Trace light through reflection, transmission and refraction

Learning goal: Predict and investigate what happens when light meets different materials. 5.PS.2 is taught across three companion lessons: this one (light paths, reflection, transmission and refraction), Investigate reflected color and warming from absorbed light (color and absorption) and Investigate sound vibrations, pitch and travel through materials (sound). Light is a form of energy that travels from a source. A lamp emits light; an ordinary book is visible when light from another source reaches the book and some reflected light enters an observer's eyes.

In a uniform transparent material, light travels along straight paths. The same is true in empty space, allowing sunlight to reach Earth. A simple model draws a line with an arrow to show the direction of light travel. The line is a selected path, not the complete beam or an object connecting the source to the observer.

An opaque card blocks transmitted light and can make a shadow on a screen. With a fixed lamp and screen, moving the card nearer the lamp generally makes its shadow larger because it blocks a wider part of the spreading light. Predict the shadow using straight paths from the source past the card's edges. The shadow is a region receiving less light, not a substance that travels out of the card.

When light meets a material, some may reflect, some may pass through and some may be absorbed. Reflection sends light back from a surface. A smooth mirror produces an orderly reflection that can form an image; a rough surface reflects light in many directions. Both a mirror and a sheet of white paper can reflect light, even though the paper does not show a clear face image.

Transparent materials let enough light through with little scattering to see a clear view. Translucent materials pass light but scatter it so the view is blurred. Opaque materials do not transmit visible light through the tested thickness. Clear plastic, frosted plastic and cardboard are useful comparison samples. Material thickness, surface condition and the kind of light matter; classify the actual sample you observe rather than assuming every object made from one material behaves identically.

Refraction is a change in light's direction as it passes between materials under suitable conditions. Light traveling at an angle from air into water can bend at the boundary and then follow a new straight path within uniform water. The part of a pencil under water can therefore appear shifted when viewed through the side. The pencil has not physically broken; light arriving at your eyes followed a changed path.

Do not draw every boundary crossing as a bend. A path entering straight along the line perpendicular to a flat boundary continues in the same direction. The lesson's diagram shows an angled path bending closer to that perpendicular reference when entering water from air. Exact angle calculations are beyond this investigation; the goal is a correct observable pattern and a clearly labeled model.

A lens uses refraction at curved surfaces to redirect light. A converging lens can focus light from a distant source into a small region; it does not create light energy. In this lesson, use an ordinary low-power classroom lamp and adult-guided optical equipment. Never look at the Sun or aim a strong beam into eyes, and do not concentrate sunlight with a lens.

Design a test that distinguishes the behaviors. Place a sample between a lamp and screen to investigate transmitted light, observe a reflected patch from the side, and compare an object's apparent position through water. Keep the lamp and observer positions consistent when comparing samples. A single dim view does not tell you whether light was absorbed, reflected away or scattered; gather observations from more than one position.

Worked light path: Lamp → page → eye is a source-to-object-to-observer sequence. The page reflects some arriving light toward the eye. Eye → page arrows would incorrectly suggest that the eye illuminates the page.

Worked evidence: A sample produces a bright but blurry patch behind it and prevents a clear view of printed letters. That supports calling the tested sample translucent. It is not opaque because some light passes through, and it is not clearly transparent because the view is scattered.

Guided investigation: In an adult-supervised classroom, use a low-power flashlight, screen, opaque card, clear plastic and translucent paper. Draw predictions, test straight paths and compare the samples without pointing light into eyes. Then place a pencil in an unbreakable clear cup of water and compare its apparent position from several viewpoints. Record actual observations separately from the model.

Independent performance: Draw and explain one reflected path and one angled air-to-water path. Label source, materials, boundary and travel arrows. Use evidence to classify tested samples, identify one limitation and improve a design for sending light to a chosen screen location. Complete the connected absorption and sound lessons for the remaining energy behaviors.