Refraction: Bending Light

Light and Optics

R-report

L23. Refraction: Bending Light

Why does a straight pencil look broken in a glass of water, and how do curved pieces of glass use that same bending to make images?

A surprising classroom phenomenon

Hold a clear glass half full of water and put a straight pencil or straw into it. Look from the side at the point where the straw meets the water. Many students notice the straw appears to kink or break at the surface so that the submerged part looks shifted. This visual puzzle is our phenomenon for the lesson: the path of light appears changed when it crosses the watery surface. You have already learned in earlier lessons that light travels in straight lines in a single uniform material; here we only add one short prerequisite: when light crosses from one material into another, something different must happen at the boundary.

• Notice where the apparent bend occurs.

• Compare the ray in air with the ray in water.

• Check how the viewing angle changes the apparent position.

What bending means: normal and change of speed

To explain the apparent kink, draw an imaginary line perpendicular to the water surface at the point where the light meets the boundary; scientists call that the normal. A light ray coming from the submerged straw toward your eye does not keep the same direction when it crosses from water into air. Instead it bends at the surface, changing direction toward or away from that normal depending on the materials. The simplest cause we use in middle school physics is change of speed: light travels more slowly in water than in air, so the ray coming out of water bends toward the normal. You do not need the full math here, only the idea that different materials change light's speed and that the boundary plus the normal help predict which way the ray bends.

A simple experiment you can try

Materials: a clear drinking glass, water, a straight pencil or plastic straw, a ruler and a sheet of paper. Place the glass on the paper and mark the rim location. Put the pencil in the glass and stand so you view the straw at an angle, not from directly above. Trace the apparent position of the straw on the paper as you look from one side, then walk a little and trace the apparent position from another viewing spot. You will see the traced lines shift. To connect the experiment to the normal, draw a perpendicular at the contact point and sketch the incoming and outgoing rays as you imagine them. The predicted observation is that rays leaving the water bend toward the normal when entering air, producing the apparent displacement of the submerged part of the straw. This hands-on test shows how refraction creates a consistent pattern of apparent position changes.

From bending rays to lenses and uses

Refraction at a flat surface explains the broken-straw effect; refraction at curved surfaces is how lenses work. A convex (bulging) lens makes rays that enter parallel to its center bend and meet at a focal point; a concave (curved inward) lens makes parallel light spread apart. Each curved surface refracts, so a lens combines two refractions in sequence to focus or diverge light. That is why eyeglasses, magnifying lenses, camera lenses, and simple magnifiers change how large or sharp an object appears. In your experiment, imagine taking many parallel rays from the straw and letting a curved glass surface bend them gradually; the same speed-change idea and the normal at each tiny patch of surface still predict the direction change. Observations you can record include where the straw appears, how that position changes with viewing angle, and whether the bending increases when you use a denser liquid. Lens uses to notice: magnifying small details, correcting vision, focusing light onto camera sensors, and concentrating sunlight in solar collectors.

• Convex lenses bring rays together to focus light.

• Concave lenses spread rays apart.

• Eyeglasses, cameras, and microscopes use these controlled bends.

Key connections

Refraction is the change in direction of light that happens when the light crosses a boundary between materials because its speed changes; using the normal at the surface helps predict whether the ray bends toward or away from that normal.

That same bending at many tiny patches of a curved surface is how lenses focus or spread light, so the simple broken-straw observation leads directly to useful technologies like eyeglasses and cameras.