Light was originally thought to be emitted or reflected corpuscles or rays, until Newton studied sunlight through a prism and concluded light was particles. However, issues arose with this theory, leading Huygens to propose light was waves. Later experiments by Young supported the wave theory. The debate continued for over 100 years as to whether light was particles or waves. The current understanding is that light exhibits properties of both particles and waves, and is a form of electromagnetic radiation that can propagate as a wave or behave as particles.

1. A
Brief
History
of
Light

2. Since the dawn of time…
Up until Sir Isaac Newton, two theories
existed as to the nature of light.
1. Objects emitted “corpuscles” of light
that entered the eye.
2. The eye emitted rays that reflected
off of objects and allowed us to see.

3. Sir Isaac Newton (1666)Newton
systematically
studied light.
Passed sunlight
through a prism
and observed the
band of colors that
emerged.
Concluded that light was made of small
particles traveling at enormous speeds.
(This explains why light travels in straight lines
and cast such sharp shadows.)

4. But why don’t the particles collide and
interact when two beams of light
crossed each other?
And, why would one color particle be
bent more than another?

5. Christian Huygens (1678)
Suggests that light is
actually small waves.
This answers the previous
questions, but begs
more…
•Why can’t light go
around obstacles then?
•And how can it travel in a
vacuum?

6. Thomas Young, 1801
Passed a beam of
light through two
closely spaced holes.
The light diffracted
demonstrating light is
more wavelike.
Newton vs. Huygens
For 100 Years the argument went on…
Particle vs. Wave

7. So they had to come up with some
explanations!
Since space is “empty”, there had to be
something else called “ether”.
Properties of ether:
– Has to be everywhere
– More dense than steel
– Anything can move through it with great
ease.

8. • Ether
– If Earth has ether around it, then Earth would
cause “ether waves” as it travels through it.
(Like a beach ball in a tub.)
– Light going with the ether waves would be
faster. Light going across the waves would
have the same velocity.
– Michelson and Morley, 1887
• Developed interferometer:
– Measures wave speeds.
• What did it show?!?!?!?!
• The speed of light is unchanging!
• Does ether exist then?

9. Electromagnetic Radiation
Current theory says that light is caused by
Electricity and Magnetism combined.
Because of this, it can travel through a
vacuum.

10. Wave - Particle Duality
Light exists as both a wave and a particle at
the same time. It shows itself however
you are looking for it.
If your experiment looks at it like a particle, it
behaves like a particle… and vice versa.