Light travels in straight lines through any single, uniform medium — but the moment it passes at an angle from air into water or glass, its direction changes abruptly at the boundary. This is refraction, and it is one of the most fun, most easily observed topics in middle school optics.

This guide starts with two refraction effects you can produce with nothing but a glass of water, then uses a draggable interactive simulation to make the patterns visible: the larger the angle of incidence, the larger the angle of refraction — and light entering a denser medium bends toward the normal.

Safety first (read this before you start)

  • If you use a laser pointer to make the light path more visible, follow the rules strictly: never aim it at eyes, people or pets, and never at mirrors, glass or other reflective surfaces. If you can manage without a laser, do.
  • A glass full of water is heavy and breakable — handle it carefully and keep it away from any electronics on the table.

Materials and equipment

  • A clear, straight-sided drinking glass and tap water;
  • A chopstick or pencil;
  • A coin and an opaque bowl;
  • Optional: a laser pointer, and water with a few drops of milk stirred in (to make the light path visible in the water).

Two effects to see first

Effect one: the "broken" chopstick.

  1. Fill a clear glass about three-quarters full of water.
  2. Dip a chopstick into the water at an angle and look at it from the side.
  3. The chopstick appears to shift, thicken, even "snap" right at the water's surface.

Effect two: the coin that vanishes and reappears.

  1. Place a coin in the center of the bottom of an empty bowl.
  2. Slowly step back and lower your line of sight until the rim of the bowl just hides the coin — then hold your head still.
  3. Have someone else pour water gently down the side of the bowl. The coin "floats" back into view.
Why this happens: light coming from the chopstick and the coin refracts at the water's surface, changing direction before it reaches your eyes. Your brain still assumes light travels in straight lines, so it "sees" the object in a different place. The interactive simulation below magnifies exactly that redirected ray.
Interactive simulation: this experiment has a drag-and-play simulator in our Interactive Lab. The lab interface is currently Chinese-only — the controls are simple sliders and switches, so it is still easy to explore.

Recording what you see

The best record for this experiment is a table of angle pairs copied from the simulator. For light going from air into water (refractive index about 1.33), you'll get a series like this:

  • Angle of incidence 0° → angle of refraction 0° (head-on entry, no bending);
  • Angle of incidence 20° → angle of refraction about 15°;
  • Angle of incidence 45° → angle of refraction about 32°;
  • Angle of incidence 70° → angle of refraction about 45°;
  • Angle of incidence 85° → angle of refraction about 48°.

Read the five rows together and two things stand out:

  • The angle of refraction is always smaller than the angle of incidence — and the two are not proportional. Going from 0° to 45° of incidence, the refraction angle grows by 32°; but going from 45° to 85° (another 40°), it grows by only 16°. So "double the incidence angle, double the refraction angle" is wrong.
  • However far you push the angle of incidence (at most 90°), the angle of refraction tops out at about 48.8°. That ceiling is the flip side of water's critical angle. Going the other way — from water into air — once the angle of incidence exceeds 48.8°, no light escapes at all. That is total internal reflection, the principle that lets optical fibers trap light inside a thread of glass.

Don't forget the real-world side of the record: the chopstick always appears to "break" exactly at the water's surface, and the lower your line of sight (the larger the angle of incidence), the bigger the offset. Looking straight down from above, the chopstick looks almost straight — which matches "no bending at 0° incidence" perfectly.

The science: why light refracts

Light travels at different speeds in different media. When light crosses from one medium into another at an angle, the change in speed makes its direction bend at the boundary — that is refraction. To describe directions, we draw a line through the point of entry perpendicular to the boundary, called the normal. The angles between the rays and the normal are the angle of incidence and the angle of refraction.

