Properties of Photons
Introduced| English |
|---|
| photons/ˈfəʊtɒnz/ |
| Planck's constant/plæŋks ˈkɒnstənt/ |
Light comes in packets
- Light is not a smooth stream -- it arrives in tiny bundles.
- Each bundle carries a fixed, exact amount of energy.
- Bluer light packs more punch per bundle than red.
- These packets explain how light and atoms trade energy.
Each photon's energy
- Light comes in packets called photons 光子.
- A photon's energy depends only on its frequency:
- Here $h$ is Planck's constant 普朗克常数.
In $E = h\nu$, the symbol $h$ is called ____ constant.
$h$ is Planck's constant, $6.63\times10^{-34}\ \text{J·s}$.
Higher frequency, more energy
- A higher-frequency photon carries more energy.
- Shorter wavelength means higher frequency means more energetic.
- A UV photon beats a red one, packet for packet.
A photon's energy is set by its...
$E = h\nu$ -- energy depends on frequency, not brightness.
A shorter-wavelength photon carries more energy than a longer-wavelength one.
Shorter wavelength means higher frequency and higher energy.
Matching photons to jumps
- An electron jumps levels only if a photon's energy fits the gap exactly.
- The photon is absorbed and the electron climbs.
- Emitting a photon drops it back down.
A photon's energy
Photon energy is proportional to frequency: bluer, higher-frequency light carries more energy per photon.
An electron absorbs a photon only if the photon's energy matches the level gap.
Energy levels are quantized, so only an exact match is absorbed.
Find the energy of a photon of frequency $5\times10^{14}\ \text{Hz}$ (with $h = 6.63\times10^{-34}$).
- $E = h\nu = (6.63\times10^{-34})(5\times10^{14})$.
- $E \approx 3.3\times10^{-19}\ \text{J}$.
A photon has $\nu = 2\times10^{15}\ \text{Hz}$ and $h = 6.63\times10^{-34}$. Its energy? Enter the coefficient before $\times10^{-18}$ J (2 decimals).
$E = h\nu = (6.63\times10^{-34})(2\times10^{15}) = 1.33\times10^{-18}\ \text{J}$.
A dim blue light versus a bright red light. Which has higher-energy photons?
Blue's higher frequency wins; brightness is just the number of photons.
A photon's energy depends on frequency (or wavelength), not on brightness -- a dim blue light still has higher-energy photons than a bright red one. Use $E = h\nu$ with frequency or $E = hc/\lambda$ with wavelength, and do not mix them. Energy comes in fixed packets, so only an exactly-matching photon causes a jump.
Light travels as photons, each carrying $E = h\nu = hc/\lambda$, where $h$ is Planck's constant. Higher frequency (shorter wavelength) means a more energetic photon, set by frequency, not brightness. An electron absorbs a photon only when its energy exactly matches the level gap.