Wave interactions with matter

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Lesson notes

Wave Interactions with Matter

When a wave meets a material, it can be **reflected**, **absorbed**, or **transmitted**.
**Reflection** occurs when a wave bounces off a surface and changes direction.
**Absorption** happens when a wave's energy is taken in by the material, often converting to heat.
**Transmission** is when a wave passes through a material, possibly with some refraction.

The Law of Reflection

For **specular reflection** (smooth surfaces like mirrors), the angle of incidence equals the angle of reflection: **θi = θr**.
The **incident ray**, **reflected ray**, and the **normal** (imaginary line perpendicular to the surface) all lie in the same plane.
The incident and reflected rays are on opposite sides of the normal.
**Diffuse reflection** occurs on rough surfaces, scattering light in many directions.

Reflection of Light

**Specular reflection** from a flat mirror produces a **mirror image** that appears reversed left-to-right.
**Curved mirrors** (spherical or parabolic) can magnify or demagnify images.
Light reflects off materials with a **higher refractive index** than the medium it travels in, it undergoes a **180° phase shift**.
Reflection also occurs at the surface of transparent media like water or glass, though less efficiently than mirrors.

Reflection of Sound and Other Waves

Sound waves reflect off surfaces, creating **echoes**; used in **sonar** to detect objects underwater.
Water waves reflect off barriers, such as a wall in a pool.
Seismic waves reflect inside Earth, helping geologists study its structure.
Radio waves reflect off the ionosphere, enabling long-distance communication.

Absorption and Transmission

**Absorption** occurs when wave energy is converted to heat; dark materials absorb more light than light ones.
**Transmission** allows waves to pass through; transparent materials (e.g., glass) transmit light well.
**Translucent** materials scatter some light, while **opaque** materials block transmission.
Sound is absorbed by soft materials like foam, reducing echoes.

Total Internal Reflection

When light travels from a **denser** to a **rarer** medium, if the angle of incidence exceeds the **critical angle**, all light is reflected back.
This is called **total internal reflection** and is used in fiber optics to transmit light signals.
It also occurs in diamonds, giving them their sparkle.

The Eye and Ear as Detectors

The **eye** detects light waves; the **cornea** and **lens** focus light onto the retina, where photoreceptors convert it to electrical signals.
The **ear** detects sound waves; the **eardrum** vibrates, transmitting vibrations through ossicles to the cochlea, which converts them to nerve impulses.
Both organs rely on wave interactions: reflection, refraction (in the eye), and absorption.

Law of reflection: angle of incidence equals angle of reflection.

Particle arrangement in solids, liquids, and gases affects wave transmission and absorption.

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Practice questions

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1.What is the law of reflection for a light ray striking a smooth surface?
Easy
- AThe angle of incidence equals the angle of reflection.
- BThe angle of incidence is greater than the angle of reflection.
- CThe angle of incidence is less than the angle of reflection.
- DThe angle of incidence is twice the angle of reflection.
2.Which type of reflection occurs when light strikes a rough surface, such as paper?
Easy
- ASpecular reflection
- BDiffuse reflection
- CTotal internal reflection
- DRegular reflection
3.When light reflects off a mirror, the incident ray, reflected ray, and normal all lie in the same plane. This is part of the:
Easy
- AFirst law of reflection
- BSecond law of reflection
- CThird law of reflection
- DLaw of refraction
4.A light ray strikes a plane mirror at an angle of 30° to the mirror surface. What is the angle of incidence measured from the normal?
Medium
- A30°
- B60°
- C90°
- D45°
5.Which of the following materials is best for producing a specular reflection of visible light?
Medium
- AA sheet of paper
- BA polished metal surface
- CA piece of wood
- DA concrete wall
6.When light travels from glass into air, total internal reflection can occur if the angle of incidence is:
Medium
- ALess than the critical angle
- BEqual to the critical angle
- CGreater than the critical angle
- DAny angle less than 90°
7.A light ray is incident on a glass-air boundary at an angle of 45°. The critical angle for glass is 42°. What happens to the ray?
Hard
- AIt is partially reflected and partially refracted into air.
- BIt undergoes total internal reflection.
- CIt is completely absorbed.
- DIt passes straight through without bending.
8.The reflection of sound waves that results in a delayed repetition of a sound is called:
Easy
- ARefraction
- BDiffraction
- CEcho
- DAbsorption