At the middle school level there are a few rules to remember — every one of them can be verified in the simulation:

  • When light passes at an angle from air into water or glass (into an optically denser medium), the angle of refraction is smaller than the angle of incidence — the ray bends toward the normal;
  • Going the other way, from water or glass into air, the angle of refraction is larger — the ray bends away from the normal;
  • As the angle of incidence increases, the angle of refraction increases too;
  • When light hits the boundary head-on (angle of incidence 0°), its direction does not change — no bending occurs;
  • Whenever refraction happens, part of the light is also reflected back into the original medium (the gray dashed ray in the simulator).

A medium's power to bend light is described by its refractive index n: the larger n is, the more sharply light bends at the same angle of incidence. Try switching the lower medium from water to glass to diamond and watch the refracted ray hug the normal more and more closely.

Refraction in everyday life

Once you understand refraction, plenty of everyday sights start making sense:

  • A swimming pool or pond looks shallower than it really is — light reflected from the bottom refracts on the way up to your eyes;
  • A chopstick in water looks "broken" — effect one of this experiment;
  • Magnifying glasses, eyeglasses and camera lenses all form images by refracting light through lenses;
  • A rainbow after the rain comes from sunlight being refracted and dispersed inside tiny water droplets.

Four things students get wrong most often

  • "The pool looks shallow because the water blocks some of the light." No. Light reflected from the bottom bends away from the normal as it leaves the water at an angle, reaching your eyes along a steeper path. Your brain traces that path backward in a straight line and "places" the bottom higher than it really is. What you see is a virtual image, shallower than the true depth. Water that looks shallow may be deep. This isn't just an exam point — it's a safety rule: never judge water depth by eye before getting in.
  • "The angle of incidence and the angle of refraction are proportional." They are not — the data series above is the counterexample. What actually stays constant is the ratio of their sines (you won't be asked to compute it in middle school, but you do need to know "not proportional").
  • "When light enters head-on, no refraction happens." Strictly, no bending happens. The light really does slow down as it enters the water — only its direction is unchanged. On a true/false question, "no refraction occurs at normal incidence" is usually marked wrong; "the direction of travel does not change" is the safe phrasing.
  • Mixing up refraction and reflection. Two things happen at the same boundary at the same time: part of the light returns to the original medium (reflection, obeying "angle of reflection equals angle of incidence"), and part enters the new medium with a changed direction (refraction). In the simulator, the gray dashed ray is the reflection and the red ray is the refraction — watching both together makes it stick.

Tips for teachers and parents

  • Timing and groups: 20 minutes, works well as a lesson opener; demonstrate for the class, then have small groups reproduce it.
  • Say it before hands touch equipment: laser pointers are handled by adults only, and never pointed at eyes.

The step most people skip. Do the "vanishing coin" before teaching any vocabulary: have students freeze the moment the coin disappears while someone else pours the water. The gasp that follows is the best lesson opener you'll get.

How to tell they really understand. If a student can draw the normal correctly on a diagram, label the angles of incidence and refraction, and state "from air into water, the angle of refraction is smaller than the angle of incidence," they've met the standard.

Take it further

  • In the simulator, push the angle of incidence very high (say 80°) and compare the refraction angles for water, glass and diamond — a direct feel for what refractive index does.
  • If you have a laser pointer and milky water, shine the beam into the water at an angle and check whether the real light path matches the simulation (mind laser safety).
  • Flip an arrow around. Draw an arrow pointing right on a piece of paper and stand it behind an empty glass — through the glass it still points right. Fill the glass with water and look again: the arrow now points left. A water-filled cylindrical glass acts like a convex lens; the refracted rays converge and cross, flipping left and right.
  • Make a test tube "disappear." Submerge a small glass test tube in cooking oil and it nearly vanishes: some cooking oils have a refractive index very close to glass, so light barely bends at their boundary — and with no bending, there is no visible boundary. We can only see transparent objects because of differences in refractive index.
  • Think about it: the Sun has already sunk below the horizon — why can we still see it? (Hint: the atmosphere gets denser toward the ground, and sunlight bends steadily as it passes through.)

When a student can use the words "normal, angle of incidence, angle of refraction, refractive index" to explain why a chopstick looks broken in water, this experiment has done its job.