory thatlight consists of tiny particles of matteremitted by a source and spread outward instraight lines.
13. He theorized that light is consists ofseries of waves with their wave fronts atright angles to the path of the rays.
14. The speed of light in a vacuum isequivalent to ___________ a. 227,000 km/s b. 299, 790 km/s c. 300, 000 km/s d. 299, 792.8 km/s
15. All radiation visible to the human eyesis called ______ a. microwave b. light c. infrared d. electromagnetic spectrum
Reference:You and the Natural World, Science 7Lilia G. Vengco, Teresita F. Religioso, Carmelita M. Capco,Gilbert C. Yang, Estrella E. Mendoza, Delia Cordero-Navaza,Bienvenido J. Valdez. 2012, Phoenix PublishingHouse Inc.