Learn Light & Photons Intuitively – Properties, What They're Made Of, How They Travel & Interact, and How We Perceive Size & Shape

 

Introduction:

Imagine you're in a boat on a calm ocean. You dip your hand into the water and wiggle your finger up and down. The wiggle creates a wave that travels outwards across the water. It's not the water itself moving from your hand to the shore. It's the energy and the pattern of the wiggle that is traveling. The water just goes up and down in place. Light is exactly like that, but instead of water, it's a wiggle in the invisible, fundamental fabric of the universe.

What is light and what it is made up of?

Light is a tiny packet of pure energy traveling through space as a wave. Think of it like the wave in a stadium. The people stay in their seats, but the wave motion travels around the stadium. Light is that traveling motion, not the stuff. If you zoom in incredibly close on a light wave, you find that energy isn't just a smooth, continuous wave. It's actually delivered in tiny, discrete packets. Imagine the wave in the stadium again. Now imagine that this wave isn't smooth, but is actually made of individual, invisible energy marbles being passed from person to person. These energy marbles of light are called photons.

Think of the electromagnetic field as an invisible ocean. When there is a disturbance like an accelerating charge, it creates a ripple in this ocean. That ripple is a photon. Think of it as the smallest possible unit of light that can exist. You can't have half a photon, light comes in these discrete energy packets. Photons are emitted when a charged particle accelerates or when a quantum system drops to a lower energy level and gets destroyed when absorbed by electrons.

When you see light, you're literally seeing photons that were created when electrons somewhere in the Sun or in a light bulb changed their energy state and emitted these little packets of pure energy. These photon travels at the speed of light and when it interacts with something like an electron, it gives up its energy and momentum. When a photon hits your retina, it gets absorbed by an electron in a molecule, transferring its energy and creating the signal your brain interprets as seeing. Photons are the energy carriers that allow atoms to interact with each other and allow us to see the world. This photon-electron interaction is fundamental to everything from vision to photosynthesis to digital cameras.

A photon is a particle or a quantum of light that has wave like properties. The photon like all quantum objects, has both particle like and wave like properties. However, it is not that the photon is a wave made of smaller particles or physical wave in the sense of water or sound. Instead, the wave nature of the photon is described by quantum mechanics as a probability wave. This wave describes the probability of finding the photon at a given point space and time. It is one of the basic building blocks of the universe.

A photon is like a mysterious entity that acts like a spread out wave when traveling through space. It acts like a localized particle when interacting with matter like light hits a surface like a solar panel and gets absorbed, knocking out an electron in photoelectric effect, it behaves like a particle delivering a precise punch of energy. But it's always the same underlying thing. we're just seeing different aspects of its nature. We can observe either wave like behavior i.e. interference and diffraction or particle-like behavior i.e. photoelectric effect, compton scattering in different experimental setups, but not both at the same time.

It's a fundamental particle of pure energy. A photon has no mass. It's not a piece of an atom like an electron or a proton. It always moves at the speed of light from the moment it's created. You can't slow down a photon. It's either moving at light speed or it doesn't exist.

The energy of a photon is given by,
E = hf,
where h is Planck's constant and
           f is the frequency of the wave.
So the wave aspect I.e. frequency is directly related to the particle aspect i.e. energy.



Photons are created in several ways, but all involve changes in the energy state of charged particle, typically electrons.

• When a charged particle like an electron accelerates, it can emit photons. This is the basis for synchrotron radiation and radio wave generation in antennas.
• When an electron in an atom drops from a higher energy level to a lower
one, the energy difference is emitted as a photon. The energy of the photon is
exactly the difference between the two levels. This is how atoms emit light in
lasers, neon signs, and the spectral lines of stars.
• When a particle and its antiparticle like an electron and a positron meet, they can annihilate and produce photons.
• Photons are emitted when atomic nuclei transition from a higher energy state to a lower one.
• Heated materials emit photons due to the thermal motion of charged particles.

In short, Light is made of photons, and each photon is like a tiny, self-contained packet of wave.


This wave has two key properties that our eyes and brain interpret.

• Wavelength (Color):

This is the distance between two wiggles. The color of the light is determined by the photon's energy which is tied to the wavelength of its wave. Long, lazy waves feel red to our eyes. Short, tight, frantic waves feel violet to our eyes. All the other colors like orange, yellow, green, blue are in between. Radio waves, microwaves, and X-rays are the exact same thing as light, just with wavelengths our eyes can't detect.

• Intensity (Brightness):

This is the height or amplitude of the wiggle. The brightness of the light is determined by the number of photons hitting your eye every second. A gentle wiggle is a dim light. A powerful, tall wiggle is a bright light.

