Why Your Shadow is Blurry, and How That Helps Us Study Alien Worlds
Featuring Diffraction and Spectroscopy
"Light is the messenger of the universe." ~ Neil deGrasse Tyson
Light itself is an extensive topic, and we cannot fathom putting it all in just one blog, so let’s go piece by piece. We know light is one of the most interesting things out there. Is it a particle or a wave? It is a whole interesting debate from the 19th century. This blog talks about one of those behaviours of light: THE WAVE NATURE.
‘Diffraction’ is one of the cool phenomena explained by the wave nature of light. It talks about how light can change shape, bend around objects, and that’s our topic for this blog!
What is Diffraction?
The textbook definition of diffraction of light says: “Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture.”
Let’s break it down into simpler terms:
Diffraction of light is when light moving in a straight line path bends around an obstacle or spreads out after passing through a small opening, and thus reaches the places it normally wouldn’t have if it were following only a straight line path.
Diffraction isn’t a phenomenon unique to light alone; all waves diffract—sound waves, water waves, and even other electromagnetic waves.
Think about that one chaotic Sunday when your neighbour’s house was undergoing construction 😩. Even with your doors and windows shut, you still heard the drilling and hammering. Why was that so? Because the little creaks around the doors and windows (the obstacles) are big enough to let the sound waves bend around and enter your room 😭 That’s diffraction in action!!!
Visually, it looks something like this:
Why is this bending of light so cool and useful enough to be studied?
Okay, real talk: if you're anything like me, halfway through the semester, mentally fried and now learning about diffraction for the first time, you might be thinking:
“Yeah, okay, waves bend… so what? Why should I care if light waves bend too 😭?”
Here’s where it gets cool, and the reason I suddenly had a whole new respect for this topic:
Astronomers use diffraction to study the composition of stars and planets!!!!!
Yesss! Just by sitting here on Earth and analysing light coming towards us from the cosmos, scientists can figure out what gases are present in a distant star or even in the atmosphere of an exoplanet! And that’s not all; this technique helps us search for bio signatures (clues of life!) in space 😳!!!
So, how exactly does bending light help us with that?
Enter: Spectroscopy
Spectroscopy is the study of how light interacts with matter. And the tool that makes this possible is a spectroscope, an optical device that splits incoming light into its spectrum (basically, a rainbow).
You may remember from our chemistry class, when we learnt that different elements can burn to produce various characteristic coloured flames? (Now the reason behind the different coloured flames is understood once we know about the various models of the atom and the behaviour of electrons, but don't worry, we won't be going into the depth of that today, it's a whole topic for another day)
For now, what's relevant is:-
Each element interacts with light in its unique way.
When we pass that coloured light through a spectroscope (where it undergoes diffraction), it doesn’t just give us a normal, plain, pretty rainbow. We see dark lines in specific places; these are called absorption lines, and they tell us which wavelengths of light are missing. These missing pieces form a kind of unique barcode for each element, a unique signature that we can use to recognise an element.
Now, here’s the magical part:
When we point a spectroscope at the light from a distant star or planet, we get a spectrum, and we look for those dark lines. If we spot a pattern that matches an element we already know (like oxygen, hydrogen, or sulphur), boom! Then we know that element is present in that star or planet’s atmosphere.
And that’s how scientists can decode the chemical composition of celestial objects, just from their light.
Now here’s where it gets even cooler: On Earth, gases like oxygen and methane are produced mainly by living organisms. So, if we find these gases in the atmosphere of a distant planet, it becomes a potential bio signature, a clue that there might be life out there.
So yeah… that simple bending of light? Not so boring after all. It's helping us search for alien life!!!
A fun Experiment to try at home!
Now, getting back to diffraction because we did go a little deep into spectroscopy there: here’s a fun little experiment you can do at home to see diffraction patterns for yourself! Yep, you don’t need a fancy lab to prove that light doesn’t always travel in a straight line.
In my senior year of high school, I did a project with my bestie to calculate the thickness of a hair strand using diffraction. Now, our project did get a bit technical and involves some formulas, but don’t worry, I’ve added a little geek section at the end for those who want to dig deeper. For now, I’ll keep it light.
While the original purpose of the experiment was to measure the thickness of the hair, it’s also just a super cool and simple way to witness diffraction in action, and to feel a little more amazed by how light behaves.
What You’ll Need:
A laser pointer
A thread or hair strand (yes, really XD)
Transparent tape
A plain wall in a dim/dark room
What To Do:
Tape the strand vertically across the opening of the laser pointer to ensure that the light hits the strand as it comes out.
