2. PARTICLE THEORY
Proposed by Sir Isaac Newton
Corpuscular Theory
Explained in Opticks published in 1704
Observed: reflection, shadows, light travelling
in straight lines
Light as small compact particles of energy
called corpuscles
Light is a particle flying through void
3. PARTICLE THEORY EVIDENCES
Light travels in straight lines
Light can travel through vacuum
According to Newton, light can’t be a wave
We can hear sound from behind an obstacle, but we don’t see
light – light shows no diffraction
Can explain reflection, refraction, rectilinear propagation
Light traveled as a shower of particles
each proceeding in a straight line
4. CORPUSCULAR THEORY OF REFLECTION
Light from a source arrives on a
mirror surface as a stream of
particles that will bounce away
from the smooth surface
A huge number of these particles
(corpuscles) are involved in a
propagating light beam
When the corpuscles touch the
Particle theory is
mirror, they bounce from different
points, reversing their order, stronger in the
producing a reversed image reflection phenomenon
5. CORPUSCULAR THEORY OF REFRACTION
Particles accelerate as they go
from air to medium of greater
optical density
Explained by forces acting on
boundaries between dif ferent
media
6. ARGUMENTS PRESENTED
Rectilinear propagation
Higher the speed, lesser the curve of the path
Corpuscles travel at high speeds that they travel in straight lines
Strong argument against wave theory – how could waves travel in
straight lines?
Reflection
When light hits a smooth surface, it’s reflected (like steel ball
bearings thrown at a smooth steel plate rebound)
Elastic particles
7. ARGUMENTS PRESENTED
Refraction
Two level surfaces, one higher than another, edges connected by a
slope
Ball is rolled on the higher surface toward the slope at a given angle
Higher surface -> down the slope -> lower surface speed up
Can be compared to light being refracted
Light particles accelerate as they go from a medium to a medium of
greater optical density
In this case, speed of light must be faster in water than in air
8. PROBLEMS WITH THE THEORY
When light hits a boundary between two media, some refract,
some reflect (characteristics of waves)
To defend: when particles reach the surface, they have fits;
some particles “decide” to go into the water while the rest
“decide” to bounce of f
9. PROBLEMS WITH THE THEORY
“Light is never known to follow crooked passages nor to bend
into the shadow” – consistent with particle theory
Proposes that light particles must always travel in straight lines
If particles encounter the edge of a barrier, shadows will be cast
Particles unblocked by the barrier will continue the straight line
For dif fraction, it is good in a macroscopic scale, but not in a
microscopic scale
When light is passed through a narrow slit, the beam spreads
and becomes wider than expected (supports wave theory)
Speed of light was proven to be slower in water