• Dim Red Light: A few low energy photons i.e. long, lazy waves arriving each second.
• Bright Violet Light: A torrent of high energy photons i.e. short, tight waves arriving each second.

So, when you see a red apple, what's happening is that white light from the sun which contains all colors hits the apple. The apple's skin is made of a material that soaks up all the light waves except the long, lazy, red ones. Those red waves bounce off and travel into your eye. Your eye detects this specific wiggle pattern and your brain interpret it as red.

So, the next time you turn on a lamp, trillions of these tiny, wave like energy bullets, each traveling at the absolute speed limit of reality, bouncing off everything in the room and carrying information directly to your eyes.

How photons navigate through a universe filled with atoms without constantly colliding?

Ordinary matter like gases, solids, etc. is composed of atoms. However, the universe also has a lot of empty space. Atoms themselves are mostly empty space. The nucleus of an atom is tiny compared to the electron cloud, and the distances between atoms in a gas like air are huge compared to their sizes. The mass or energy is concentrated in these tiny spaces, separated by vast emptiness. The atoms in the air are spaced far apart. In air at sea level, there are about 1019 molecules per cubic centimeter, but that still means that the average distance between molecules is about 10,000 times the size of an atom. So, most of the volume is empty space. There's just a very low probability that a photon will interact with any given atom in air.

Then how photons interact?

While, photons are packets of electromagnetic energy. They travel at the speed of light. When photons travel through a medium like air, water, or glass, they can indeed interact with atoms and molecules. Not every atom will interact with every photon. The probability of interaction depends on the energy of the photon and the atomic structure. Photons only get absorbed when their energy exactly matches an electron's possible energy transitions. Think of atoms like picky bouncers at a club. They only let in photons with the exact right energy ID. Most visible light photons don't have the right ID for air molecules, so they just walk right through. For visible light, the air is mostly transparent because the energy of visible photons does not match the energy differences between electronic energy levels in nitrogen and Oxygen, the main components of air. Therefore, most visible light photon pass through air without being absorbed.

What Happens When a Photon Does Interact?

If a photon has the right energy to be absorbed by an atom i.e. matching an electronic transition, it can be absorbed. The atom then moves to a higher energy state. The atom may then re-emit a photon in a random direction i.e. scattering or after a delay i.e. fluorescence, etc. Alternatively, energy might be converted into heat. If the photon is not absorbed, it can still be scattered, which is what makes
the sky blue. But even in scattering, the photon is not collapsing with the atom in the sense of two solid objects hitting each other. It's an electromagnetic interaction.

Usually photons do travel in straight lines through vacuum and through transparent materials like air. In perfect vacuum, nothing exists to interact with, so they continue forever perfectly in straight lines at light speed.

While in air, because no electric charge exists. Photons don't interact with the electromagnetic force that affects electrons. They only interact when they hit an electron's probability cloud in just the right way. In a medium, they can be absorbed and re-emitted, or scattered, which may change their direction. However, in a transparent medium like air, the scattering is minimal for visible light, so light appears to travel in straight lines. In air, most visible light photons pass through unaffected. A tiny fraction get scattered thats why the sky is blue. Even fewer get absorbed. When photons do interact, they can be absorbed or scattered, which may
change their direction or energy.

In solid objects, dense materials have electrons with energy levels that match visible light. These photons get absorbed or reflected. That's why you can't see through walls.

Look at the journey of a photon from chair to eye:

Photons from emitting source hits the chair. Electrons in chair absorb some photons, reflect others. Reflected photon enters air. 99.99...% passes through air molecules unaffected, while 0.00...% gets scattered i.e. changes direction slightly. Some gets absorbed and becomes heat. Surviving photons enter your eye. Retinal molecules absorb them because perfect energy match. Congratulations, you can see chair. In air at sea level, a visible light photon will travel about 30-100 meters on average before hitting an air molecule. Since most things we look at are much closer, most photons make it to our eyes.

So photons travel straight through the empty spaces between atoms, only occasionally interacting when the quantum conditions are just right. It's not that they're dodging atoms, they're mostly just passing through the vast emptiness that makes up what we call solid matter.

How we see objects and why photons from the object reach our eyes without being absorbed by the air, if photons are constantly being absorbed by electrons?

