Science · 7th Grade · Energy and Matter in Motion · Weeks 1-9

Light Waves and Refraction

Students investigate how light bends as it passes from one medium to another, exploring the phenomenon of refraction and its applications.

• Common Core State Standards
• MS-PS4-2

About This Topic

Refraction occurs when a light wave changes speed as it passes from one medium into another, causing it to bend at the boundary. The degree of bending depends on the angle of incidence and the optical density of the two materials. This principle, addressed in MS-PS4-2, is the same physics behind eyeglasses, camera lenses, magnifying glasses, and the apparent bending of a straw in a glass of water. US 7th graders connect refraction to optical phenomena they encounter daily.

Students apply Snell's Law qualitatively to predict the direction of bending: light bends toward the normal when entering a denser medium (moving from air into glass) and away from the normal when entering a less dense medium. Lenses exploit this property in predictable ways. Converging (convex) lenses bend parallel light rays toward a focal point, while diverging (concave) lenses spread them apart, forming the basis of corrective eyewear and optical instruments.

Refraction is a topic where hands-on investigation produces immediate visible results that diagrams cannot replicate. When students trace ray paths through actual lenses and glass blocks, the abstract geometry becomes a physical reality they can measure, predict, and verify in real time. Active learning turns what looks like a confusing set of rules into a coherent, testable model.

Key Questions

1. Predict how light will bend when passing through different transparent materials.
2. Explain the phenomenon of refraction using real-world examples.
3. Analyze how lenses use refraction to focus or disperse light.

Learning Objectives

• Predict the path of a light ray as it passes from one medium to another, given the angle of incidence and the relative optical densities of the media.
• Explain the phenomenon of refraction using the concept of light changing speed as it crosses a boundary between different materials.
• Analyze how convex and concave lenses modify parallel light rays to converge or diverge them, respectively.
• Compare the visual distortions caused by refraction in everyday scenarios, such as a straw in water or objects viewed through a glass block.

Before You Start

Properties of Light Waves

Why: Students need to understand that light travels in waves and has properties like direction and speed before investigating how these change.

Angles and Measurement

Why: Accurate prediction and analysis of light bending require students to be comfortable with measuring and identifying angles.

Key Vocabulary

Refraction	The bending of a light wave as it passes from one medium to another, caused by a change in the speed of light.
Medium	A substance or material through which light travels, such as air, water, or glass.
Angle of Incidence	The angle between an incoming light ray and the normal (an imaginary line perpendicular to the surface) at the point where the ray strikes.
Normal	An imaginary line perpendicular to a surface at the point where a light ray strikes or reflects.
Convex Lens	A lens that is thicker in the middle than at the edges, causing parallel light rays to converge at a focal point.
Concave Lens	A lens that is thinner in the middle than at the edges, causing parallel light rays to diverge.

Watch Out for These Misconceptions

Common Misconception

Light always bends toward the normal at any boundary.

What to Teach Instead

Light bends toward the normal only when moving into a denser medium. When moving into a less dense medium, it bends away from the normal. Having students test both directions using a glass block and a ray box, first going air to glass then glass to air, makes this direction rule concrete and avoids overgeneralizing.

Common Misconception

Refraction only happens with glass and water.

What to Teach Instead

Refraction occurs at any boundary where light changes speed, including between air masses of different temperatures, which causes mirages. The shimmering appearance of hot pavement on a summer day is refraction happening in plain air. Connecting to mirages helps students see refraction as a universal optical effect, not a glass-specific one.

Active Learning Ideas

Inquiry Circle: The Penny in the Cup

Groups place a coin just out of sight at the bottom of an opaque cup. Students add water slowly and observe the coin appear as refraction bends the light path. They sketch ray diagrams showing where the light actually comes from versus where their eye perceives the penny to be, then drain the cup and watch the coin disappear again.

20 min·Small Groups

Think-Pair-Share: Predicting Ray Direction

Students are given diagrams of light rays approaching a boundary between air and glass at different angles. They predict the direction of the refracted ray and compare predictions with a partner before the class checks using a laser pointer and a glass block, revising any incorrect predictions with the correct geometric rule.

20 min·Pairs

Inquiry Circle: Lens Focal Length Lab

Groups use a convex lens, a meter stick, and a distant light source to find the focal length by measuring where the image comes into sharp focus. They test how moving the object closer or farther changes where the image forms and record how the image size and orientation changes, connecting physical results to the converging lens model.

40 min·Small Groups

Gallery Walk: Refraction in Real Devices

Stations show cross-section diagrams of a camera lens, corrective eyeglasses, a fiber optic cable, and a refracting telescope. Student groups annotate where refraction is occurring at each surface, what the device needs the light to do, and how the curved shape of the lens determines the direction and degree of bending.

30 min·Small Groups

Real-World Connections

• Opticians use the principles of refraction to design eyeglasses and contact lenses that correct vision problems like nearsightedness and farsightedness by precisely bending light rays to focus on the retina.
• Microscope manufacturers and telescope designers rely on understanding how lenses refract light to create instruments that magnify distant objects or reveal microscopic details, essential for scientific research and exploration.
• Photographers use lenses in cameras, which are carefully shaped to refract light, to capture clear images by focusing light onto the sensor or film.

Assessment Ideas

Quick Check

Present students with diagrams showing light rays passing from air into water at various angles of incidence. Ask them to draw the refracted ray, indicating whether it bends toward or away from the normal, and to briefly explain their reasoning.

Exit Ticket

Provide students with a scenario: 'A diver looks up at a fish in the air above the water.' Ask them to sketch the path of light from the fish to the diver's eye, showing how refraction affects what the diver sees, and to label the point where refraction occurs.

Discussion Prompt

Pose the question: 'How do magnifying glasses and binoculars use refraction differently to help us see things better?' Facilitate a class discussion where students explain the role of convex and concave lenses in each device.

Frequently Asked Questions

What causes refraction of light?

Refraction occurs when light changes speed as it crosses from one medium to another. The change in speed causes the wavefront to bend at the boundary. The angle of bending depends on how different the two media's optical densities are and the angle at which the light arrives at the surface.

How does a lens use refraction to form an image?

A convex lens refracts light at both its curved surfaces so that parallel incoming rays converge at a single focal point. The curved shape is precisely calculated so that light from different parts of the beam bends by exactly the right amount to meet at one spot, forming a sharp, focused image.

What are some real-world examples of refraction for 7th graders?

Eyeglasses use shaped lenses to refract light and correct vision. A straw appears bent in water because the image of the submerged portion reaches your eye along a different path than the portion above water. Mirages on hot roads occur when light bends through air layers at different temperatures near the pavement surface.

How does active learning help students understand light refraction?

Refraction follows geometric rules that students can verify themselves with simple equipment. When students trace rays through physical lenses, predict the direction of bending before they test it, and explain their results to peers, they build a working model of the physics rather than memorizing a definition. MS-PS4-2 calls for exactly this kind of model-based thinking.