In 1801 Thomas Young let one beam of light through two narrow slits and caught the result on a screen. Not two bright lines — a whole row of them, evenly spaced, with darkness in between. Waves arriving in step add; waves arriving half a wavelength out of step cancel. Particles do not do that. The pattern is the argument, and it is why this chapter is called the wave nature of light.

The spacing between neighbouring bright fringes is $\Delta y = \dfrac{\lambda L}{d}$ — read aloud delta-y equals lambda L over d. Delta is the Greek letter, said “DEL-ta”, and here it means “the gap between”. Four quantities, four units: $\Delta y$ is the fringe spacing measured on the screen, $\lambda$ is the light's wavelength, $L$ is the distance from the slits to the screen, and $d$ is the separation between the two slits. Every one of the four is a LENGTH, and in the formula every one of them must be in metres.

Which is precisely where this lesson bites. A real bench hands you a wavelength in nanometres ($10^{-9}$ m), a slit separation in millimetres ($10^{-3}$ m), fringes in millimetres and a bench length in metres — four prefixes in one sentence. Convert every length to metres BEFORE it enters a socket, compute, then convert the answer once, at the end, into whatever the ask demands. Push the units through your rearrangement as a check: metre × metre ÷ metre must leave a metre. If it does not, the rearrangement is wrong, no appeal — though units that DO work out never prove you right. The check is a one-way street.

Read the formula for its behaviour, too. Redder light spreads the fringes; a further screen spreads them; TIGHTER slits spread them. That last one surprises everyone, and it is the denominator saying so.