What is the difference between specular and diffuse reflection?

Specular reflection occurs on smooth surfaces where parallel rays are reflected in a single direction, creating a clear image. Diffuse reflection occurs on rough surfaces where light is scattered in many directions, preventing image formation.

How does transmission differ from absorption?

Transmission involves the wave passing through the medium and emerging on the other side, often with reduced amplitude. Absorption involves the wave's energy being transferred to the particles of the medium (usually as heat) and not emerging.

What is the difference between the angle of incidence and the angle of reflection?

The angle of incidence is the angle between the incoming ray and the normal. The angle of reflection is the angle between the outgoing ray and the normal. According to the law of reflection, these two angles are always equal.

What is the most common error when measuring angles in ray diagrams?

The most common error is measuring the angle between the ray and the surface boundary. The correct method is to measure the angle between the ray and the normal line (the line perpendicular to the surface).

What happens if you forget to draw arrows on a ray diagram?

Without arrows, the diagram does not show the direction of energy transfer, making it impossible to distinguish between the incident ray (incoming) and the reflected ray (outgoing).

Why is it incorrect to say a red object "contains" red light?

Objects do not contain light; they interact with it. A red object appears red because it reflects red wavelengths of light to the observer's eye while absorbing all other visible wavelengths.

State the Law of Reflection.

The angle of incidence is equal to the angle of reflection ($i = r$). This applies to all types of waves reflecting off a surface.

Define the "Normal" in the context of wave reflection.

The Normal is an imaginary construction line drawn perpendicular ($90^\circ$) to the surface at the exact point where the incident ray hits the boundary.

What is wave absorption?

Absorption is the process where a wave's energy is transferred to the energy stores of the medium it is entering (e.g., increasing the thermal energy of particles), rather than being reflected or transmitted.

Why do transmitted waves often have a lower amplitude than the incident wave?

Energy is conserved at the boundary. Since some energy is almost always reflected or absorbed by the material, less energy remains for the transmitted wave, resulting in a lower amplitude.

Wave Behavior at Boundaries: Reflection, Absorption & Transmission | AQA GCSE Physics

1. Fundamental Boundary Interactions

The Boundary Principle: When a wave travels from one medium to another, it does not simply stop; its energy must be conserved and redistributed.

Three Outcomes: The interaction results in a combination of three processes:

Reflection: The wave bounces off the boundary and remains in the original medium.
Transmission: The wave passes through the boundary and propagates into the new medium (often accompanied by refraction).
Absorption: The wave's energy is transferred to the particles of the new medium, usually dissipating as heat.

Simultaneity: In most real-world scenarios, all three occur simultaneously. For example, a window reflects some light, absorbs a small amount as heat, and transmits the rest.

2. The Physics of Reflection

Definition: Reflection occurs when a wave hits a boundary and changes direction to stay in the incident medium.

The Law of Reflection: This fundamental principle governs all reflected waves (light, sound, water). It states:

Formula: $\theta_i = \theta_r$

Where $\theta_i$ is the angle of incidence and $\theta_r$ is the angle of reflection.

Surface Texture Effects:

Specular Reflection: Occurs on smooth, flat surfaces (e.g., mirrors). Parallel incident rays are reflected parallel to each other, creating a clear image.
Diffuse Reflection: Occurs on rough surfaces. Incident rays are scattered in many different directions because the local normal varies across the surface. This is why you cannot see your reflection in a piece of paper, despite it being white.

Ray diagram showing reflection. An incident ray strikes a surface at an angle i relative to the dashed normal line. It reflects off at an equal angle r. The normal is perpendicular to the surface.

3. Transmission and Absorption

Transmission

Definition: The process where a wave passes through a material and emerges from the other side.
Transparency: The degree of transmission depends on the material's transparency. A transparent material transmits a high percentage of light; an opaque material transmits none.
Amplitude Change: Transmitted waves often have lower amplitude than the incident wave because some energy is lost to reflection or absorption at the boundary.

Absorption

Definition: Energy is transferred from the wave to the particles of the medium.
Mechanism: In light, this occurs when the light's frequency matches the energy levels of electrons in the material. In sound, the wave energy is converted into kinetic energy (vibration) of the particles.
Result: The absorbed energy is typically re-emitted or dissipated as thermal energy (heat).

4. Color and Selective Absorption

Perception of Color: The color of an opaque object is determined by which wavelengths of light are reflected and which are absorbed.

Selective Reflection: An object appears a specific color (e.g., red) because it reflects that wavelength of light while absorbing all other visible wavelengths.
White Objects: Reflect all visible wavelengths equally.
Black Objects: Absorb all visible wavelengths; no light is reflected to the eye.

5. Ray Diagram Methodology

The Normal Line: A construction line drawn perpendicular ( $90^\circ$ ) to the surface at the point where the wave hits. It is usually drawn as a dashed line.

Measurement Rule: All angles in wave optics are measured relative to the normal, never relative to the surface.

Angle of Incidence ( $i$ ): The angle between the incoming ray and the normal.
Angle of Reflection ( $r$ ): The angle between the outgoing ray and the normal.

Drawing Conventions: Rays are drawn as straight lines with arrows indicating the direction of travel. Incident rays point towards the boundary; reflected rays point away.

6. Key Distinctions

Concept	Definition	Key Characteristic
Reflection	Wave bounces back	$i = r$ ; stays in same medium
Transmission	Wave passes through	Enters new medium; speed often changes
Absorption	Energy transfers to matter	Wave disappears; medium gains thermal energy
Specular	Smooth surface reflection	Clear image; parallel rays stay parallel
Diffuse	Rough surface reflection	No image; rays scatter in all directions

7. Exam Strategy & Tips

The "Normal" Trap: The most common error is measuring the angle between the ray and the surface. Always draw the normal first and measure from there. If given the angle to the surface (e.g., $30^\circ$ ), calculate the angle of incidence as $90^\circ - 30^\circ = 60^\circ$ .
Arrow Direction: Marks are often lost for missing arrows on rays. Always check that your incident ray points in and reflected ray points out.
Conservation: Remember that Incident Energy = Reflected + Transmitted + Absorbed. If a question asks why a transmitted wave is quieter or dimmer, the answer is usually that some energy was reflected or absorbed.

Wave Behavior at Boundaries: Reflection, Absorption & Transmission

Summary

1. Fundamental Boundary Interactions

2. The Physics of Reflection

3. Transmission and Absorption

Transmission

Absorption

4. Color and Selective Absorption

5. Ray Diagram Methodology

6. Key Distinctions

7. Exam Strategy & Tips

Wave Behavior at Boundaries: Reflection, Absorption & Transmission

Summary

1. Fundamental Boundary Interactions

2. The Physics of Reflection

3. Transmission and Absorption

Transmission

Absorption

4. Color and Selective Absorption

5. Ray Diagram Methodology

6. Key Distinctions

7. Exam Strategy & Tips