How does wavelength affect the interaction of EM waves with glass?

Glass interacts differentially across the spectrum: it transmits visible light, absorbs ultraviolet (UV), and reflects infrared (IR) radiation.

What is the difference between specular reflection and scattering (diffuse reflection)?

Specular reflection occurs on smooth surfaces where rays reflect in a single direction, forming clear images. Scattering occurs on rough surfaces where rays reflect in many directions, making the object visible from various angles without a clear image.

How does the behavior of a wave change when moving from a less dense to a more dense medium?

The wave slows down and its wavelength decreases. Consequently, the ray bends towards the normal line at the boundary.

What is the difference between transmission and absorption in terms of energy?

In transmission, the wave energy passes through the medium with minimal interaction. In absorption, the wave energy is transferred to the particles of the medium, typically increasing its temperature.

What is a common mistake when measuring the angle of incidence or reflection?

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 light changes when it enters water?

Frequency is determined by the source of the wave and remains constant regardless of the medium. Only the velocity and wavelength change to maintain the relationship $v = f\lambda$.

What error occurs if you assume a 'transparent' object transmits all EM radiation?

Transparency is frequency-specific. An object like glass may transmit visible light but completely absorb ultraviolet radiation, meaning it is only transparent for a specific range of the spectrum.

Refraction is the change in direction of a wave as it passes from one medium to another due to a change in its speed.

What is the 'Normal' in wave interaction diagrams?

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

What defines an 'Opaque' material?

An opaque material is one that does not transmit any light; it either reflects or absorbs all incident radiation in the visible spectrum.

EM Waves & Matter | OCR GCSE Physics

EM Waves & Matter

Summary

The interaction between electromagnetic (EM) waves and matter is defined by how energy is transferred, redirected, or permitted to pass through a medium. These interactions—transmission, absorption, reflection, and refraction—are determined by the wave's frequency and the physical properties of the material, such as its atomic structure and optical density.

1. Core Interaction Mechanisms

Transmission: This occurs when an EM wave passes through a material without being significantly absorbed or reflected. The transparency of a medium is frequency-dependent; for instance, glass is transparent to visible light but opaque to ultraviolet radiation.
Absorption: Energy is transferred from the EM wave to the particles of the substance, often increasing the internal energy (heat) of the material. This happens most efficiently when the wave's frequency matches the resonant energy levels of the material's electrons.
Reflection: The wave 'bounces' off the boundary between two media. The nature of the surface determines whether the reflection is clear (specular) or diffused (scattering).
Refraction: This is the change in direction of a wave as it crosses a boundary between media of different densities, caused by a change in the wave's velocity.

Diagram showing an incident EM wave splitting into reflected, refracted, and absorbed components at a boundary.

2. The Physics of Refraction

3. Differential Interaction and Color

Selective Absorption: Opaque objects appear a specific color because they reflect that specific wavelength while absorbing all others. For example, a green leaf reflects green light and absorbs red and blue light to drive photosynthesis.
Color Filters: These materials work by transmitting only specific wavelengths. A red filter allows red light to pass through (transmission) while absorbing all other frequencies of the visible spectrum.
Total Reflection and Absorption: An object that reflects all visible wavelengths equally appears white, while an object that absorbs all visible wavelengths appears black.

4. Reflection Types: Specular vs. Scattering

Specular Reflection: Occurs on smooth surfaces (like mirrors) where parallel incident rays are reflected in a single, uniform direction. This preserves the phase relationship of the waves, allowing for the formation of clear images.
Scattering (Diffuse Reflection): Occurs on rough surfaces where incident rays are reflected in many different directions. While the law of reflection still applies at the microscopic level, the macroscopic result is a lack of a clear image, which is how we see most non-shiny objects.

5. Key Distinctions in Media Behavior

6. Exam Strategy & Tips