Light

Prepared by: Victor R. Oribe

Light: Particle or Wave?

Sir Isaac Newton advocated the particle theory that light consists of tiny particles of matter emitted by a source and spreads outwards in straight lines called rays.

Christian Huygens, Dutch mathematician and scientist, theorized that light consists of a series of waves with their wave fronts at right angles to the path of the rays

Five properties of light were developed:

a. rectilinear propagation of lightb. Reflection of wavesc. refraction of lightd. interferencee. diffraction

Rectilinear propagation inside an unchanging medium.

Reflection of waves from the surface of the medium

Refraction, the bending of waves when a change of speed takes place

Interference, when two waves of opposite phases are superimposed

Diffraction, the bending of waves around corners

During the time of Newton and Huygens, only the first three were recognized and observed.

a) Rectilinearb) Reflectionc) Refraction

Spite of scientific discussions in defense of the theories, the problems were unresolved for more than a hundred years.

In the early 19th century, the last two properties of light, interference and diffraction, were observed for the first time

Since these two unusual occurrences could not be clarified and explained by the particle theory, the wave theory become more acceptable.

The Photoelectric Effect

At the turn of the 20th century, it was observed that electrons were emitted by some metals whenever light with certain frequency was made 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 the increase in the velocity of the photoelectrons, according to Huygens's theory.

This did not happen. Instead an increase in intensity increased the number of electron emitted and their velocities are the same as those emitted at lower light intensity.

Even Newton’s theory could not explain why velocities of the electrons remained constant in spite of an increase in intensity of the light.

Photoelectric effect did not help in solving the dilemma of which theory was correct.

In spite of these apparent contradicting theories of light, technological advances have been made.

Florescent lamp have been invented making use of the photon theories of Max Planck and Albert Einstein.

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 and C. H. Townes, consists of an active material which is excited by an external source to disturb the normal distribution of electrons in the material and move them to higher energy levels.

As the electrons fall back to their lower levels, they give off excess energy in the form of radiation.

When the energy is lost by absorption or transmission, the laser beam oscillates.

But this beam differs from an ordinary light beam.

It does not diffuse and the energy waves emitted by the stimulated atoms travels in the same direction, at the same frequency, and in perfect step with the stimulating radiation, which results in greatly magnified intensity

The Quantum Theory

Max Planck presented the theory that electrons absorbed energy only in discrete quantities proportional to their frequencies.

Albert Einstein adopted Planck's concept and formulated the quantum theory which helped Niels Bohr build a workable and acceptable model of the atomic structure.

Bohr’s concept of discrete energy levels provided the French physicist Louis de Broglie with the theory of the dual nature of light.

De Broglie suggested that light had both particles and wavelike properties.

This theory of De Broglie is known as wave mechanics.

Wave Mechanics accepted Einstein’s idea of the interchangeability of mass and energy (E=mc2 )

Einstein said that in every mechanical system, waves are associated with mass particles.

Speed of Light

The first attempt to measure the speed of light was made by Galileo Galilei.

Galileo and his assistant, each carrying a lamp, positioned themselves separately on two hilltops.

They tried to measure the time it takes the light from one lamp to travel between the two hilltops.

The assistant was to flash his lantern as soon as he saw Galileo flash his. Galileo tried to measure the time between the flash of his lantern and the time when he received the light from his assistant’s lantern.

The experiment was not successful because light traveled so fast that for distances on the earth’s surface within one’s visual range, the time involved in its transmission was so short as to be negligible and not 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 at 277,000 km/s.

Professor Albert A. Michaelson refined the figure proposed by Roemer by publishing a report on his Mt. Wilson and Mt. San Antonio in California.

His computations showed the speed of light in air as 299, 790 km/s.

In a vacuum the speed of light is slightly faster at 299,792.8 km/s

The common value of 300,000 km/s or 3.0 x 108 m/s may be used in ordinary computations with very little error.

At this velocity, the earth’s circumference (40,000 km) could be transverse seven and one-half times in one second.

The light reflected from the moon 384 000 km away from us would reach us in 1.3 seconds.

The light from the sun would take roughly eight minutes to travel to the earth.

The distance traveled by light in one year is called a Light-year.

This is the unit of distance used by astronomers to

measure distances in the

universe.

Light : An Electromagnetic Wave

Electromagnetic wave tells us that electric and magnetic fields are present in light.

In 1856, James Clerk Maxwell was doing a theoretical study of electromagnetic waves.

He realized that a changing electric field gives rise to a changing magnetic field which in turn creates a changing field, and so on.

