High School
SAT
ACT
University
MCAT
LSAT
DAT
Wize High School Grade 12 Physics Textbook > Light as a Particle

Photons

Thin FilmsPrevious Section
The Compton Effect (Light = Particle)Next Section
Popular Courses
Find My Course
0:00 / 0:00

Photons

  • Light "particles" are called photons. They are actually packets of energy.

Photon energy:

The energy of a photon is given by:

 E=hf \boxed{ \ E=hf \ }


where hh is Planck's constant and has the value:

 h=6.626×10−34 J⋅s=4.136×10−15 eV⋅s\boxed{\ h=6.626\times10^{-34}\ J\cdot s =4.136\times10^{-15}\ eV\cdot s }


This tells us that energy is quantized - and we are entering the field of quantum mechanics!


Wize Concept
The smallest amount of energy corresponds to a single photon, and we cannot have fractions of hfhf, only integer multiples of it.


Exam Tip
Switch between wavelength and frequency using c=λfc=\lambda f to get two versions of the energy equation: E=hf=hcλE=hf=\dfrac{hc}{\lambda}

Photon momentum:

The photon doesn't have mass, so we can't use p=mvp=mv, instead we define the momentum of a photon as:

p=hλ\boxed{p = \frac{h}{\lambda}}

Practice: Energy of Photons

Ozone formation in the upper atmosphere starts when oxygen molecules absorb UV radiation.

Ozone absorbs radiation of wavelengths 220 – 290 nm, thus protecting organisms from this radiation. What is the energy of the most energetic of these photons?

A. 9.03×10−19J9.03\times10^{-19}J
B. 9.03×10−28J9.03\times10^{-28}J
C. 6.85×10−19J6.85\times10^{-19}J
D. 6.85×10−28J6.85\times10^{-28}J
E. None of the above

Practice: Frequency of Photons

A nitrogen gas laser emits photons with a frequency of 8.9×1014Hz8.9\times10^{14}Hz. If a nitrogen laser pulse gives off 3.83×10−3J3.83\times10^{-3}J
of energy, how many photons does the pulse contain?

A. 34.09×1011 photons34.09\times10^{11}\ photons
B. 2.32×1011 photons2.32\times10^{11}\ photons
C. 2.32×1017 photons2.32\times10^{17}\ photons
D. 6.49×1015 photons6.49\times10^{15}\ photons
E. None of the above