What is the primary difference between reflection and refraction in terms of wave behavior?

Reflection involves a wave bouncing off a boundary and staying within the original medium, while refraction involves the wave crossing the boundary into a new medium and changing direction due to a change in speed.

How does the behavior of an EM wave change when moving from a high-density medium to a low-density medium?

The wave increases in speed and its path bends away from the normal line, resulting in an angle of refraction that is larger than the angle of incidence.

What is the difference between how visible light and gamma rays interact with atoms?

Visible light typically interacts with the outer electrons of an atom, causing transitions between energy levels, whereas high-energy gamma rays can interact directly with the nucleus of the atom.

What is a common mistake when measuring the angle of incidence in a ray diagram?

Students often measure the angle between the ray and the surface of the material. The correct measurement is always between the ray and the normal (the line perpendicular to the surface).

Why is it incorrect to say that the frequency of an EM wave changes during refraction?

Frequency is determined by the source of the wave and remains constant as the wave passes through different media; only the speed and wavelength change to accommodate the new medium's properties.

What error occurs if you assume all EM waves interact with glass in the same way?

This ignores wavelength dependency; while glass is transparent to visible light (transmission), it is largely opaque to ultraviolet radiation (absorption) and infrared (reflection/absorption).

Define 'Refraction' in the context of EM waves.

Refraction is the change in direction of an EM wave as it passes from one medium to another, caused by a change in the wave's velocity.

What is the 'Normal' in optics?

The normal is an imaginary line drawn perpendicular ($90^{\circ}$) to the surface of a boundary at the point where an incident ray strikes it.

Define 'Absorption' at the atomic level.

Absorption is the process where an atom takes in the energy of an EM wave, typically causing an electron to move to a higher energy level.

Why do EM waves slow down when they enter a material like glass?

The waves interact with the charged particles (electrons and nuclei) within the material, which creates a delay in the propagation of the wave compared to its travel in a vacuum.

EM Waves & Matter | AQA GCSE Science

Combined Science Trilogy / Physics

Waves

EM Waves & Matter

Summary

The interaction between electromagnetic (EM) waves and matter is defined by how energy is transferred, redirected, or absorbed as waves encounter different media. These interactions—transmission, reflection, refraction, and absorption—are determined by the wave's frequency and the atomic structure of the material.

1. Definition & Core Concepts

Electromagnetic Waves: These are transverse waves consisting of oscillating electric and magnetic fields that transfer energy from a source to an absorber without requiring a physical medium.
Interaction Types: When an EM wave hits a boundary between two substances, it can be transmitted (passes through), reflected (bounces off), refracted (changes direction), or absorbed (energy is taken in by the material).
Wavelength Dependency: The specific interaction depends heavily on the wavelength of the EM wave; for instance, a material like glass may transmit visible light but absorb ultraviolet radiation.

Diagram showing an incident EM ray hitting a boundary, splitting into a reflected ray and a refracted ray that bends toward the normal.

2. Underlying Principles of Refraction

3. Atomic Interactions: Absorption and Emission

Electron Transitions: When an EM wave is absorbed, its energy is transferred to an electron, causing it to move from a lower energy level to a higher one (excitation).
Photon Emission: Conversely, when an electron moves from a higher energy level back to a lower one, it emits an EM wave with a frequency corresponding to the energy difference between the levels.
Frequency Specificity: Atoms can only absorb or emit specific frequencies of EM waves that match the energy gaps between their internal energy levels, which explains why materials have unique absorption spectra.

4. Key Distinctions

Interaction	Description	Resulting Effect
Reflection	Wave bounces off the surface	No change in speed or wavelength
Refraction	Wave enters medium and bends	Change in speed and wavelength
Absorption	Energy is transferred to atoms	Material increases in internal energy
Transmission	Wave passes through unchanged	Energy continues to propagate

Density and Bending: When moving from a less dense to a more dense medium (e.g., air to glass), the wave bends towards the normal; when moving from more dense to less dense, it bends away from the normal.

5. Exam Strategy & Tips