Turn off the lights and dim the room.
Point the laser at a plain wall, and observe what happens.
You’ll see a pattern of bright and dark bands around the central spot of the laser. You'll see something similar to this:
Now you might be thinking: why go through all this with a laser and a tiny hair? Why not just use a torch and a pencil? Well, here’s the thing: to see a diffraction pattern clearly, a few conditions have to be just right:
We need monochromatic light (light of a single colour), which is why lasers are perfect. White light tends to spread into a rainbow and blur the pattern.
The size of the object matters too. The size of the obstacle should be comparable to the wavelength of the wave; this is the reason why you can hear with the doors shut but cannot see (because light waves have shorter wavelengths than sound waves and aren't able to diffract around all the objects so easily).
And finally, the distance between the object and the screen or wall affects how spaced out and visible the pattern is.
And yes, diffraction doesn’t only happen in cool experiments. It’s happening around you all the time!
Take shadows, for example. You might have noticed that the edges of your shadow aren’t perfectly sharp. That’s because diffraction occurs even there, light bends slightly around your body, softening the edge of the shadow.
(We don’t see dark and light bands like we do in the laser experiment because:
Our bodies are way too big compared to the wavelength of light, hence not allowing the bending waves to cause interference to form patterns,
We’re usually using white light, which scatters,
And the conditions just aren’t ideal for forming clear patterns.)
But still, diffraction is happening, and those fuzzy edges are its quiet little signature 😌
Geek Corner
For those who want to understand the mathematics behind my experiment and dig a little deeper :)
feel free to skip this part if you're not into the mathematics behind the diffraction experiment <3
Let’s Understand the Setup Through the Experiment We Did!
Here’s a diagram that shows what was happening in our hair-diffraction experiment:
In this case:
a = thickness of the hair strand (this is the value we wanted to find!)
D = distance between the light source and the wall (since we taped the hair onto the laser pointer, this is just the distance between the laser and the wall where the pattern appeared)
p = path difference = wavelength of the laser light wavelength of the laser light (this is constant and known for red laser, usually around 650 nm)
x = distance between the central bright fringe (central maxima) and one of the dark fringes
central maxima = the brightest part of the pattern, right at the centre
bright fringe/maxima = the light bands, the red spots here that we saw
dark fringe/minima = one of the dark bands you see on either side of the central bright one
Mathematics behind dark and bright fringes:
Whenever two light waves meet, they can either add up (constructive interference) or cancel out (destructive interference), and what decides that is the path difference (the difference in distance each wave has travelled).
General Rule (For All Wave Interference Setups):
Constructive interference (bright fringes) occur when:
Path difference = 0, λ, 2λ, 3λ,..., nλ
→ Waves arrive in phase, crest meets crest.
Destructive interference (dark fringe) occurs when:
Path difference = λ/2, 3λ/2, 5λ/2,..., (2n+1)λ/2
→ Waves arrive out of phase, crest meets trough.
This applies to all types of interference — double slit, single slit, even sound or water waves.
In single slit diffraction, you might’ve seen that dark fringes are calculated using nλ and bright fringes use (2n+1)λ/2. That feels backwards, right?
In this setup, it’s not two separate slits interfering; it’s different parts of the same slit interfering with each other. Imagine dividing the slit into equal halves, at certain angles, the wavelets from one half perfectly cancel out those from the other half. That’s when destructive interference (dark fringe) happens at nλ.
So the formulas aren’t flipped; the physics underneath is the same. It just shows up differently because of how we define the sources of interference in each case.
Now we use this formula to calculate the thickness of the hair strand using the condition for dark fringes:
a⋅sinθ=n⋅p
Note: we can also measure the bright fringes instead, but the formula for bright fringe formation is
a⋅sinθ=(2n+1)⋅p
hence to keep the calculations really simple we chose to calculate wrt dark fringes instead.
where;
a = thickness of hair strand (the variable we have to find)
θ = angle at which the dark fringe appears
p = wavelength of the light
n = order of the dark fringe (1st, 2nd, etc.)
Now,
p = wavelength of the laser = 650 nm (typical for red laser light)
D = distance between the laser and the wall (you should try 3–4 different distances to get a more reliable average, because that's how experiments are done!)
x = distance between the central bright fringe and the first dark fringe, which you measure using a scale
Once you’ve got all the values, simply plug them into the formula to calculate the thickness of the hair strand!!!
And just like that, with nothing but a laser, a hair, and a ruler, you’ve measured something as tiny as a human hair using the wave nature of light!
Comments