Light, Sound and Materials
Specular Reflection and Diffuse Scattering
When a beam of light hits a smooth, polished surface such as a mirror, all the rays reflect at the same angle — the angle of incidence equals the angle of reflection, both measured from the normal. This is called specular reflection. Because all rays reflect uniformly, a clear image is formed.
Most everyday surfaces are not perfectly smooth. An uneven surface scatters incoming light in many different directions — this is diffuse reflection. This is why you can see a piece of paper from any angle but cannot see your reflection in it.
Refraction, Prisms and Dispersion
Light is refracted when it changes speed at a boundary. When white light passes through a glass prism, the different colours within it travel at slightly different speeds in glass, so they refract by slightly different amounts. Violet light (shortest visible wavelength) is refracted the most; red light (longest visible wavelength) is refracted the least. The beam spreads into the colours of the spectrum. This spreading is called dispersion.
White light is a mixture of all visible colours, ranging from violet (shortest wavelength) to red (longest wavelength). A prism disperses white light into a spectrum because different wavelengths travel at different speeds in glass, causing different amounts of refraction. Violet refracts the most; red refracts the least.
Colour, Absorption and Transmission
A coloured filter works by transmitting light of some wavelengths and absorbing light of others. A red filter transmits red wavelengths and absorbs all others, which is why only red light passes through it.
An object’s apparent colour depends on which wavelengths it scatters (reflects diffusely). An object appears white if it scatters all colours equally; it appears black if it absorbs all colours and scatters none. A red tomato scatters red wavelengths and absorbs the rest. This means the colour you perceive depends on the light falling on the object: a red object under only blue light will appear dark (because there is no red light to scatter).
A convex lens (converging lens) is thicker at the centre. It refracts parallel rays inwards so they meet at a focal point on the far side. Convex lenses are used in cameras, eyes and magnifying glasses. A concave lens (diverging lens) is thinner at the centre. It refracts parallel rays outwards so they appear to spread from a point on the same side as the incoming light. Concave lenses are used in glasses to correct short-sightedness.
A real image is formed when refracted rays actually converge at a point; it can be projected onto a screen. A virtual image appears to come from a point but the rays do not actually meet there; it cannot be projected.
Sound Transmission Through Solids
Sound travels as a longitudinal wave, and it travels better through solids and liquids than through air — because the particles are closer together and transmit vibrations more efficiently. This is why you can hear sound through a wall more easily when you press your ear against it.
The bones in the middle ear are a biological example of sound transmission through a solid. The three small bones (malleus, incus and stapes) transmit vibrations from the eardrum to the fluid of the inner ear. These bones transmit sound most efficiently in the frequency range from about 1 kHz to about 3 kHz — which corresponds well with the frequencies most important for understanding speech. This is why human hearing is most sensitive over this range of frequencies.
Sound travels faster and more efficiently through solids and liquids than through gases, because particles are closer together and transmit vibrations more readily. The bones in the middle ear transmit sound vibrations most efficiently in the 1 kHz to 3 kHz range, the range that matters most for understanding speech.
The ways in which sound waves are transmitted, reflected and refracted as they pass through different materials are exploited in several technologies. In medical ultrasound imaging, high-frequency sound waves above the range of human hearing are directed into the body. Reflections from boundaries between different tissue types return to a detector and are used to build up an image.
SONAR (Sound Navigation And Ranging) works on a similar principle in water: pulses of sound are sent out from a vessel and the time for reflections to return is used to calculate the depth of the sea floor or the position of objects such as fish shoals. Earthquake (seismic) waves, which travel through the Earth, have revealed details of the Earth’s layered internal structure because different types of seismic wave are reflected and refracted differently at boundaries between rock layers. The differences in velocity, absorption and reflection between wave types allow scientists to map structures hidden from direct observation.
Transmission, Absorption and Scattering
When a wave meets a material, three things can happen: it may be transmitted (pass through), absorbed (its energy transferred to the material) or reflected and scattered. Most real encounters involve a combination of all three, in proportions that depend on the wavelength of the wave and the nature of the material. A clear glass window transmits visible light but absorbs some infrared; a coloured glass transmits selected wavelengths and absorbs others; an opaque wall absorbs all visible light and transmits none.
The principle extends to sound. Soft furnishings in a concert hall absorb sound, reducing echo and reverberation. Hard surfaces reflect sound, which can cause unwanted echoes. Acoustic engineers designing concert halls and recording studios use knowledge of sound transmission, absorption and reflection to control how sound behaves in a space. The same considerations apply to building construction: thick, dense walls transmit less sound than thin partitions because more of the wave energy is absorbed within the material.