Revision notes

## Wave Interactions with Matter Waves (light, sound, water) interact with materials in three main ways: **reflection**, **absorption**, and **transmission**. ### Reflection - **Reflection** is the change in direction of a wave at a boundary between two media, causing the wave to return into the original medium. - **Law of Reflection**: The angle of incidence equals the angle of reflection (θi = θr). The incident ray, reflected ray, and normal all lie in the same plane. - **Specular reflection** occurs on smooth surfaces (e.g., mirrors) and forms clear images. **Diffuse reflection** occurs on rough surfaces and scatters light. - Reflection can cause **echoes** (sound) and is used in sonar, radar, and seismic studies. ### Absorption and Transmission - When a wave hits a material, part of its energy may be **absorbed** (converted to heat) or **transmitted** through the material. - **Transparent materials** (e.g., glass, water) transmit most light; **opaque materials** absorb or reflect it. - The amount of reflection and transmission depends on the **refractive index** of the materials. Light reflects more when moving from a low to high refractive index. ### Light and Sound - **Light** reflects off mirrors (specular) and most surfaces (diffuse). The **eye** detects reflected light to see objects. - **Sound** reflects off hard surfaces, creating echoes. The **ear** detects these reflected waves. ### Key Formulas/Definitions - **Law of Reflection**: θi = θr (angles measured from the normal). - **Normal**: an imaginary line perpendicular to the surface at the point of incidence. ### Examples - Mirror: specular reflection forms a reversed image. - Echo: sound reflects off a distant wall. - Water: light reflects off the surface (partial) and transmits into the water (refraction).

Reflection: The change in direction of a wave at an interface between two different media so that the wavefront returns into the medium from which it originated.
Law of Reflection: The angle of incidence equals the angle of reflection (θi = θr); the incident ray, reflected ray, and normal all lie in the same plane.
Specular reflection: Reflection from a smooth surface where the wave is reflected in a single direction, forming a clear image (e.g., mirror).
Diffuse reflection: Reflection from a rough surface where the wave is scattered in many directions, retaining energy but losing the image.
Absorption: The process by which a wave's energy is taken up by a material, often converted to heat.
Transmission: The passage of a wave through a material.
Normal: An imaginary line perpendicular to the surface at the point of incidence, used to measure angles of incidence and reflection.
Angle of incidence: The angle between the incident wave and the normal to the surface.
Angle of reflection: The angle between the reflected wave and the normal to the surface.
Refractive index: A measure of how much a material slows down light; differences in refractive index cause reflection and refraction at boundaries.

Flashcards

What are the three main things that can happen when a wave hits a material?

The wave can be reflected, absorbed, or transmitted through the material.

Define reflection of a wave.

Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated.

State the law of reflection.

The angle of incidence equals the angle of reflection, and the incident ray, reflected ray, and normal all lie in the same plane.

What is the difference between specular and diffuse reflection?

Specular reflection (e.g., from a mirror) preserves the image; diffuse reflection scatters light in many directions, losing the image but retaining energy.

Give an example of reflection of sound waves.

Echoes are caused by the reflection of sound waves off surfaces.

What is the normal in the context of reflection?

The normal is an imaginary line drawn perpendicular to the reflecting surface at the point where the incident ray strikes.

What happens to light when it travels from one medium to another with a different refractive index?

Some light is reflected from the interface, and the remainder is refracted (transmitted) into the new medium.

What is total internal reflection?

Total internal reflection occurs when light traveling from a denser medium to a less dense medium strikes the boundary at an angle greater than the critical angle, and all light is reflected back.

How does the phase of reflected light change when reflecting off a material with higher refractive index?

The reflected light undergoes a 180° phase shift.

How does the phase of reflected light change when reflecting off a material with lower refractive index?

The reflected light remains in phase with the incident light.

What type of surface produces specular reflection?

A very smooth surface, like a mirror or calm water, produces specular reflection.

What type of surface produces diffuse reflection?

A rough or uneven surface, like paper or a wall, produces diffuse reflection.

What is absorption of a wave?

Absorption occurs when a wave's energy is taken up by the material, often converting to heat.

What is transmission of a wave?

Transmission is when a wave passes through a material, continuing to propagate on the other side.

How does the human eye detect light?

The eye detects light when reflected or emitted light enters the eye and is focused onto the retina, where photoreceptor cells convert it into signals for the brain.

How does the human ear detect sound?

The ear detects sound when sound waves cause the eardrum to vibrate; these vibrations are transmitted through the middle ear to the cochlea, where hair cells convert them into electrical signals.

What is an echo?

An echo is a reflected sound wave that is heard after the original sound, caused by reflection off a distant surface.

What is sonar?

Sonar uses sound wave reflection to detect objects underwater by sending out pulses and listening for echoes.

What happens to a wave when it is absorbed by a material?

The wave's energy is transferred to the material, often increasing its temperature.

Give an example of transmission of light through a material.

Light passing through a glass window is an example of transmission.

What is the critical angle in total internal reflection?

The critical angle is the angle of incidence above which total internal reflection occurs when light travels from a denser to a less dense medium.

How does a mirror reflect light?

A mirror has a smooth metallic coating that reflects most incident light specularly, producing a clear image.

What is the difference between the incident ray and the reflected ray?

The incident ray is the incoming wave that strikes the surface; the reflected ray is the wave that bounces off the surface.

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

They are equal: angle of incidence = angle of reflection.

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