Total Internal Reflection Experiment With Water

Hold a glass of water above your head and look up at the underside of the surface. It is a mirror. Not a partial reflection, not a glare: a complete, silvery mirror you cannot see the ceiling through. That is total internal reflection, and this total internal reflection experiment turns it into something you can see and measure in an afternoon. The showpiece version sends a beam down a falling stream of water. That version honestly needs a laser, so this guide leads with two routes that do not.

Key Takeaways

• Light going from water into air cannot escape if it strikes the surface at more than 48.6° from the normal. All of it reflects back.
• The reliable laser-free versions are a clear acrylic rod, a thick slab of clear jelly, and looking up at a water surface from below.
• The famous water-stream light pipe really does need a narrow, collimated beam. A torch gives only a faint glow in the first few centimetres, and saying otherwise sets you up to fail.
• You can measure the critical angle and get within a couple of degrees of 48.6°, which is a genuine physics measurement at home.
• Best for ages 9 to 15. Any laser step is adult-held, aimed into a sink, and never near anyone’s eyes.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Clear acrylic rod or thick clear tube	20–30 cm	Plastics supplier, model shop, or a clear acrylic ruler	A 30 cm length of clear PVC hose from a hardware shop, filled with water and plugged
Clear gelatine	2 sachets, set in a shallow tray	Supermarket, unflavoured	Flavoured clear jelly works; opaque or milky jelly does not
Bright LED torch with a tight beam	1	Hardware shop	A phone torch, which is more diffuse and gives a weaker effect
Milk	1 drop per litre of water	n/a	This is what makes the beam path visible. One drop, not two
Clear 2 L bottle	1	n/a	For the water-stream version
Drill bit or hot skewer, 4–5 mm	1	n/a	Adult only. Used once, to make the hole in the bottle
Semicircular dish or a clear food box	1	n/a	For the critical-angle measurement; a round glass bowl works
Printed protractor	1	Print one at A4 size	Draw one with a compass and ruler
Class 2 red laser pointer, under 1 mW	1, optional	n/a	Adult-held only. See the safety table. The experiment works without it

Time: prep 20 min plus 2 hours for jelly to set, experiment 30 min, cleanup 10 min Cost: a clear acrylic rod costs a few pounds; the jelly version costs the price of two sachets Mess level: do the water-stream part in a sink or outside

Do the Experiment: A Total Internal Reflection Experiment

Step 1 - Make the surface into a mirror

Fill a clear glass two-thirds with water, hold it above eye level and look upward through the side of the glass at the underside of the surface.

What you should see: A perfect mirror. The bottom of the glass is reflected in the surface and the ceiling is invisible through it. Lower the glass until your eye is level with the surface and the mirror disappears. That transition is the critical angle, found with your head.

Step 2 - Pipe light through acrylic

Press the torch hard against one end of the clear acrylic rod in a dark room, and look at the other end. Then bend the beam path by using a curved acrylic ruler or a gently coiled clear hose full of water.

What you should see: The far end glows brightly while the sides stay comparatively dark. Light entering within the acceptance angle keeps striking the walls at steeper than the critical angle and cannot get out, so it is funnelled along the rod even around a gentle curve. Sharp bends leak, which is informative.

Step 3 - Set clear jelly and shine through it

Make up two sachets of clear gelatine with 20% less water than the packet says, so it sets firm, and pour it into a shallow tray to a depth of 3 cm. Chill for two hours, then cut a slab about 4 cm wide and 15 cm long. Press the torch to one end in a dark room.

What you should see: The slab lights up along its length and glows at the far end, with visible bright patches where the beam strikes the top and bottom faces and bounces. Jelly has a refractive index close to water, so this is the water experiment in solid form, and it is the best laser-free way to actually see the bouncing.

Step 4 - Measure the critical angle

Fill a clear semicircular dish, or a round glass bowl, with water and one drop of milk per litre, and stand it on a printed protractor with the centre of the flat face at the protractor’s origin. In a dark room, send a narrow torch beam in through the curved side so it travels through the water and arrives at the flat face from inside. Rotate the dish and watch the beam that emerges.

What you should see: At shallow angles from the normal, a beam exits and bends away from the normal. As you rotate, the exiting beam gets dimmer and swings closer to the surface, and then vanishes abruptly, at which point all the light reflects back into the water. Read the angle. Expect 46° to 51° against a true value of 48.6°.

Step 5 - Build the water stream and ask for the prediction

An adult drills a clean 4 to 5 mm hole in the side of a clear 2 L bottle, about 3 cm up from the base. Fill the bottle with water plus one drop of milk, cap it, and set it on the edge of a sink with the hole pointing into the basin. Before anything else, ask: “If I shine a beam straight through the bottle and out of the hole, will the light go straight on and hit the far wall, or will it follow the curve of the water?”

