Waves & optics

CMB Spectrum Thermometer

How can a small foreground make a precise cosmic thermometer read too warm?

12 min

Waves & optics

CMB Sky and Angular Power

Why does a broad telescope beam remove small sky patterns before large ones?

12 min

Waves & optics

Spontaneous Raman Spectrometer

Do molecular lines keep the same energy shift when the laser changes?

18 min

Waves & optics

Optical Tweezers: Brownian Calibration

How does a bead’s thermal motion reveal the stiffness of an optical trap?

18 min

Waves & optics

Attosecond RABBIT Metrology

How can electron spectra reveal the duration of an attosecond pulse train?

20 min

Waves & optics

Radio Echoes from the Ionosphere

Why does a radio echo suggest a higher layer than the height where the wave actually turns?

15 min

Waves & optics

Single-Pass Optical Amplifier

Why does an inverted medium stop giving exponential gain?

20 min

Waves & optics

Passive Optical Resonator

How can two mirrors with no gain between them pick out frequencies and store light?

20 min

Waves & optics

Gain Curve Meets Longitudinal Modes

Which cavity modes survive after they saturate the gain?

25 min

Waves & optics

Stable Resonator Geometry

Does a stable cavity guarantee that its beam fits through the aperture?

25 min

Waves & optics

Alignment & Mode Matching

Why can a centered beam still reflect from a resonant cavity?

25 min

Waves & optics

Mode Locking

Can a laser reach a much higher peak power without any more energy?

25 min

Waves & optics

Semiconductor Laser Diode

When does injected current become coherent output?

25 min

Waves & optics

Ti:Sapphire Ultrafast Oscillator

What keeps an ultrashort pulse alive after every round trip?

25 min

Waves & optics

Raman & Brillouin Lasers

Can transparent glass become a laser gain medium?

25 min

Waves & optics

Random Laser

Can scattering hold light long enough to lase?

25 min

Waves & optics

Thermal Lensing & High-Power Scaling

Can better cooling alone fix the beam of a high-power laser?

25 min

Waves & optics

External Feedback & Laser Chaos

When does a delayed reflection destabilize a laser?

25 min

Waves & optics

Tunable Laser & Mode-Hop-Free Scan

Why does rotating a grating alone cause mode hops?

25 min

Waves & optics

Coherent Beam Combining

In coherent beam combining, does locking the phases fix every error?

25 min

Waves & optics

Microresonator Kerr Comb

When can a laser-driven microring keep a short pulse circling inside it?

25 min

Waves & optics

Pound–Drever–Hall Frequency Lock

Can reflection tell a laser which way to tune?

25 min

Waves & optics

Laser Linewidth & Phase Noise

How does phase noise become a laser linewidth?

25 min

Waves & optics

Injection Locking

How can a weak master laser make a stronger slave laser follow its frequency?

25 min

Waves & optics

Regenerative & Multipass Amplifier

When should a pulse circulating in a regenerative amplifier be let out?

25 min

Waves & optics

Nonlinear Frequency Conversion

Does a longer crystal always make more new light?

25 min

Waves & optics

Chirped Pulse Amplification

Can a short pulse be amplified without its peak power getting dangerously high?

25 min

Waves & optics

Fiber Laser & FBG Cavity

Does more fiber always give more useful output?

25 min

Waves & optics

DFB, DBR & VCSEL Architectures

Where should feedback sit to select a laser mode?

25 min

Waves & optics

Laser Medium Zoo

How do different laser media reach population inversion?

25 min

Waves & optics

Optical Frequency Comb

What must you measure before a frequency comb can work as a ruler for light?

25 min

Waves & optics

Laser Dynamics & Relaxation Oscillation

Why does a laser overshoot before it settles?

25 min

Waves & optics

Q-Switched Giant Pulse

How can stored inversion become a giant pulse?

25 min

Waves & optics

Output Coupler Optimization

Which output mirror gives a laser the most output power?

