Question

How is it possible for an atom to emit visible light even though the atom is smaller than the wavelength of visible light?

Answer 1

There are many different possible light sources - hot objects, incandescent gas, lasers, LEDs, phosphors etc.
Light is made of photons of a particular energy that we can see. Those are generally made by electron transitions between atomic orbitals - excite an atom by, for example, hitting it (i.e. heat) and an electron is pushed to a higher energy orbital. Then it jumps down and emits a photon of a particular color that corresponds to the energy difference between the two orbitals. That makes light of one particular colour, e.g. yellow in a sodium streetlamp. Light produced in low-pressure gas, like in fluorescent tubes or the atmosphere of the sun is like that - a mixture of very specific colours that we interpret as just one depending on the proportions (we can't see a full spectrum, just 3). So a sodium lamp actually puts out 2 different yellow wavelengths, but we see just one.

In a fluorescent tube, the gas actually puts out ultraviolet (black light), but the inside of the tube is painted with different phosphors that glow in UV. Their atoms are excited by the UV and the electrons jump down and emit a photon of visible light. By changing the mix of chemicals in the phospor, you can adjust the colour of the light. So we can make "daylight" tubes, "cool white" etc. by adding more or less red and yellow phosphors.

Inside the sun, the pressure is much higher. That broadens the emission spectrum of light emitted by individual atoms. Some of that is just doppler shift because the atoms are moving very fast in random directions. So light from the sun is smeared out to a continuous spectrum compared to what you get from a gas discharge. The overall colour we see depends on how much red, green and blue there is, which depends on the temperature (hotter objects emit more blue). That's called blackbody radiation. Incandescent lights and red-hot objects are like that.

If we shine a white light, like the sun, on something green, like grass, it looks green because it absorbs red and blue light. Atoms in things can work backwards - absorb particular colours of light that correspond to the energy difference between two atomic orbitals, as well as emit those colours when the atom is excited. So in grass, the chlorophyll in the leaves absorbs light to convert it into energy to run the plant - the opposite of a lamp.