Waves, light and sound, Year 11: the electromagnetic spectrum and refraction

microwaves; ultraviolet

HintOne is also used to heat food; the other lies just past the violet end of what you can see.

The whyThe seven groups run from long wavelength and low frequency (radio) to short wavelength and high frequency (gamma). The spectrum is continuous: the groups merge into one another with no gaps. Infrared lies just beyond red light and ultraviolet just beyond violet, which is where their names come from.

The wavelength gets shorter and the frequency gets higher

HintAll of them share one speed in a vacuum, so the two quantities must change in opposite ways.

The whyBecause all electromagnetic waves travel at the same speed in a vacuum, wave speed = frequency × wavelength means a shorter wavelength must go with a higher frequency. Radio waves have the longest wavelength and the lowest frequency; gamma rays have the shortest and the highest.

They are all the same

HintStarlight and radio signals sent out together from a distant galaxy also arrive together.

The whyEvery electromagnetic wave travels at the same velocity through a vacuum, and through air: about 300 000 000 m/s. They are also all transverse waves, and all transfer energy from a source to an absorber. What differs along the spectrum is wavelength and frequency.

transverse

HintIt is the same kind of wave as a ripple on water, not the same kind as sound.

The whyUnlike sound, electromagnetic waves need no material to travel through, which is how sunlight crosses space. They carry energy from a source (the Sun, a lamp, a transmitter) to whatever absorbs them (your skin, a solar panel, an aerial).

red

HintThe colours are listed in the same direction as the whole spectrum: long wavelength first.

The whyVisible light runs from red (longest wavelength, lowest frequency) to violet (shortest wavelength, highest frequency). Red therefore sits next to infrared and violet next to ultraviolet. Our eyes detect only this narrow band of the whole spectrum.

Highest: matt black. Lowest: shiny silver

HintDark, dull surfaces and bright, polished ones sit at opposite ends of the ranking.

The whyAll four faces are at the same temperature because they share the same hot water, so any difference in the readings comes from the surface alone. Dark, matt surfaces are the best emitters of infrared radiation; light, shiny surfaces are the poorest. This is the 'radiated' half of the infrared required practical.

distance

HintA radiant heater feels weaker the further back you stand.

The whyThe reading on an infrared detector falls quickly as it moves away from the source. If the gap changed between faces, a difference in reading could be due to the gap and not the surface. The gap is a control variable; the type of surface is the independent variable and the detector reading is the dependent variable.

The matt black plate: it absorbs more of the infrared radiation, while the shiny plate reflects most of it

HintThink which car feels hotter after an hour in a sunny car park: a dark one or a silver one.

The whyGood emitters of infrared are also good absorbers: dark, matt surfaces take in most of the radiation that falls on them, while light, shiny ones reflect it. The plates must be the same size, material and distance from the heater so that the surface is the only thing that differs. This is the 'absorbed' half of the required practical.

A wave front

HintIn a ripple tank, each bright line on the screen is one of these.

The whyWave fronts give a bird's-eye view of a wave: a set of parallel lines, one for each crest, one wavelength apart. A ray is the arrow at right angles to them that shows which way the wave is going. Higher-tier pupils use diagrams of these lines to explain refraction.

The part of each wave front that enters the glass first slows down first, while the rest is still moving faster in air, so the wave front swings round to a new direction

HintPicture a row of marchers stepping from tarmac onto mud, one end of the row before the other.

The whyRefraction is caused by the change in speed. Meeting the boundary at an angle, one end of a wave front is in the slower material while the other end is still in the faster one, so the slower end falls behind and the whole front turns. Going into the slower material, the ray bends towards the normal.

shorter

HintUse the wave equation with one quantity fixed and one going down.

The whyWave speed = frequency × wavelength. The frequency is set by the source and does not change at a boundary, so a lower speed must mean a smaller wavelength. On a wave-front diagram this shows as the wave fronts being drawn closer together in the glass.

It slows down but does not change direction

HintAsk whether any part of a wave front reaches the glass before the rest.

The whyEvery part of each wave front crosses the boundary at the same moment, so all of it slows together and nothing makes it turn. The wave fronts get closer together but stay parallel to the surface. Refraction, the turning of the ray, happens only when the wave meets the boundary at an angle.

How a substance absorbs, transmits, refracts or reflects electromagnetic waves depends on their wavelength

HintAsk what differs between the three kinds named, given that all are members of one family.

The whyThe same substance can be transparent to one part of the spectrum and opaque to another: ordinary window glass lets visible light through but absorbs most of the ultraviolet that causes sunburn. A prism splitting white light is the same rule at work, since each wavelength is refracted by a different amount.