[Physics] How exactly is white light a combination of several wavelengths?

frequencyopticsvisible-lightwavelength

I have read that light is an electromagnetic wave. Every ray of light has a specific wavelength. The colour perceived by any observer is dependent upon the wavelength of the incident light.

What I don't understand is that how do electromagnetic waves of different wavelengths combine to form a single wave of another wavelength? Simply put, I have the following two related questions to ask:

  • When we look upon a, say, completely white object, what is the composition of the individual rays that strike our eyes? Are those rays waves formed by the addition of waves corresponding to individual wavelengths that constitute white colour? I get that white light is composed of all wavelengths of visible light, but how are those wavelengths combined into a single unit which we call white light?
  • If it is so, then how are prisms able to disperse light into its constituent colours?

Also, as a side question, how does all of this relate to light being composed of photons?

Best Answer

The OP's restated question is

I get that white light is composed of all wavelengths of visible light, but how are those wavelengths combined into a single unit which we call white light? Are they just sticking to each other like a bundle of sticks, perhaps with spacing in between them?

It appears that the OP is visualizing a light "wave" as a "ray" comprising a line with tiny wiggles -- maybe something like a guitar string.

In classical electromagnetic theory, actually a light wave is more like a wave on water (though not confined to a 2D surface). A water wave can be a single frequency, in which case the spacing between ripples is constant and the shape of the ripples is a perfect sinusoidal wave. If the water wave is comprised of multiple frequencies, the spacing and shapes of the ripples vary from place to place (and of course they vary in time as well).

If we were to draw lines in the direction the ripples are moving at each point, the lines would correspond to what we think of as "rays"; but those lines are not objects, they are only a way to describe the motion of a point on a wave.

Light waves (from the classical perspective) similarly correspond to ripples in electric and magnetic field amplitude and direction, within a 3D volume. "Light rays" are not real objects, they are just a way to describe the motion of the wavefronts of light. A mix of colors/wavelengths/frequencies amounts to a mix of spacing between the ripples, and to variations in the shapes of the ripples. This answers the first part of your question.

The second part of your question:

If it is so, then how are prisms able to disperse light into its constituent colours?

This is very different from the first part of your question. So: remember that a "ray" is really just a way to describe the motion of a point on a wave. This works fine for waves that never change shape or spacing, but gets ambiguous when the wave is not a perfect sinusoid. Suppose each "ripple" has an extra bump on it. In that case we can imagine that the overall (3D) wave is actually the sum of two (3D) waves, where one wave has half the wavelength of the other. Every light wave can be resolved into a superposition of different-wavelength waves, which may be traveling in the same direction or in different directions.

When light of a single wavelength enters a prism, its direction changes because its speed changes. But the speed of light in glass is dependent on the wavelength, so the new direction is different for each wavelength. In the case above, when a wave is composed of two waves having different wavelengths but the same speed, incident on the prism from the same direction, the longer-wavelength component ends up moving in a different direction from the shorter-wavelength component.

It's still all one complicated wave, but the wave segregates into different regions where the ripples have different spacing. "Rays" drawn to describe the motion of the ripples will indicate that the shorter-spacing ripples are moving in a direction different from the direction of the longer-spacing ripples.