Narration script

Unit three: light as a wave. Light is an electromagnetic wave, made of oscillating electric and magnetic fields travelling at about three hundred million metres per second. Its wavelength decides its colour and how it interacts with materials. Silicon solar cells care about a fairly narrow window, roughly three hundred to twelve hundred nanometres.

You will learn to describe a wave by its wavelength, frequency, speed and amplitude, to place ultraviolet, visible and infrared on the spectrum, to explain refraction and interference, and to use the grating equation, which is the principle behind the spectrometer you will build in Lab A.

Why does a solar engineer need this? Anti-reflection coatings are interference devices. Textured surfaces and cover glass rely on refraction. Quantum efficiency, spectral response and the solar spectrum are all plotted against wavelength. Fluency in nanometres comes first.

A wave is a travelling disturbance that carries energy without carrying matter along. For light, what oscillates is the electric field, with a magnetic field at right angles. Wavelength is the distance between crests, usually given in nanometres, which are billionths of a metre. Frequency is how many crests pass per second. In vacuum every wavelength travels at the same speed, c. And amplitude is the height of the oscillation; brightness goes as amplitude squared. The key relationship is: speed equals wavelength times frequency. Short wavelength means high frequency.

Here are two waves side by side, a red one near seven hundred nanometres and a blue one near four hundred and forty. Both move at exactly the same speed. The blue one simply oscillates faster, because its crests are closer together.

Quick example. Green light at five hundred and fifty nanometres has a frequency of c divided by the wavelength, about five point four five times ten to the fourteen hertz. That is roughly half a million billion oscillations every second.

Radio, microwaves, infrared, visible, ultraviolet, X-rays and gamma rays are all the same phenomenon at different wavelengths. Our eyes see only about three hundred and eighty to seven hundred and fifty nanometres.

This chart lays out the whole electromagnetic spectrum on a logarithmic axis, with wavelength along the bottom and photon energy along the top. The yellow strip is the slice of sunlight that reaches the ground, roughly two hundred and eighty to four thousand nanometres, the range tabulated in the standard solar spectrum. Notice how tiny it is compared to the whole spectrum, and that it extends well beyond the visible into the infrared.

This table connects regions to solar cells. Ultraviolet is only about five percent of standard sunlight's power, but it is what degrades plastics like encapsulants. The visible band carries about half the power, and silicon absorbs it strongly. Near-infrared, out to about eleven hundred nanometres, is still absorbed by silicon, but weakly near the edge, which is why cells need light trapping. And about nineteen percent of the power lies beyond silicon's absorption edge; it passes straight through the cell unused and mostly ends up as heat. We computed those percentages ourselves from the standard spectrum table, in unit eight.

When light enters a material it slows down, to c divided by n, where n is the refractive index. The frequency stays the same, so the wavelength inside shrinks. At a boundary, light bends, and part of it reflects. Look at the indices in the table: air is one, solar glass and encapsulant about one point five, silicon nitride about two, and crystalline silicon nearly four. The bigger the jump in index at a surface, the more light bounces off. Bare silicon reflects over thirty percent of visible light. That is why every commercial cell is textured and coated.

Interference is what happens when two waves overlap. Crest on crest gives a brighter result; crest on trough cancels. A thin transparent film, like the roughly seventy-five nanometre silicon nitride layer on a solar cell, reflects light from both its top and bottom surfaces. If the film is a quarter of a wavelength thick inside the material, those two reflections cancel. That is why solar cells look dark blue. Red and green reflections are suppressed, and a bit of blue and violet still gets reflected. The colour comes from the coating's thickness, not from a blue material.

Now your first instrument, the diffraction grating. It has many fine parallel lines, a distance d apart. Light leaves at angles where waves from neighbouring lines arrive in step: d times the sine of the angle equals a whole number times the wavelength. Each colour leaves at its own angle, so white light fans out into a rainbow.

With a five hundred lines per millimetre grating, the line spacing is two micrometres. Green at five hundred and fifty nanometres appears at about sixteen degrees, red at about twenty and a half, and violet at about eleven and a half. In Lab A you will use exactly this to calibrate a homemade spectrometer.

Misconceptions. Infrared is not heat; it is light our eyes cannot detect, and any absorbed light becomes heat. A cell's blue colour comes from interference, not pigment. And light slowing down in glass does not lose energy; its frequency, and so its photon energy, is unchanged.

On the factory floor, anti-reflection coatings are deposited with nanometre thickness control. Spectral measurements are everywhere in test labs. And ultraviolet exposure is a standard stress test for degradation.

Work through the review questions. In the next unit we discover that light is not only a wave. It also comes in packets called photons, and that changes everything for solar cells.