Light Behavior Essentials: Rays, Media, and What We Measure

A working model of light: rays and wavefronts

In this course we will use two complementary pictures of light, chosen because they are practical for predicting what you see in lenses, mirrors, and everyday scenes.

Ray model (geometric optics)

A ray is an imaginary line that shows the direction light energy travels. Rays are especially useful when objects and apertures are much larger than the wavelength of light (typical for everyday optics). With rays, you can draw where light goes, where it reflects, and where it bends at boundaries.

• Rays travel in straight lines in a uniform medium (like still air or uniform glass).
• Rays change direction only when the medium changes (at a boundary) or when the medium varies gradually (like hot air above a road).

Wavefront idea (without heavy math)

A wavefront is a surface (often drawn as a line in 2D diagrams) connecting points where the light wave is in the same “phase” (think: crests lined up). Rays are drawn perpendicular to wavefronts. This helps you reason about bending: when a wavefront enters a new medium and one side slows down first, the wavefront pivots, and the ray direction changes.

What we measure: wavelength, frequency, speed, refractive index

Light can be described by a few key quantities. You do not need equations beyond the conceptual relationships below, but we will keep the ideas consistent.

Wavelength (λ)

Wavelength is the distance between repeating points of the wave (crest-to-crest). In visible light, different wavelengths correspond to different perceived colors (roughly: shorter wavelengths look “bluer,” longer look “redder”).

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Frequency (f)

Frequency is how many wave cycles pass a point each second. Frequency is set by the source (a lamp, LED, the Sun) and is extremely important because it typically stays the same when light crosses from one medium to another.

Speed in a medium (v)

Light travels at different speeds in different materials. In vacuum it travels at c (the maximum speed for light). In materials (air, water, glass) it travels slower: we call that speed v.

Refractive index (n)

The refractive index tells you how much light slows down in a medium compared with vacuum:

n = c / v

Higher n means slower light in that material and (often) stronger bending at boundaries.

How these relate conceptually

• When light enters a new medium, its frequency stays the same (the source “sets” it).
• If the speed changes but frequency stays the same, the wavelength must change in that medium.
• So: slower medium (higher n) → shorter wavelength in that medium.

We will use this idea repeatedly: boundaries change speed and wavelength, not frequency.

Dispersion (why colors bend differently)

In many materials, n depends slightly on wavelength. This is called dispersion. It is why a prism spreads white light into colors and why some lenses show color fringes near edges. You do not need the detailed formula; just remember: different colors can refract by different amounts.

Optical interfaces: what happens at a boundary

An optical interface is a boundary between two regions where light’s speed (and thus refractive index) differs, such as air–glass, air–water, or glass–plastic. When a ray hits an interface, several things can happen at once.

1) Reflection

Some light can bounce back into the original medium. For smooth surfaces (like a mirror or calm water), reflection is mostly specular (mirror-like): rays leave in a single well-defined direction.

Law of reflection (angle rule): the angle of reflection equals the angle of incidence, measured from the normal.

2) Refraction

Some light can transmit into the second medium and change direction because its speed changes. This bending is refraction. You will later use Snell’s law for calculations, but for now keep the qualitative rule:

• Entering a higher index medium (slower): ray bends toward the normal.
• Entering a lower index medium (faster): ray bends away from the normal.

3) Absorption

Some light energy can be converted to other forms (often heat) inside the material. Dark materials absorb more; clear materials absorb less. Absorption can depend strongly on wavelength (e.g., tinted glass).

4) Scattering

Light can be redirected in many directions by small-scale irregularities or particles. Scattering is why frosted glass looks “milky,” why fog reduces contrast, and why dusty air makes light beams visible.

Angle and sign conventions we will use throughout the course

To avoid confusion later (especially with lenses and mirrors), we will adopt consistent conventions now.