Step 6 - Run the stream version

Uncap the bottle so the stream flows in an arc into the sink. In a darkened bathroom, press the torch to the bottle wall directly opposite the hole, aiming through the water at the hole.

What you should see, honestly: With a torch, a faint glow in the first 5 to 10 cm of the stream and a brighter spot where it lands. That is the real result, and it is not the dramatic glowing arc in photographs. With an adult holding a Class 2 red laser aligned through the bottle at the hole, the stream lights up along its whole arc, because a laser beam is narrow enough to stay inside it. Then let the stream break into droplets by lowering the water level: the light stops being guided exactly where the smooth stream breaks up, which proves the guiding depends on a continuous surface and not on water being “sticky” to light.

What’s Actually Going On

Refraction at a boundary obeys Snell’s law: n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal. Water’s refractive index for yellow light at 20 °C is 1.3330, tabulated in the formulation published by Allan Harvey, J. S. Gallagher and J. M. H. Levelt Sengers in the Journal of Physical and Chemical Reference Data in 1998. Air is 1.0003.

Now go from water into air, so n₁ > n₂. Snell’s law requires sin θ₂ = (n₁/n₂) sin θ₁, which is larger than sin θ₁, so the ray bends away from the normal on exit. Increase θ₁ and θ₂ grows faster, until θ₂ reaches 90° and the refracted ray would be running flat along the surface. The angle at which that happens is the critical angle, θc = arcsin(n₂/n₁) = arcsin(1/1.3330) = 48.6°.

Beyond that angle, Snell’s law has no solution: sin θ₂ would have to exceed 1. There is no transmitted ray. All the energy goes into the reflected ray, which is why the reflection is total rather than partial, and why the surface above your glass of water looks like a mirror rather than a window with glare. For common glass at n = 1.52 the critical angle is 41.1°, and for acrylic at n = 1.49 it is 42.2°, which is why an acrylic rod guides light so well.

Inside a stream or a rod, a ray entering near the axis strikes the side wall well beyond the critical angle, so it reflects, crosses, reflects again and zig-zags onward. Nothing is holding the light in; it simply has nowhere else to go. That matters, because two popular explanations are wrong: light is not “attracted” to water or “bent by” the stream, and the stream is not a tube with walls. Break the stream into drops and the guiding stops instantly.

Two changes separate this from a real optical fibre. The index contrast in a fibre is between a glass core and a slightly lower-index glass cladding, not glass and air, so the guiding survives scratches and contact. And at the scale of a single-mode core, around 9 micrometres, the ray picture stops being adequate and the light is better described as waveguide modes. The practical breakthrough was purity, not geometry: Charles Kao shared the 2009 Nobel Prize in Physics for groundbreaking achievements concerning the transmission of light in fibres for optical communication.

How to Explain It to Your Kid

Ages 5–8: The water surface can be a mirror

Give them the glass of water held overhead and let them find the mirror themselves, then lower it until the mirror disappears. Let them do that ten times. The idea to land is simple: looking at water from a steep angle lets you see through, looking along it turns it into a mirror. Then let them light up the jelly slab, which is the part they will remember.

Ages 9–12: There is an angle where light gives up trying to leave

Introduce the critical angle and the number 48.6° for water. Do the measurement in Step 4 and compare. Then connect it to a real object: the light carrying this web page to their home almost certainly travelled through glass fibre for part of its journey, and the same rule kept it inside. The idea of light being steered rather than just reflected also underpins how a TV remote’s infrared beam gets where it needs to go.

Ages 13+: Derive it, then find the acceptance angle

Have them derive θc = arcsin(n₂/n₁) from Snell’s law and explain why it only exists going from high to low index. Then push further: what is the largest angle at which light can enter the end of the rod and still be guided? That is the acceptance angle, it depends on both indices, and it is the geometric root of the numerical aperture quoted on every fibre datasheet. Finally, ask why a real fibre uses a glass cladding instead of air.

The question to ask: “If we filled the rod with a liquid that had a higher refractive index than the rod itself, would it still guide light? Say what would happen and why, before we look it up.”

Why Teaching This Early Pays Off

This is the first place many children meet a threshold phenomenon: nothing, nothing, then an abrupt and complete change at one value of a continuous variable. Most of what they have met so far is gradual. Thresholds reappear in photoelectric effects, activation energies and resonance, and having one they can watch flip on a protractor is a good first encounter.

It is also the clearest case in school optics where the mechanism explains a technology the child uses daily. Fibre to the home, undersea cables and hospital endoscopes all run on the same two lines of algebra. That connection is worth making explicitly, because Galili and Hazan’s 2000 analysis of learners’ optics knowledge found that students’ models stay fragmented when each phenomenon is taught as a separate rule rather than as a consequence of one principle.

Be honest about the limits. One afternoon does not teach waveguide theory, and the ray model here breaks down at the scale real fibres work at. Saying so beats pretending the simple picture is the whole picture.

