6.6.2.2 Electromagnetic Wave Properties and Refraction — Refraction and Wave Properties — AQA Combined Science Trilogy 8464

(HT only) Explain that various substances may absorb, transmit, refract, or reflect electromagnetic waves distinctly depending directly on the applied wavelength.
(HT only) Understand that physical effects, such as refraction, intrinsically occur due to the shifting velocity of waves as they transverse through differing substances.
Construct accurate ray diagrams vividly illustrating the refraction of a wave positioned at the direct boundary between two different media.
(HT only) Use comprehensive wave front diagrams to effectively explain refraction in terms of wave speed changes occurring during medium-to-medium travel.
Describe and perform a practical investigation studying how the amount of infrared radiation absorbed or radiated by a surface is heavily influenced by the nature of that specific surface.

What happens when electromagnetic waves meet matter? (HT only)

Electromagnetic waves can interact with a material in several ways. Transmission means that the wave passes through the material. Absorption transfers energy from the wave to the material. Reflection sends a wave back from a boundary. A transmitted wave may also undergo refraction: a change in direction caused by a change in wave speed.

These are not necessarily separate alternatives. At a window, for example, some visible light passes through the glass while some is reflected from its surface. Light entering the glass at an angle can also be refracted.

The interaction depends on both the material and the wavelength. A material that transmits one part of the electromagnetic spectrum may absorb or reflect another. For example, ordinary window glass transmits visible light but absorbs some ultraviolet wavelengths. Being transparent to visible light does not mean being transparent to every electromagnetic wave.

Representing refraction with rays

A ray is a line showing the direction in which a wave travels. Its arrowhead shows the direction of travel. Ray diagrams let us represent what happens where two media meet, such as the boundary between air and glass.

To describe a ray's direction at a boundary, draw a normal: an imaginary line at right angles to the boundary, passing through the point where the ray meets it. The angle of incidence is between the incoming ray and the normal. The angle of refraction is between the refracted ray and the normal.

For light travelling from air into glass, the refracted ray bends towards the normal, so its angle to the normal becomes smaller. For light travelling from glass into air, it bends away from the normal, so its angle to the normal becomes larger.

Three ray diagrams compare visible light crossing a flat air–glass boundary. From air into glass, the refracted ray is closer to the perpendicular normal, so the refraction angle r is smaller than the incidence angle i. From glass into air, r is larger than i. A ray incident along the normal continues straight without bending. Arrowheads show travel direction. — Light bends towards the normal entering glass from air, and away from the normal leaving glass for air. Along the normal, it does not bend.

Constructing a refraction ray diagram

For a ray entering glass from air:

Draw the boundary and label the air and glass on opposite sides.
Draw a straight incident ray reaching the boundary, with an arrow pointing towards it.
At the point of incidence, draw a dashed normal perpendicular to the boundary.
From that same point, draw the refracted ray inside the glass, closer to the normal than the incident ray. Add an arrow pointing away from the boundary.
Label the incident and refracted rays, and mark their angles to the normal if needed.

For a ray leaving glass and entering air, use the same construction but draw the outgoing ray farther from the normal. If tracing a ray through a rectangular glass block, draw a separate normal at each boundary where the ray crosses.

A ray travelling along the normal continues straight through the boundary: it does not bend. This is an important exception to the idea that crossing a boundary always changes direction.

Why a change of medium can cause bending (HT only)

All electromagnetic waves have the same speed in a vacuum, but their speed in a material can be different. Visible light travels more slowly in glass than in air.

When a wave enters a medium where it travels more slowly, it bends towards the normal, provided it meets the boundary at an angle. When it enters a medium where it travels faster, it bends away from the normal. The speed change, rather than simply the name of the material, determines the bending direction.

The wave's frequency stays the same when it crosses the boundary. Its wavelength changes with its speed. Using $v = f\lambda$ , a lower speed at the same frequency means a shorter wavelength; a higher speed means a longer wavelength.

Explaining refraction with wavefronts (HT only)

A wavefront joins points at the same stage of a wave's cycle, such as points along a crest. In a diagram of straight waves, successive wavefronts are parallel lines. The direction of travel is perpendicular to these lines, and the spacing between successive crests represents the wavelength.

Two wavefront diagrams show light entering glass from air. At oblique incidence, wavefronts slope upwards to the right in air and become closer to horizontal in glass. A continuous highlighted wavefront has one end already in glass, slowing first, while its other end remains in faster air. The propagation ray is perpendicular to the wavefronts and bends towards the normal. Wavefront spacing decreases in glass, indicating shorter wavelength and lower speed, while frequency remains unchanged. At normal incidence, horizontal wavefronts cross together: their spacing decreases but the vertical travel direction does not change. — At an angle, one end of a wavefront slows before the other, turning the wavefront. At normal incidence, all parts slow together.

Consider a wave entering glass from air at an angle. One end of a wavefront reaches the glass first and slows down, while the other end is still travelling faster in air. During this interval, the end still in air travels farther. This turns the wavefront, changing the direction of travel towards the normal.

Once the wave is in the glass, the wavefronts are closer together because the wavelength is shorter. The frequency has not changed.

For travel from glass to air, one end speeds up before the other, producing bending away from the normal. At normal incidence, the wavefront is parallel to the boundary: all parts cross and change speed together. Its spacing changes, but its orientation does not, so there is no bending.

Ray diagrams — all tiers

Draw and label the boundary, incident ray, normal and refracted ray; add direction arrows.
The normal is perpendicular to the boundary. Measure angles from it.
Air → glass: towards the normal.
Glass → air: away from the normal.
Along the normal: no bending.

Interactions with matter — HT only

Whether electromagnetic waves are absorbed, transmitted, reflected or refracted depends on wavelength and material. Different interactions can occur together.

Speed and wavefronts — HT only

Slower medium: towards the normal; shorter wavelength.
Faster medium: away from the normal; longer wavelength.
Frequency stays unchanged: $v = f\lambda$ .
At oblique incidence, one end of a wavefront changes speed before the other, turning the wavefront and the direction of travel.
At normal incidence, all parts change speed together: no change in direction.
Rays are perpendicular to wavefronts; crest-to-crest spacing represents wavelength.

Measure angles between each ray and the normal, not between the ray and the boundary.

Draw rays with a ruler, add arrowheads and place the change in direction exactly at the boundary.

Use the change in wave speed to decide the bending direction; entering a material does not always mean bending towards the normal.

For a wavefront explanation, state that one end changes speed before the other, then link this to the change in direction.

The change in direction of a wave caused by a change in its speed when it passes between different media.

A substance or material through which a wave travels, such as air, water or glass.

The passage of a wave through a material.

The transfer of energy from a wave to a material.

The return of a wave from a boundary into the medium it came from.

An imaginary line perpendicular to a boundary at the point where a ray meets it.

The angle between the incoming ray and the normal.

The angle between the transmitted, refracted ray and the normal.

A line joining points at the same stage of a wave's cycle, such as points along a crest.