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With the help of the impulse of a particle and the impulse of a(n electromagnetic) wave, evidence for light being a stream of particles and vice versa can be found.
In classical physics, the impulse/momentum of a “normal particle” is defined as:
(p
p
→
(
particle
)
=
m
⋅
v
→
{\displaystyle {\vec {p}}({\text{particle}})=m\cdot {\vec {v}}}
Louis de Broglie found that the wavelength of an electromagnetic wave
λ
{\displaystyle \lambda }
Planck’s constant
h
{\displaystyle h}
p
{\displaystyle p}
(dBλ
p
(
EM
)
=
h
λ
⋅
λ
÷
p
λ
=
h
p
substitute
p
=
m
⋅
v
λ
=
h
m
⋅
v
⋅
m
⋅
v
÷
λ
m
⋅
v
=
h
λ
which is equivalent to
p
(
EM
)
=
p
(
particle
)
{\displaystyle {\begin{aligned}p\left({\text{EM}}\right)&={\frac {h}{\lambda }}&\cdot \lambda \div p\\\lambda &={\frac {h}{p}}&{\text{substitute }}p=m\cdot v\\\lambda &={\frac {h}{m\cdot v}}&\cdot m\cdot v\div \lambda \\m\cdot v&={\frac {h}{\lambda }}\quad \quad &{\text{which is equivalent to}}\\p\left({\text{EM}}\right)&=p\left({\text{particle}}\right)\end{aligned}}}
Therefore, we conclude that an electromagnetic wave is just a stream of particles and vice versa.
