Interactions of Waves

Middle Years Programme - Unit 3, Chapter 15, Lesson 3: Interactions of Waves

Statement of Inquiry

  • Discoveries of principles depend on the interactions of systems in the environment.

Objectives

  • Recognize the different ways waves interact with matter.
  • Differentiate between reflection, refraction, and diffraction.
  • Understand different types of interference.

Types of Material

Materials are categorized based on their interaction with light:

  • Transparent: Allows all light to pass through.
  • Translucent: Allows partial light to pass through.
  • Opaque: Allows no light to pass through.

Interaction with Matter

Key interactions include absorption, transmission, reflection, refraction, and diffraction.

Wave Behaviors

  • Reflection: Light bounces off a surface.
  • Transmission: Light passes through a material.
  • Absorption: Light is absorbed by a material and doesn't reflect or transmit.

1- Reflection

  • When a wave hits a new medium, part of it reflects, causing a change in direction.
  • Examples: Echo (sound wave), reflection of light, reflection on a rope.
  • Law of Reflection: The angle of incidence is equal to the angle of reflection.

Law of Reflection

  • The angle of incidence equals the angle of reflection.
  • Angle of incidence: The angle between the incoming wave and the normal (a line perpendicular to the surface).
  • Angle of reflection: The angle between the reflected wave and the normal.
  • If the angle of incidence is 45°, the angle of reflection is also 45°.

2- Refraction

  • When a wave hits a new medium, part of it is reflected (as per the law of reflection), and part enters the new medium.
  • The part entering the new medium changes speed, thus changing direction (bending), which is called refraction.
  • If the wave hits the new medium at a right angle, no bending occurs.

Refraction of Light Waves

  • Light bends when it enters water at an angle because the side of the wave that enters the water first slows down.
  • When light enters from a denser medium (solid) to a less dense medium (liquid or gas), the refracted ray deviates away from the normal (it becomes faster).
  • When light enters from a less dense medium (gas) to a denser medium (solid or liquid), the refracted ray deviates towards the normal (it becomes slower).

3- Diffraction

  • When a wave hits an edge of a barrier, it bends, or when it passes through an opening, it spreads out. This interaction is called diffraction.

Wave Interference

  • When two waves meet at the same location, they overlap, which is called interference.

1- Constructive Interference

  • Occurs when two waves meet and combine to form a wave with a larger amplitude.
  • When the crest of one wave overlaps the crest of another wave, constructive interference occurs.
  • After interference, waves continue as if they had not met.

2- Destructive Interference

  • Occurs when two waves meet at the same position and combine to form a wave with a smaller amplitude.
  • When the crest of one wave overlaps the trough of another wave, destructive interference occurs.
  • After interference, waves continue as if they had not met.

Standing Waves

  • A standing wave forms when the incoming wave and the reflected wave have the right frequency. When they meet, they combine to form a standing wave that appears to stand still.
  • Standing waves consist of nodes and antinodes.
  • Nodes: Parts of the standing wave where destructive interference occurs.
  • Antinodes: Parts of the standing wave where constructive interference occurs.