Here are the answers to the quiz: 1. b 2. d 3. b 4. c 5. b 6. a 7. B 8. B 9. A 10. B 11. B 12. Isaac Newton 13. Christian Huygens 14. d 15. b

1. Light
Prepared by: Victor R. Oribe

2. 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

3. Five properties of light were
developed:
a. rectilinear propagation of light
b. Reflection of waves
c. refraction of light
d. interference
e. diffraction

4. Rectilinear propagation inside an unchanging
medium.

5. Reflection of waves from the surface of the
medium

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

7. Interference, when two waves of opposite
phases are superimposed

8. Diffraction, the bending of waves around
corners

9. During the time of Newton and
Huygens, only the first three
were recognized and observed.
a) Rectilinear
b) Reflection
c) Refraction

10. 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.

11. 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.

12. 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.

13. 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

14. 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.

15. 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

16. 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.

17. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. 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.

25. 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.

26. 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.

27. 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.

28. 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.

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

30. 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 )

31. 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.

33. 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?

34. 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.

35. 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.

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

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

38. 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.

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

40. Translucent
Opaque
Transparent

41. Reflection of Light
Reflected light is a light that is bounced back
from the surface of an object.

42. 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.

43. The amount of reflected light depends on three
factors:
1. The kind of medium the object is made.

44. 2. The smoothness of the surface of the medium.

45. 3. The angle at which the light strikes the
surface.

46. 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.

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

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

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

50. 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.

51. Quiz

52. 1. Mirrors affect light by
_____________
a. refracting it
b. reflecting it
c. changing its speed
d. changing its color

53. 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.

54. Refer to the figure:
3. The incident ray on the reflecting surface
is _____
a. AO
b. NO
c. OB
d. NB

55. Refer to the figure:
4. The reflected ray on the reflecting surface
is _____
a. AO
b. NO
c. OB
d. NB

56. Refer to the figure:
5. The normal drawn from the point of
incidence is_________
a. AO
b. NO
c. OB
d. NB

57. Refer to the figure:
6. The angle of incidence is_________
a. AON
b. AOB
c. i
d. r

58. Write A if the object is Luminous
and B if the object is Illuminated.
7. Planets
8. Comet
9. Star
10. Asteroid
11. A meteor

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

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

61. 14. The speed of light in a vacuum is
equivalent to ___________
a. 227,000 km/s
b. 299, 790 km/s
c. 300, 000 km/s
d. 299, 792.8 km/s

62. 15. All radiation visible to the human eyes
is called ______
a. microwave
b. light
c. infrared
d. electromagnetic spectrum

63. Reference:
You and the Natural World, Science 7
Lilia 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.