25 min

Waves & optics

Gaussian Beam & Beam Quality

Can a lens make a poor beam diffraction limited?

25 min

Waves & optics

Transverse Laser Modes

Which beam pattern wins when a laser's transverse modes compete for gain?

25 min

Waves & optics

Build & Debug Your First Laser

When a laser you built won't lase, how do you find the fault?

25 min

Waves & optics

Laser Diagnostic Bench

Which measurements tell laser light apart from fluorescence and amplified spontaneous emission?

25 min

Waves & optics

Laser Noise Startup Workbench

Where does the first photon in a laser cavity come from?

20 min

Waves & optics

Population Inversion Workbench

Why can stronger pumping still fail to produce gain?

15 min

Waves & optics

Three Light–Matter Transitions

Does stimulated emission always mean amplification?

15 min

Waves & optics

Laser vs Lamp vs LED

Can a very bright light still make interference fringes with poor contrast?

15 min

Waves & optics

When Two Waves Meet

Why do passing waves leave a stable fringe pattern?

6 min

Waves & optics

Draw Any Wave with Circles

How can steady rotations rebuild almost any periodic signal?

12 min

Waves & optics

Light after a Narrow Slit

Why does a narrower opening spread light farther?

8 min

Waves & optics

How a Lens Redirects Light

How do three principal rays locate and size an image?

7 min

Waves & optics

Listen to Spacetime with Light

How can a length change smaller than a proton become a readable signal?

15 min

Waves & optics

How One Opening Redraws a Water Wave

Why does narrowing an opening spread the transmitted wave through a wider angle?

9 min

Waves & optics

Two Refractions inside a Thick Lens

What does the thin-lens model miss when thickness matters?

12 min

Waves & optics

A Rainbow Only Nanometres Thick

Why does a colorless film only hundreds of nanometres thick develop flowing color?

11 min

Waves & optics

How a Laser Builds inside a Cavity

Which waves return between two mirrors and keep growing?

15 min

Waves & optics

Optical Boundary Explorer

What happens to light's speed, direction and power when it crosses into a new material?

11 min

Waves & optics

Hall of Mirrors

Which virtual images can an observer actually see as mirrors move?

11 min

Waves & optics

Mirror Engineering Challenge

Can a few mirrors steer a beam to a fixed target just by getting the angles right?

13 min

Waves & optics

Curved Mirror Observatory

How does mirror shape move the focus and change off-axis image quality?

13 min

Waves & optics

Caustic Factory

Why does light reflected or refracted by a curved surface gather into sharp, bright curves?

15 min

Waves & optics

Floating Image Mirascope

Where do two facing parabolic mirrors form the floating image you can see but not touch?

15 min

Waves & optics

Fermat Path Finder

Why can one rule about travel time explain reflection, refraction and curved light rays?

13 min

Waves & optics

Underwater Vision Observatory

How can one flat water surface make a fish look shallower, fool a spear, squeeze the sky and distort photos?

11 min

Waves & optics

Window Ghost Observatory

Why does window glass barely shift the view through it, yet show ghost images at night?

11 min

Waves & optics

N-BK7 Prism Spectrometer Observatory

How do you set up a glass prism so that where each color lands tells you its wavelength?

13 min

Waves & optics

Rainbow Caustic Observatory

How can one water drop create primary and secondary bows with a dark band between them?

13 min

Waves & optics

Mirage & Green-Flash Observatory

How can layers of warm and cool air make distant objects sink, rise or appear several times?

15 min

Waves & optics

GRIN Geometry Observatory

How can a smoothly varying refractive index guide and focus light with no surfaces inside it?

15 min

Waves & optics

Eclipse & Transit Shadow Observatory

How do the sizes of a light source and of the body blocking it shape shadows, eclipses and transits?

13 min

Waves & optics

Unilluminable Room

Can a room with mirrored walls have a spot that a single small lamp inside never lights?

