understand that a photon is a quantum of electromagnetic energy

📚 What is a photon? A photon is a tiny, indivisible packet of electromagnetic energy. Think of it as a “light bullet” that travels at the speed of light, $c = 3.0 \times 10^8 \,\text{m/s}$.

The energy of a single photon is given by the famous equation:

$E = hu$

where $h = 6.63 \times 10^{-34}\,\text{J·s}$ is Planck’s constant and $u$ is the frequency of the light.

Because frequency and wavelength are related by $c = \lambda u$, we can also write:

$E = \frac{hc}{\lambda}$

So, the shorter the wavelength, the higher the energy.

Even though photons have no mass, they still carry momentum:

$p = \frac{h}{\lambda}$

Think of a photon as a tiny “push” that can change the motion of an object, like a gentle breeze that nudges a feather.

🔍 Remember: In exam questions you’ll often be given either wavelength or frequency. Use the appropriate formula:

• If given $u$, use $E = hu$.
• If given $\lambda$, use $E = \dfrac{hc}{\lambda}$.

For momentum, always use $p = \dfrac{h}{\lambda}$. No mass needed!

Check units carefully – energy in joules or eV, momentum in kg·m/s.

Imagine a train where each car is a photon. The train moves at light speed. The energy of the train depends on how many cars (photons) it has and how fast each car is moving (frequency). The momentum is like the push each car gives to the train’s front.

When the train stops (light is absorbed), the energy and momentum are transferred to the material, causing effects like heating or the photoelectric effect.