Light, Reflection & Refraction
Light as an Electromagnetic Wave
Light is a transverse electromagnetic wave. It is part of the electromagnetic spectrum and travels at approximately \(3 \times 10^8\) m/s in a vacuum. Like all EM waves, light transfers energy from a source to an observer.
When light meets a surface or boundary, it can be reflected, refracted, transmitted, or absorbed. The behaviour depends on the wavelength of the light and the properties of the substance.
When electromagnetic waves meet a boundary between materials, they may be absorbed, transmitted, refracted, or reflected. The way they behave varies with wavelength.
Separate Physics only. The law of reflection, ray diagrams for reflection and the difference between specular and diffuse reflection are not examined on Combined Science. Refraction, including the glass block core practical below, is examined.
Reflection
When light reflects off a smooth surface (like a plane mirror), it obeys the law of reflection: the angle of incidence equals the angle of reflection. Both angles are measured from the normal — an imaginary line perpendicular to the surface at the point where the ray hits.
Smooth surfaces produce specular reflection, giving clear images. Rough surfaces produce diffuse reflection, scattering light in many directions.
To draw a ray diagram for reflection: (1) Draw the mirror as a straight line. (2) Draw the normal at 90° to the surface at the point where the incident ray hits. (3) Draw the incident ray arriving at the surface. (4) Measure the angle of incidence (between the incident ray and the normal). (5) Draw the reflected ray at the same angle on the other side of the normal.
Always use a ruler and protractor in the exam. Label the incident ray, reflected ray, normal, angle of incidence (\(i\)), and angle of reflection (\(r\)).
Refraction
Refraction occurs when light passes from one transparent medium into another (for example, from air into glass). The light changes speed at the boundary. If it hits the boundary at an angle to the normal, it also changes direction.
When light passes from a less dense medium (like air) into a more dense medium (like glass), it slows down and bends towards the normal. When it passes from a more dense medium into a less dense medium, it speeds up and bends away from the normal.
Light bends towards the normal when it slows down (entering a denser medium) and away from the normal when it speeds up (entering a less dense medium). A ray hitting the boundary along the normal passes straight through without bending.
In this core practical, you investigate how light refracts as it enters and exits a rectangular glass block.
Method: Place the glass block on paper and draw around it. Shine a ray of light at different angles into the block using a ray box. Mark the incident ray, the point where it enters the block, the point where it exits, and the emergent ray. Remove the block and draw the refracted ray through the block by joining the entry and exit points.
Key observations: The light bends towards the normal on entering the glass (it slows down) and bends away from the normal on leaving (it speeds up). The emergent ray is parallel to the incident ray but displaced sideways. If the angle of incidence is 0° (along the normal), no bending occurs.
The independent variable is the angle of incidence. The dependent variable is the angle of refraction. Control variables include the same glass block and the same light source.
How EM Waves Interact with Matter
Different substances absorb, transmit, refract, or reflect electromagnetic waves in ways that vary with wavelength. For example, glass transmits visible light but absorbs ultraviolet. The atmosphere transmits visible light and radio waves but absorbs most gamma rays and X-rays.
The differences in velocities of electromagnetic waves in different substances cause refraction. When EM waves pass from one material to another where they travel at a different speed, the change in speed can cause a change in direction.
Changes within atoms and their nuclei can generate electromagnetic radiation across a wide frequency range. For example, when electrons in an atom move from a higher energy level to a lower one, they emit EM radiation — the frequency depends on the energy difference.
Conversely, atoms can absorb EM radiation, causing electrons to move to higher energy levels or, at very high frequencies, causing changes within the nucleus itself. Radio waves can be produced by oscillations in electrical circuits, and can themselves induce oscillations in a receiving circuit.
In the core practical, light bends towards the normal when entering glass (slowing down) and away from the normal when leaving (speeding up). The emergent ray is parallel to the incident ray but displaced sideways.
Measuring the Speed of Waves
The speed of sound in air can be measured by timing an echo over a known distance, or by using two microphones connected to a timer. The speed of ripples on water can be measured using a ripple tank: create waves of known frequency and measure the wavelength using a strobe light or ruler, then calculate speed using \(v = f \times \lambda\).
Exam Tip: In the glass block practical, always draw the normal first, then measure angles from the normal (not from the surface). Remember that the emergent ray comes out parallel to the incident ray. If asked to explain refraction, link the change in direction to the change in speed — do not just say "light bends" without explaining why.
Light is a transverse electromagnetic wave that can be reflected, refracted, transmitted, or absorbed when it meets a boundary. Reflection follows the law of reflection (angle of incidence = angle of reflection). Refraction occurs because light changes speed when moving between media — it bends towards the normal when slowing down and away when speeding up. The core practical with a glass block demonstrates these principles. Different materials interact with EM radiation differently depending on wavelength, and changes in atoms can both generate and absorb electromagnetic radiation.