Light and the electromagnetic spectrum Revision Notes

Electromagnetic Wave Interactions

Substances can interact with electromagnetic (EM) waves in four main ways: absorption, transmission, reflection, and refraction. These interactions are not constant but vary significantly depending on the wavelength of the EM radiation and the properties of the material.

Wavelength Dependence

The behaviour of a substance towards EM waves is highly dependent on the wavelength. For instance, a material might be transparent to visible light (transmitting it) but opaque to X-rays (absorbing them). This wavelength-dependent interaction is fundamental to many technological applications and natural phenomena, such as how Earth's atmosphere filters sunlight.

Examples of Interactions

Consider different materials: glass transmits visible light but reflects some and absorbs much of the UV spectrum. Metals typically reflect a broad range of EM waves. Water largely transmits visible light but absorbs infrared radiation. The specific atomic and molecular structure of a substance determines its interaction profile across the electromagnetic spectrum.

Velocity Changes and Refraction

Electromagnetic waves, such as visible light, travel at their highest speed in a vacuum. When these waves pass into a new substance, like from air to glass, their velocity changes because different substances have different optical densities. If the wave enters the new medium at an angle to the normal, this change in speed causes a change in direction—this is called refraction.

Refractive Index and Wave Behaviour

The refractive index measures how much the speed of light is reduced inside a material compared to a vacuum. A higher refractive index means a greater reduction in light speed and, for a given entry angle, greater bending of the wavefront. Refraction is the underlying principle behind the operation of optical devices like lenses, prisms, and fibre optics.

Angle of Incidence and Refraction

If an EM wave enters a different medium exactly along the normal (angle of incidence is 0°), only its speed changes—not its direction. Refraction, or bending of the wave path, only occurs if the angle of incidence is not 0° (not along the normal). The amount of bending increases with the angle of incidence, assuming the materials have different refractive indices.

Emission of Radiation

Every object, regardless of its temperature, continuously emits thermal radiation. This radiation is a form of electromagnetic waves, with infrared radiation being a common example for objects at room temperature. The emission is not dependent on light shining on the object, but rather on its internal thermal energy.

Temperature's Influence on Radiation

The temperature of an object dictates both the amount and the type of radiation it emits. As temperature rises, the total intensity (power emitted per unit area) of the radiation increases sharply. Furthermore, the distribution of wavelengths in the emitted radiation shifts: hotter objects emit a greater proportion of shorter-wavelength radiation, including visible light and ultraviolet, while cooler objects primarily emit longer-wavelength infrared radiation. This explains why a hot iron glows red, then orange, and eventually white as its temperature increases.

Energy Absorption and Emission

All objects continuously absorb and emit thermal radiation. The rate at which an object absorbs and emits energy depends on several factors, including its surface area, surface properties (e.g., colour, texture), and temperature difference with its surroundings. Dark, dull surfaces are efficient absorbers and emitters, while light, shiny surfaces are poor absorbers and emitters.

Achieving Constant Temperature

A body maintains a constant temperature when the average power it absorbs from its surroundings is exactly equal to the average power it radiates. This condition is known as thermal equilibrium. When this balance is disturbed, the object's temperature will change until a new equilibrium is reached or until the rates of absorption and emission once again become equal.

Imbalance and Temperature Change

If a body absorbs more power than it radiates, its temperature will increase until the radiation rate matches the absorption rate. The hotter the object, the more power it radiates. Conversely, if a body radiates more power than it absorbs, its temperature will decrease until the absorption rate matches the radiation rate, or it reaches the temperature of its surroundings.