Light exhibits properties of both particles and waves. Early theories proposed light as particles (Newton) or waves (Huygens), but the photoelectric effect showed light acting as particles, emitting electrons. Later, the dual nature of light was recognized, with wave-like behavior in interactions with other light and particle-like behavior in interactions with matter. Light can be described as either a particle (photon) or wave, depending on the situation.

2. 2) Properties of light
3) Theories about light

4. • Light travels in straight lines:
Las e r

5. • Light travels VERY FAST – around
300,000 kilometres per second.
At this speed it can
go around the world 8
times in one second.

6. • Light travels much faster than sound. For example:
2) Thunder and lightning
start at the same time,
but we will see the
lightning first.
2) When a starting pistol
is fired we see the
smoke first and then
hear the bang.

7. • We see things because they reflect light
into our eyes:
Homework

8. • Luminous and non-luminous objects
A luminous object is one that produces light.
A non-luminous object is one that reflects light.
Luminous objects Reflectors

9. • Shadows
Shadows are places where light is “blocked”:
Rays of light

10. • Light travels in straight lines
• Light travels much faster than sound
• We see things because they reflect light into our eyes
• Shadows are formed when light is blocked by an object

12. • Aristotle (384 - 322 B.C.), an ancient Greek thinker, thought that
we saw the world by sending “something” out of our eye and that
reflected from the object.
• In Plato’s time (427 – 347 B.C.), the reflection of light from smooth
surfaces was known. He was also a Greek.
• The ancient Greeks (about 200 A.D.) also first observed the
refraction of light which occurs at the boundary of two transparent
media of different refractive indices.
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14. In the17th century, two scientists had different
views about the nature of light ……
Light is
particles
No! Light
is waves
Isaac Newton Christian Huygens
1643 - 1727 1629 - 1695
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15. I think light is a stream of tiny
particles, called Corpuscles …
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16. • Corpuscular Theory of Light
 states that light consists of
tiny particles that traveled in
straight lines through space.

17. • Newton’s particle theory of light dominated optics during the
18th century.
• Most scientists believed Newton’s particle theory of light
because it had greater explanatory power.
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18. states that light is considered to
consists electromagnetic waves
that travel in a vacuum at the a
speed of 3.0 x 10 m/s8

19. (source: Treatise on light, published by Huygens in
1690)
I think light is emitted as a series of
waves in a medium he called “aether”
(“aether” commonly also called “ether”)
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20. states that light is a form of wave
disturbance which starts from a
vibrating body is transmitted
through space at great speed.

21. Wave theory
of light? “No
way!”
Albert Einstein
1879 - 1955 21

22.  states that light is a form of
energy in discreet units called
quanta. The word quantum
means the smallest possible unit
of energy.

23.  states that light is composed
of bundles of wave energy
known as photons

24.  The photoelectric effect is observed when light
strikes a metal, and emits electrons.
Einstein used the idea of photons (light consists of tiny
particles) to explain results which demonstrate the
photoelectric effect.
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25. Light is not particles,
not waves, but BOTH!
Louis de Broglie
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1892 - 1987

26.  states that light can be both a particle
and a wave. If light interacting with
light, wave behavior tends to appear
dominant and if light is interacting
with matter, particle or photon
behavior appears dominant.