Normals and angles

• The normal is an imaginary line perpendicular to the surface at the point where the ray hits.
• All incidence, reflection, and refraction angles are measured from the normal, not from the surface.
• Angles are taken as positive magnitudes in basic reflection/refraction diagrams; the geometry (which side of the normal) is handled by the drawing.

“Toward” and “away from” the normal

• Toward the normal means the refracted ray makes a smaller angle with the normal than the incident ray.
• Away from the normal means the refracted ray makes a larger angle with the normal than the incident ray.

Ray arrows and media labels

• Always draw arrows showing the direction of travel.
• Label the two media (e.g., air, water, glass) so you can reason about which side is higher index.

Predict-then-check activities (everyday optics)

These short activities are designed to build intuition. For each one: (1) predict using the rules above, (2) check by observing, (3) explain using rays, normals, and interfaces.

Activity A: Glass of water — “broken” straw and apparent depth

What you need: a clear glass, water, a straw or spoon, and a coin (optional).

Step-by-step

• Fill the glass with water and place the straw so part is in air and part is in water.
• Look from the side at the waterline where the straw crosses the surface.
• Now move your head left/right and slightly up/down while keeping your eyes on the crossing point.

Predict

• At the air–water interface, rays from the underwater part will refract as they exit into air.
• Because light goes from higher index (water) to lower index (air), rays bend away from the normal.
• Your brain traces rays backward in straight lines, so the underwater portion should appear shifted, making the straw look “broken.”

Check

• You should see a kink at the surface and a change in apparent position of the submerged part.

Explain (ray sketch guidance)

• Draw the water surface as a line, then draw a normal at the point where a ray exits.
• Draw an incident ray in water heading toward the surface; then draw the refracted ray in air bending away from the normal.
• Extend the refracted ray backward into the water with a dashed line: where it seems to come from is the apparent position.

Activity B: Shiny spoon — curved mirror behavior

What you need: a shiny metal spoon.

Step-by-step

• Look at your reflection in the inside of the spoon (the concave side).
• Move the spoon closer and farther from your face.
• Flip the spoon and look at the back (the convex side).

Predict

• The spoon is a reflective interface: most of what you see is specular reflection.
• Concave side: as distance changes, the image can switch between upright and inverted (because reflected rays can converge and cross).
• Convex side: the image should remain upright and appear smaller (reflected rays diverge).

Check

• You should observe an inversion on the concave side at some distance, and a consistently upright, reduced image on the convex side.

Explain (angle convention practice)

• Pick one point on the spoon and imagine the local normal (perpendicular to the surface at that point).
• Use “angle in equals angle out” with respect to that normal to reason about where the reflected ray goes.

Activity C: Window reflection — when glass acts like a mirror

What you need: a window (daytime and nighttime observations are ideal).

Step-by-step

• In daytime, look at a window from an angle and notice both the scene outside and faint reflections of the room.
• At night (or with a darker outside), look again: the reflection becomes stronger.
• Change your viewing angle: look more “grazing” along the glass surface.

Predict

• At each air–glass interface, some light reflects and some transmits.
• When outside is dark, transmitted light from outside is weak, so the reflected indoor light dominates and the window looks mirror-like.
• At more grazing angles, reflection tends to become more noticeable.

Check

• You should see stronger reflections at night and at shallow viewing angles.

Explain (interface outcomes)

• Identify two interfaces: air→glass and glass→air. Each can reflect and transmit.
• What you perceive depends on relative brightness of the two sides plus how much reflection occurs at your viewing angle.

Quick self-check: classify what you’re seeing

Use this checklist when you encounter a new optical situation in later chapters.

• Is there an interface? Identify the two media and which likely has higher refractive index.
• What outcomes are present? Reflection, refraction, absorption, scattering (often more than one).
• Where is the normal? Draw it and measure angles from it.
• Does the ray bend toward or away from the normal? Decide based on higher-to-lower or lower-to-higher index.
• Could scattering be hiding the geometry? If the surface is rough or the material is cloudy, expect diffuse directions and reduced image sharpness.