Before You Start: What Parents Should Know

Best age	9 to 15. Ages 6 to 8 can do Steps 1 to 3 with an adult
Supervision	Adult required for drilling the bottle and for any step involving a laser
Real hazards	Lasers and eyes. Retinal damage from a laser can be permanent and painless. If you use one at all, it must be a low-power red Class 2 pointer, held by an adult, aimed horizontally into a sink and never toward a face, a window, a mirror or an aircraft. Do not buy a high-power green pointer from a market stall or marketplace listing; there is no way to verify its actual output, and laser products are regulated for exactly this reason. Water on a bathroom floor is a slip hazard; drilled plastic leaves sharp edges
Don’t	Don’t substitute a laser for the torch in a child’s hands under any circumstances, and don’t let anyone aim any beam along the water stream toward another person’s eye. The experiment is designed so that the laser is optional

When It Doesn’t Work

• The water stream shows no glow at all with a torch. Expected. A torch is too divergent to stay inside a 4 mm stream. Add one drop of milk, darken the room completely, and look at the first few centimetres. For the full effect you need a collimated beam.
• The acrylic rod leaks light along its length. It is scratched, dusty or frosted, so rays scatter below the critical angle and escape. Wipe it, and prefer a moulded finish to a sawn one.
• The jelly is cloudy and glows everywhere. Too much scattering. Use unflavoured clear gelatine, make it with 20% less water than the packet states, skim any foam off the top before it sets, and chill it properly.
• You cannot find a sharp critical angle in Step 4. Your beam is too wide. Tape a card with a 1 mm slit over the torch. Also check your milk dose: more than one drop per litre scatters the beam so much that the transition smears out.

Take It Further

• Measure the critical angle for three materials. Water, cooking oil and a clear acrylic block, each with the protractor method, then compute n = 1/sin θc and compare with published values. Oil comes out near 1.47, close to acrylic and far from water.
• Chain all three light-steering tricks. Reflect the beam with the two mirrors from a homemade periscope, refract it through the water in the broken pencil experiment, then guide it down the rod. Three different mechanisms, one beam, one afternoon.

What to Watch For Over the Next 3 Months

• Week 4: They notice that a bubble underwater looks silvery, or that the inside of a swimming pool’s surface is a mirror when you look up from below.
• Month 2: They start talking about fibre optic cables as glass that keeps light inside rather than as wires that carry light, which is the correct mental model.
• Month 3: If they ask why a diamond sparkles more than glass, that is the right next question: diamond’s refractive index of about 2.42 gives a critical angle near 24°, so light entering the top is nearly trapped and bounces repeatedly before leaving.

Frequently Asked Questions

Do I need a laser for this to work?

No. Steps 1 to 4 all work without one, and the jelly slab and acrylic rod give a visible light pipe with just a torch. Only the full-length glowing water stream needs a collimated beam, and that step is optional for a reason.

What exactly is the critical angle for water?

48.6°, measured from the normal to the surface, using a refractive index of 1.3330 for water and 1.0003 for air. For glass at 1.52 it is 41.1° and for acrylic at 1.49 it is 42.2°. A higher index gives a smaller critical angle and therefore better light guiding.

Why does the glowing stream stop glowing where it breaks into drops?

Because the guiding depends on a continuous smooth surface at which rays can reflect. Once the stream becomes separate droplets, each drop presents curved surfaces at all sorts of angles and scatters light in every direction instead of reflecting it forward.

Is a real optical fibre just a very thin water stream?

The principle is related but the engineering differs. A fibre guides light at the boundary between a glass core and a lower-index glass cladding rather than glass and air, and at single-mode scales the ray picture must be replaced by waveguide modes.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. Harvey, A. H., Gallagher, J. S., & Levelt Sengers, J. M. H. (1998). “Revised Formulation for the Refractive Index of Water and Steam as a Function of Wavelength, Temperature and Density.” Journal of Physical and Chemical Reference Data, 27(4), 761–774. https://doi.org/10.1063/1.556029
2. Georgia State University. “Total Internal Reflection.” HyperPhysics. https://hyperphysics.gsu.edu/hbase/phyopt/totint.html
3. The Physics Classroom. “Total Internal Reflection.” https://www.physicsclassroom.com/class/refrn/Lesson-3/Total-Internal-Reflection
4. The Nobel Foundation. “The Nobel Prize in Physics 2009.” https://www.nobelprize.org/prizes/physics/2009/summary/
5. US Food and Drug Administration. “Laser Products and Instruments.” https://www.fda.gov/radiation-emitting-products/home-business-and-entertainment-products/laser-products-and-instruments
6. Galili, I., & Hazan, A. (2000). “Learners’ knowledge in optics: interpretation, structure and analysis.” International Journal of Science Education, 22(1), 57–88. https://doi.org/10.1080/095006900290000
7. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165