15 min

Waves & optics

Solar Concentrator & Étendue

Why can't any lens or mirror concentrate sunlight onto an arbitrarily small spot?

15 min

Waves & optics

Illumination & Radiometry Studio

How do you light a flat surface evenly when light fades with distance and angle?

13 min

Waves & optics

Ray Cloaking Challenge

Can a set of lenses steer light around an object and leave the background looking unchanged?

15 min

Waves & optics

Branched Light in Random Media

Why does light crossing gently uneven material gather into branches instead of spreading evenly?

15 min

Waves & optics

Gravitational Lens Telescope

How can light bent around an unseen mass reveal how heavy it is?

17 min

Waves & optics

Thin Lens Imaging Bench

How do three principal rays locate every real and virtual thin-lens image?

11 min

Waves & optics

Thick Lens Designer

What does the thin-lens approximation miss for a lens with real thickness?

17 min

Waves & optics

Compound Optics Workbench

How can one matrix tell you where a chain of lenses forms its image and how much it magnifies?

17 min

Waves & optics

Parfocal Zoom Challenge

Can moving lens groups change focal length while the image plane stays fixed?

17 min

Waves & optics

Camera Obscura

What size of pinhole gives the sharpest picture?

13 min

Waves & optics

Photography Optics Playground

From one camera position, what do focal length and aperture change in a photo?

13 min

Waves & optics

Lens Aberration Clinic

How do you identify a lens aberration from its blur and fix it without creating another?

17 min

Waves & optics

Anamorphic & Cylindrical Optics

How can a lens stretch or squeeze an image in one direction only?

15 min

Waves & optics

Human Eye & Corrective Optics

What lens corrects short sight, long sight, presbyopia or astigmatism?

13 min

Waves & optics

Build a Microscope

What sets the smallest detail a microscope can resolve?

13 min

Waves & optics

Build a Telescope

How do aperture, optical layout, and obstruction determine what a telescope can resolve?

13 min

Waves & optics

Binocular Prism Lab

How do binocular prisms turn the image upright, and why can a roof prism blur it?

17 min

Waves & optics

Telecentric Metrology Camera

Can a camera measure an object's size correctly when the image is out of focus?

15 min

Waves & optics

Tilt-Shift & Scheimpflug Studio

How does tilting the lens rotate the plane of focus and reshape depth of field?

15 min

Waves & optics

Pupils, Stops & Vignetting

What do the different openings inside a lens system each limit?

17 min

Waves & optics

Projector Optics Lab

Why can a bigger light source fail to brighten a projector, and why does a tilted screen also blur?

17 min

Waves & optics

Smartphone Camera Module

What do folding, autofocus and stabilization each do in a thin phone camera, and what can none of them fix?

17 min

Waves & optics

VR Headset Optics

Why can a VR headset change depth while focus stays fixed, and why does a small eye shift cut off the corners?

15 min

Waves & optics

Ghost Image, Pepper’s Ghost & HUD

Where does a half-mirror image appear, and why can a bright background or the mirror’s edge hide it?

13 min

Waves & optics

Light-Field Camera & Display

Can one light-field photo be focused on either near or far objects, and what does that cost?

15 min

Waves & optics

Coherent Wave Composer

Why do a phasor diagram, a wavefront picture and a detector reading all describe the same interference?

11 min

Waves & optics

Young Double-Slit Bench

How do slit width, slit spacing, phase and coherence each change a double-slit pattern?

11 min

Waves & optics

Thin-Film Color Studio

Why does a thin film reflect color, and why does the color shift when you tilt it?

13 min

Waves & optics

Interference Surface Metrology

How can interference fringes map the shape of a surface, down to tiny defects?

13 min

Waves & optics

Michelson Precision Lab

How can counting fringes measure displacement, wavelength, and refractive index?

13 min

Waves & optics

LIGO Interferometer

How does a kilometer-long interferometer turn a passing gravitational wave into a measurable signal?