Typically, there is a light source in the environment, such as the sun or a lamp. This light source emits photons which are packets of light energy that travel in all directions. When these photons hit the object i.e. chair, they interact with the atoms in the chair. The electrons in the chair's atoms can absorb the photons and then re-emit new photons or reflect some photons in a process called scattering. The chair's material reflects photons with modified properties. A red chair absorbs most colors but reflects red photons. These reflected red photons travel to your eye. The specific wavelengths i.e. colors of the re-emitted photons depend on the material of the chair, which is why the chair appears to have a certain color. Some of these re-emitted or reflected photons travel in the direction of your eye. They pass through the air between the chair and your eye. The photons enter the cornea of your eye, pass through the lens, and finally hit the retina at the back of the eye. In the retina, there are specialized cells called rods and cones that contain light-sensitive molecules. When a photon is absorbed by one of these molecules such as rhodopsin in rods,it causes a chemical change that triggers a nerve impulse. This impulse is sent to the brain, which interprets it as vision.

Why don't the photons get absorbed by the air, specifically by the electrons in the molecules of the air on their way from the chair to your eye?

It depends on energy levels of the electrons in the air molecules and the wavelength of the photons. The air is mostly composed of nitrogen (78%) and oxygen (21%), with trace amounts of other gases. The molecules in the air have specific energy levels that their electrons can occupy. For an electron to jump to a higher energy level, it must absorb a photon with exactly the right amount of energy i.e. corresponding to the difference between two energy levels. The photons that make up visible light have energies that correspond to wavelengths between about 400 nm (violet) and 700 nm (red). It turns out that the energy of visible light photons does not match the energy gaps in the electrons of nitrogen and oxygen molecules. Therefore, the air is transparent to visible light. The photons pass through without being absorbed.

Air is not transparent to all wavelengths. Some UV photons have enough energy to be absorbed by oxygen and ozone in the upper atmosphere which is good because it protects us. Some IR wavelengths are absorbed by greenhouse gases like carbon dioxide and water vapor, which is why the atmosphere is not completely
transparent to IR. So, in summary, the photons of visible light which are the ones we see with do not have the right energy to be absorbed by the electrons in the air molecules. Therefore, they can travel long distances through the air without losing energy.

Even though air is transparent to visible light, there are some processes that can
affect light traveling through the air like rayleigh scattering, caused by scattering of light by molecules and  small particles in the air. It is more effective for shorter wavelengths. This is why the sky appears blue: the blue light from the sun is scattered in all directions, so when we look at the sky away from the sun, we see this scattered blue light. While nitrogen and oxygen don't absorb visible light, other gases like water vapor, carbon dioxide, and ozone can absorb specific wavelengths, but mostly in the infrared and ultraviolet. If there are dust particles, smoke, or water droplets in the air, they can absorb or scatter light. This is why on a hazy day, distant objects appear blurred.

We know how we see objects, but how we perceive the size and shape of objects, and the role of photons and electrons in this process?

Imagine we have big red chair and a small green chair, how we see difference in shape and size between a big red chair and a small green chair, and whether electrons escape during the process of reflection.

The material of the chair has specific properties at the atomic/molecular level. The electrons in the material are bound to atoms or molecules and have certain energy levels. When a photon hits the material, it can be absorbed if its energy which depends on its wavelength matches the energy difference between two electron energy levels. If not, it might be reflected or transmitted. For the red chair, the material absorbs most of the non-red wavelengths and reflects red wavelengths (620-750 nm).
Similarly, the green chair reflects green wavelengths (495-570 nm) and absorbs others. The reflected photons then travel to our eyes. The color we perceive is determined by the wavelength of the photons that enter our eyes. Now, about the shape and size:

The pattern of photons that reach our eyes from different parts of the chair forms an image on our retina. The brain processes this image to infer the shape and size of the chair. Photons coming from the top of the chair and from the bottom of the chair come from different directions and hit different parts of the retina. The lens of the eye focuses the light, so that the pattern of photons on the retina corresponds to the spatial distribution of the reflected light from the chair. So, the wavelength (color) and the direction (shape and size) of the photons are both important.

Is electrons escape during these process?

Generally, no. In the process of reflection or absorption and re-emission of photons, the electrons in the material do not escape. They are bound to the atoms or molecules of the material. When a photon is absorbed, it excites an electron to a higher energy level. Then, when the electron falls back to a lower energy level, it may emit a photon this is called fluorescence or phosphorescence. However, in the
case of reflection, the process is often elastic scattering like in a mirror or inelastic scattering where the photon is absorbed and then re-emitted at the same or similar wavelength, but without losing the electron.

In most solid materials, the electrons are bound and cannot escape easily unless the photon energy is very high like in the photoelectric effect, which requires UV light for some materials. For visible light and typical chair materials, the energy of the photons is not enough to eject electrons i.e., cause the photoelectric effect. So, the electrons remain in the material. However, note that in the photoelectric effect, which is a different phenomenon, photons with sufficient energy can indeed eject electrons from a material. But for everyday objects like chairs and under visible light, this does not occur because the energy of visible light photons is too low to overcome the work function, the energy needed to eject an electron of common materials.