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Wave Interactions: Reflection, Refraction & Diffraction
Wave Interactions
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Waves carry energy from one place to another, and they change behavior when they meet a surface, enter a new material, or pass through an opening. Reflection, refraction, and diffraction explain many things students can observe, such as echoes, bent-looking straws in water, and sound traveling around a doorway. These interactions happen with water waves, sound waves, light waves, and many other types of waves. Learning them helps connect diagrams and formulas to real-world motion and energy transfer.
Reflection happens when a wave bounces off a boundary, refraction happens when a wave changes speed and direction in a new medium, and diffraction happens when a wave spreads out after passing an edge or gap. The amount of bending or spreading depends on wave speed, wavelength, angle, and the size of the opening or obstacle. Scientists and engineers use these ideas in mirrors, lenses, eyeglasses, cameras, sonar, medical ultrasound, and wireless communication. A good wave diagram shows wavefronts, rays, boundaries, angles, and labeled changes in speed or direction.
Key Facts
- Wave speed is related to frequency and wavelength by v = fλ.
- Reflection follows the law of reflection: angle of incidence = angle of reflection.
- Refraction occurs when a wave changes speed as it enters a different medium.
- When a wave slows down entering a new medium, it bends toward the normal line.
- Diffraction is strongest when the gap size is about the same as the wavelength.
- Frequency stays the same when a wave crosses into a new medium, but speed and wavelength can change.
Vocabulary
- Reflection
- Reflection is the bouncing of a wave off a surface or boundary.
- Refraction
- Refraction is the bending of a wave as it changes speed when entering a new medium.
- Diffraction
- Diffraction is the spreading of waves as they pass around an obstacle or through an opening.
- Wavelength
- Wavelength is the distance from one crest of a wave to the next crest.
- Normal line
- The normal line is an imaginary line drawn perpendicular to a surface at the point where a wave hits.
Common Mistakes to Avoid
- Confusing reflection with refraction is wrong because reflection means a wave bounces back, while refraction means it enters a new medium and bends.
- Measuring angles from the surface is wrong because reflection and refraction angles are measured from the normal line.
- Thinking frequency changes during refraction is wrong because the wave source controls frequency, while speed and wavelength change in the new medium.
- Assuming diffraction only happens through tiny holes is wrong because diffraction happens around edges and through openings, but it is most noticeable when the opening is close to the wavelength.
Practice Questions
- 1 A water wave has a frequency of 4 Hz and a wavelength of 0.50 m. What is the wave speed?
- 2 A light ray hits a flat mirror at an angle of incidence of 35 degrees measured from the normal. What is the angle of reflection?
- 3 A sound wave can be heard around a classroom doorway, but a beam of light from a flashlight does not spread around the doorway as much. Use wavelength and diffraction to explain why.