Physics: Sound and light

A wave is a disturbance that transfers energy from one place to another without transferring matter. We can classify waves into two main types. Transverse waves, like light or ripples on water, have oscillations (vibrations) that are perpendicular (at 90°) to the direction of energy transfer. Longitudinal waves, like sound, have oscillations that are parallel to the direction of energy transfer, consisting of compressions and rarefactions. To describe a wave, we use key properties: Amplitude (A) is the maximum displacement from the equilibrium position, determining the wave's energy. Wavelength (λ) is the distance for one complete wave cycle. Frequency(f) is the number of complete waves passing a point per second, measured in Hertz (Hz). The Period (T) is the time it takes for one complete wave to pass. The speed of a wave(v) is related to its frequency and wavelength by the wave equation: v = f × λ.

Sound is a form of energy produced by vibrating objects. When an object, like a guitar string or a speaker cone, vibrates, it pushes and pulls on the surrounding particles of a medium (like air). This creates a series of compressions (areas where particles are bunched together) and rarefactions (areas where particles are spread apart). This pattern of compressions and rarefactions travels outwards as a longitudinal wave. Because sound relies on particle vibrations to travel, it requires a medium. This can be a solid, a liquid, or a gas. Sound travels fastest in solids, where particles are tightly packed and vibrations are passed on quickly, and slowest in gases, where particles are far apart. In a vacuum, where there are no particles, sound cannot travel at all. This is why space is silent.

The characteristics of a sound are directly related to the properties of its wave. Pitch describes how high or low a sound is and is determined by the wave's frequency. A high frequency produces a high-pitched sound (like a whistle), while a low frequency produces a low-pitched sound (like a bass drum). Loudness is our perception of the sound's intensity and is determined by the wave's amplitude. A wave with a large amplitude carries more energy and sounds louder. An echo is a reflection of a sound wave. When a sound wave hits a hard, flat surface, like a wall or a cliff, it bounces back. We can use the time it takes for an echo to return to calculate the distance to the reflecting surface. The total distance travelled by the sound is to the object and back, so the distance to the object is half of this total distance.

Light is a form of energy that travels as a transverse wave. It is part of a much larger family of waves called the electromagnetic (EM) spectrum. All waves in the EM spectrum are transverse, travel at the same incredibly high speed in a vacuum (approximately 300,000,000 m/s or 3.0 x 10⁸ m/s), and do not require a medium. The spectrum is a continuous range of wavelengths and frequencies, arranged in order. From lowest frequency (and longest wavelength) to highest frequency (and shortest wavelength), the order is: Radio waves, Microwaves, Infrared, Visible Light, Ultraviolet, X-rays, Gamma rays. Visible light is the only part of the spectrum that our eyes can detect. It consists of a range of colours, which can be remembered by the acronym ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet), with red having the lowest frequency and violet the highest.

Reflection is what happens when a wave, such as light, hits a surface and bounces off. The Law of Reflection governs this process and has two parts. First, the angle of incidence is equal to the angle of reflection (i = r). These angles are always measured relative to the normal, an imaginary line drawn perpendicular (at 90°) to the surface at the point where the light ray hits. Second, the incident ray, the reflected ray, and the normal all lie in the same flat plane. When you look into a plane (flat) mirror, you see an image that appears to be behind the mirror. This is a virtual image, meaning the light rays do not actually come from the image location; they only appear to. Images in a plane mirror have specific properties: they are upright, the same size as the object, laterally inverted (left and right are swapped), and the image is located as far behind the mirror as the object is in front of it.

Refraction is the bending of light as it passes from one transparent medium to another, for example, from air into glass. This bending happens because the speed of light changes when it enters a different medium. Light travels more slowly in optically denser materials (like water or glass) than it does in less dense materials (like air). When a light ray enters a denser medium at an angle, it slows down and bends towards the normal. Conversely, when it leaves the denser medium and enters a less dense one, it speeds up and bends away from the normal. If a light ray enters a new medium along the normal (at 90° to the surface), it changes speed but does not change direction. This phenomenon is responsible for many optical effects, such as a straw in a glass of water appearing bent or the apparent shallowness of a swimming pool.