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If you have ever walked down an older street at night and noticed that everything looks slightly eerie and washed out, you were likely standing under a low-pressure sodium vapour lamp. These lights cast a distinct, deep yellow glow that makes a red car look black and a blue jacket appear like a muddy grey.
The reason for this lies in the physics of how atoms release light, and how our eyes interpret colours. It is a perfect real-world demonstration of quantum mechanics playing out on a street corner.
Why do some street lights glow orange yellow?
Before modern white LEDs took over, many cities relied on low-pressure sodium (LPS) lamps for street lighting. Inside the glass tube of an LPS lamp, an electric current passes through vaporised sodium metal.
When the electrons in the sodium atoms absorb energy from this current, they jump from their normal resting state to a higher energy state. However, the laws of physics dictate that they cannot stay there for long. As the electrons drop back down to their original, stable energy level, they release the excess energy as a photon of light.
Because atoms have distinct, fixed energy levels, the energy of that emitted photon is always exactly the same. For sodium, this energy drop corresponds almost entirely to a specific wavelength: 589 nanometres (nm). In the visible spectrum, 589 nm sits squarely in the yellow-orange band. Because the lamp uses sodium in an excited state to produce light at a characteristic wavelength near 589 nm, almost all the light it emits is this single colour.
This sharply peaked output is a prime example of an emission spectrum. Every element in the periodic table has its own unique spectrum, acting like a barcode for that atom. You can read more about how scientists first discovered these atomic fingerprints in our post on Joseph von Fraunhofer and the dark lines in sunlight.
Why do colours look dull under yellow street lights?
We see the colour of an object because of the light it reflects. A red car looks red in daylight because sunlight contains all the colours of the rainbow. When sunlight hits the car, the paint absorbs the blue, green, and yellow light, but it reflects the red light back to your eyes.
When you park that same red car under a low-pressure sodium lamp, the only light available to illuminate it is yellow (589 nm). Because there is no red light in the beam for the car's paint to reflect, the car absorbs the yellow light and reflects almost nothing at all. To your eyes, the bright red car appears black or dark brown.
Lighting engineers measure how accurately a light source reveals colours using the Color Rendering Index (CRI). Daylight reveals colours perfectly, making objects look natural. Because LPS lamps emit only one colour and offer no red or blue wavelengths to bounce off objects, low-pressure sodium lighting actually has a negative CRI. This means the colours of objects illuminated by it are difficult to distinguish.
To understand how different materials break apart light into its component colours, see our guide on the dispersion of white light by a glass prism, explained for Class 10.
Connecting phenomena in physics
In a traditional classroom, students often learn about atomic emission spectra in chemistry, and human colour vision in biology, without ever seeing how they connect in the real world. A street light stripping the colour from a jacket is a perfect practical example of how these concepts overlap.
Understanding the link between an atom's energy levels and the light we see takes more than just reading the textbook definition. Being able to change the variables, excite different atoms, and see the resulting wavelengths helps make the physics concrete. This is where the Fermi Simulation Lab comes in, providing interactive simulations for the ideas that are easier to see than to read.
A practical experiment for the street
You can test this colour-subtraction effect yourself if you can find a true low-pressure sodium street light. They are becoming rare as LEDs replace them, but they can still be found at large junctions, railway crossings, or industrial estates where seeing the shape of the road is more important than identifying the colour of a vehicle.
Take a handful of brightly coloured objects—such as red, blue, and green markers, or brightly coloured sweet wrappers—to the light at night.
Mix them up and try to identify which colour is which without using a torch or your phone screen. You will find it nearly impossible to tell the red and green apart.
Turn on your phone's white LED torch and shine it on the objects.
Watch as the full spectrum of white light instantly restores the colours that the sodium lamp had erased.
This simple demonstration proves that the colour we perceive is not just a permanent property of the object itself, but depends entirely on the spectrum of light hitting it.