17 min

Waves & optics

Mach–Zehnder Sensor

Can a Mach–Zehnder’s two outputs tell a refractive-index change from absorption or polarization marking?

17 min

Waves & optics

Fabry–Pérot Passive Cavity

How can two highly reflective mirrors let all the light through, and what does loss do to the resonance?

17 min

Waves & optics

Sagnac Gyroscope

How can two beams in the same fiber detect rotation, and why can’t brightness alone tell which way it turns?

17 min

Waves & optics

Division-of-Wavefront Interferometers

Why is the fringe dark where Lloyd’s mirror meets the screen, but bright at a biprism’s center?

17 min

Waves & optics

Low-Coherence Interferometry & OCT

How can broadband light reveal the depth a reflection came from inside a sample?

15 min

Waves & optics

Stellar Interferometer

How can two telescopes measure a star too small for either one to resolve?

15 min

Waves & optics

Speckle Lab

Why does coherent light scattered by a diffuser look grainy, and can averaging images smooth it out?

15 min

Waves & optics

Holography Studio

How can a plate that records only brightness keep the phase needed to rebuild a three-dimensional wavefront?

17 min

Waves & optics

Moiré & Fringe Metrology

How can broad fringe patterns reveal a tiny shift or the shape of a surface?

15 min

Waves & optics

Huygens–Fresnel Propagation Lab

How does a diffraction pattern change from close to the aperture to far away?

13 min

Waves & optics

Aperture Diffraction Studio

How is the diffraction from a hole related to that from an obstacle of the same shape?

13 min

Waves & optics

Crystal Diffraction & Reciprocal Space

How can the spots and rings in a diffraction pattern reveal a crystal’s lattice?

17 min

Waves & optics

Diffraction-Grating Spectrometer

What sets how finely a grating spectrometer can separate two close wavelengths?

13 min

Waves & optics

Diffractive Lens Workshop

How does a diffractive lens focus light, and what do finer phase steps improve?

17 min

Waves & optics

Talbot Carpet

Why does light behind a grating repeat the grating’s pattern at certain distances?

15 min

Waves & optics

Optical Fourier Processor

How can a mask between two lenses keep only an image’s edges, blur it or erase a repeating pattern?

17 min

Waves & optics

Abbe Resolution Lab

Why can tilting the light reveal stripes a microscope misses, and why can blocking some light fake finer ones?

13 min

Waves & optics

PSF–OTF–MTF Imaging Lab

Can a lens swap the bright and dark parts of stripes, and can pixels invent a stripe period that isn’t there?

17 min

Waves & optics

Phase Microscopy Lab

Can a transparent cell be seen without staining, and is the relief it seems to have real?

15 min

Waves & optics

Schlieren Flow Viewer

How does a knife edge make invisible air flow visible, and why does turning it make some features vanish?

15 min

Waves & optics

Pupil Engineering & Coronagraph

How can a telescope block a star’s light and still see a planet right next to it?

15 min

Waves & optics

Phase Retrieval Challenge

Can you recover the phase of light from brightness measurements alone?

15 min

Waves & optics

Coded-Aperture Camera

How can a camera with a patterned mask instead of a lens form an image?

15 min

Waves & optics

Computational Deblurring Lab

When does deconvolution recover detail, and when does it only amplify noise or model error?

17 min

Waves & optics

Adaptive Optics Observatory

How can measured wavefront slopes drive a mirror fast enough to sharpen a turbulent star image?

15 min

Waves & optics

Ptychography & Coherent Diffraction Imaging

How can overlapping illumination recover both the amplitude and the phase of an object without a lens?

15 min

Waves & optics

Polarization State Explorer

How can a Jones vector, an ellipse and Stokes measurements all describe the same polarization state?

13 min

Waves & optics

Polarized Reflections & Sky

Why does a polarizing filter usually need different angles to darken water glare and blue sky?

13 min

Waves & optics

Birefringent Crystal Lab

Why does a birefringent crystal split one ray into two side-by-side rays?