James Clerk Maxwell developed equations showing the relationship of electricity and magnetism.

Based from these equations, he found that the net result of these interchanging fields was to create a train of invisible waves of electric and magnetic fields that could propagate through space.

These waves are called electromagnetic (EM) waves.

Using the equations developed by Maxwell they found out that EM waves travels at the speed of 3.0 x 108 m/s, the same as the measured speed of light in a vacuum.

James Clerk Maxwell thought that there was a relationship between electric and magnetic field.

He concluded that light waves are only a particular type of general category called electromagnetic waves.

Electromagnetic Spectrum

Maxwell’s theory of light was fully accepted after EM waves were first created and detected experimentally by Heinrich Hertz in 1887.

Hertz give experimental evidence that light and electromagnetic waves have the same nature and that they travel at the same speed and exhibit the same properties such as refraction, reflection, diffraction, and interference.

The difference in some properties was found to be due to their different wavelength.

The electromagnetic waves make up what is called collectively as the Electromagnetic Spectrum.

Each wavelength is different from another in its wavelength, but all waves travel in vacuum with the speed of light.

The wavelengths vary from 3 x 107 m at high frequencies.

In Angstrom unit (A0 ), the common unit to measure wavelengths is from 3 x 1017 A0 (1 A0 = 1.0 X 10-10 m).

It is found that the wavelength of visible light range from 3.5 x 10-7 m to 7.5 x 10-7 m (3500 A0 to 7500 A0 )

The visible spectrum includes all radiation visible to the human eye and is commonly called Light.

All radiation above and below the visible waves have wavelengths that are too short or too long for the eye to 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 and Illuminated 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 a certain area.

When light strikes the surface of an object, several things may happen.

If the radiation is turned back by the surface without entering the object, the light is said to be REFLECTED.

Some of the light that falls on an object may be absorbed.

The radiant energy is transformed into molecular motion which warms the object.

All objects that absorb light experience a rise in temperature.

Most of the objects around us are not emitters of light but instead as reflectors of light.

Objects which allow light to pass through are said to transmit light and are described as TRANSPARENT

Some other substances can transmit light but allow the light to scatter or diffuse, making it difficult to distinguish objects that are behind them. This substances are called TRANSLUCENT.

Substances that do not transmit light at all, like stones and wood are classified as OPAQUE objects.

Translucent

Transparent

Opaque

Reflection of LightReflected light is a light that is bounced back from the surface of an object.

Part of the light coming from the source, passing through the air, and touching the boundary of a medium is reflected.

The rest of the light enters into the medium and is partially absorbed and partially transmitted.

The amount of reflected light depends on three factors:

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 the surface.

The ratio of the amount of light reflected by a surface to the amount of light falling on it is called Reflectance of the surface and is usually expressed as a percentage.

Polished metals will have a higher percentage of reflectance than wood; smooth glass will reflect more light than rough glass.

The more perpendicular the angle of the light, the less the reflectance.

Law of Reflection

1. The incident ray, the reflected ray, and the normal to the reflecting surface lie on the same plane.

2. The angle of incidence, and the angle of reflection are equal.

Regular and Diffuses Reflection

Regular reflection

When the reflection takes place from a perfect plane surface it is called Regular Reflection.  

Regular reflection

When in incident beam of light falls on an irregular surface, the reflected light will dispersed in all directions.

The dispersal of reflected light is called Diffusion.

Quiz

1. Mirrors affect light by _____________

a. refracting itb. reflecting itc. changing its speedd. 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. AOb. NOc. OBd. NB

Refer to the figure:

4. The reflected ray on the reflecting surface is _____

a. AOb. NOc. OBd. NB

Refer to the figure:

5. The normal drawn from the point of incidence is_________

a. AOb. NOc. OBd. NB

Refer to the figure:

6. The angle of incidence is_________a. AONb. AOBc. id. r

Write A if the object is Luminous and B if the object is Illuminated.

7. Planets8. Comet9. Star10. Asteroid11. A meteor

12. He advocated the particle theory that light consists of tiny particles of matter emitted by a source and spread outward in straight lines.

13. He theorized that light is consists of series of waves with their wave fronts at right angles to the path of the rays.

14. The speed of light in a vacuum is equivalent to ___________

a. 227,000 km/sb. 299, 790 km/sc. 300, 000 km/sd. 299, 792.8 km/s

15. All radiation visible to the human eyes is called ______

a. microwaveb. lightc. infraredd. 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 Publishing House Inc.