13 min

Waves & optics

Waveplate & Poincaré Sphere Lab

How does a waveplate move a polarization state on the Poincaré sphere?

13 min

Waves & optics

Optical Activity Polarimeter

What can a rotating analyzer really tell us about an unknown chiral solution?

13 min

Waves & optics

Photoelastic Stress Viewer

How do colorful polarization fringes map the principal-stress difference inside a loaded object?

15 min

Waves & optics

LCD Pixel Microscope

How can three voltage-controlled liquid-crystal shutters make one colored pixel?

17 min

Waves & optics

TIR Phase & Fresnel Rhomb

How can total internal reflection, which returns all the light, act as a waveplate that works across many wavelengths?

15 min

Waves & optics

Faraday Isolator

Why can a Faraday isolator pass light going forward but block the same light coming back?

17 min

Waves & optics

Electro-Optic Modulator

How can a voltage across a crystal control the brightness of a light beam?

17 min

Waves & optics

Optical Fiber: Rays to Modes

When does the picture of rays bouncing inside a fiber stop working, so that light must be treated as modes?

13 min

Waves & optics

Fiber Pulse Propagation

How does dispersion reshape an ultrashort pulse as it travels along a fiber?

17 min

Waves & optics

Evanescent Directional Coupler

How can light cross from one waveguide into a nearby one that it never touches?

15 min

Waves & optics

Microring & Whispering-Gallery Resonator

Why does a small ring beside a waveguide make the transmitted light dip at certain wavelengths?

15 min

Waves & optics

Photonic Crystal Band Structure

How can a periodic dielectric block some frequencies of light, yet trap or guide light at defects?

15 min

Waves & optics

Boundary Wave Tunnel

How can light still get through a narrow gap when it should be totally internally reflected?

15 min

Waves & optics

Beam Shift Microscope

Why does a reflected light beam shift slightly, both within and out of the plane of incidence?

15 min

Waves & optics

Gaussian Beam & Gouy Phase

How do the width and phase of a focused Gaussian beam change as it passes through the focus?

17 min

Waves & optics

Self-Healing Beam Arena

How do Bessel and Airy beams rebuild themselves after part of them is blocked?

15 min

Waves & optics

Light in Disorder

How can interference in disordered material send extra light straight back, or stop light from spreading sideways?

15 min

Waves & optics

Optical Vortex & OAM

Can a bright petal pattern carry zero average orbital angular momentum?

15 min

Waves & optics

Vector Beams & Optical Skyrmions

What makes the polarization pattern across a beam an optical skyrmion?

15 min

Waves & optics

Photonic Nanojet & Hook

How can a tiny transparent particle squeeze light into a very narrow jet, or bend it into a hook?

15 min

Waves & optics

Superoscillation Designer

How can band-limited waves add up to a hotspot smaller than the diffraction limit?

15 min

Waves & optics

Surface Plasmon Resonance Lab

How does surface plasmon resonance detect a change in a sample's refractive index?

15 min

Waves & optics

Negative Refraction & Superlens

How does a negative-index slab bend light the wrong way and recover fine detail that normally fades out?

15 min

Waves & optics

Topological Photonic Edge State

Why is a topological edge mode so hard for defects to remove?

15 min

Waves & optics

Nonlinear Light Shapes Its Own Medium

When can a bright beam change a material's refractive index enough to focus itself and stop spreading?

15 min

Waves & optics

Absorption, Spontaneous & Stimulated Emission

What decides whether atoms absorb resonant light or amplify it?

13 min

Waves & optics

Pumping & Population Inversion

Why can't pumping a two-level system produce population inversion, when three- and four-level schemes can?

13 min

Waves & optics

Laser Startup from Noise

How does a laser grow from random noise into one dominant mode?

17 min

Waves & optics

Stable Resonator & Transverse Modes

How should two mirrors be curved and spaced to make a stable cavity that favors the fundamental mode?

17 min