# Explanation: When light waves propagate through and around objects whose dimensions are much greater than the wavelength of the light,

My textbook, Fundamentals of Photonics, 3rd edition, by Teich and Saleh, says the following:

When light waves propagate through and around objects whose dimensions are much greater than the wavelength of the light, the wave nature is not readily discerned and the behaviour of light can be adequately described by rays obeying a set of geometrical rules. This model of light is called ray optics.

I don't understand what is meant by this part:

When light waves propagate through and around objects whose dimensions are much greater than the wavelength of the light, the wave nature is not readily discerned ...

What is meant by "through and around objects whose dimensions are much greater than the wavelength of the light"? And why is the wave nature not readily discerned?

I would greatly appreciate it if people could please take the time to clarify this.

• IMO it's poorly worded. There's some phenomena that you can describe while ignoring the wave-like nature of light, and just modelling it as bundles of rays. There's other phenomena that can only be described in terms of waves. I think the author didn't try hard enough to define those two categories. The dimensions of the "objects" that constitute an interferometer, for example, are all much greater than the wavelength of light, but you absolutely can not explain how an interferometer works without talking about waves. Nov 5, 2019 at 18:04
• @SolomonSlow Oh, so by "dimensions" it is referring to the physical measurements of the objects? Nov 5, 2019 at 18:42
• Yes. I believe that the author is using "dimensions" to mean physical length measurements. Nov 5, 2019 at 18:47
• @SolomonSlow hmm, but how does such a thing relate to the wavelength of light? I apologise if these are rudimentary questions, but I’ve just begun studying photonics. Nov 5, 2019 at 18:48

"Dimensions" here refers to (quoting from Oxford dictionaries) "a measurable extent of a particular kind, such as length, breadth, depth, or height", e.g. "the final dimensions of the pond were 14 ft x 8 ft".

Light has an intrinsic dimension associated with it - the wavelength. The wavelength of ordinary light is on the order of a few hundred nanometers, which is of course way smaller than the dimensions of ordinary life, so the wave nature of light is not readily discerned, and ray optics works.

However if you take light and make it move through or around things that are only a few hundred nanometers wide, then the wave nature of light will manifest. Examples of these phenomena are interference and diffraction, which you'll encounter later in the book.

As for why the wave nature isn't readily discerned in this case, you'll also encounter the reasons later in the book. Briefly, there are equations such as $$d \mathrm{sin} \theta = \lambda$$, where $$\lambda$$ is the wavelength of light. Because the wavelength of light is so small, the left hand side of the equation is very small as well, and the diffraction angle $$\theta$$ is also approximately zero.

If this all sounds like Greek to you, don't worry about it, you'll learn about it soon.

Wave nature means to see a sinusoidal variation. Example, water waves.Their wave length is within dimensions so that the peaks and troughs can be distiguished by naked eye, from centimeters to maybe a hundred meters. One would have difficulty seeing water waves of milimeter wavelength.

through and around objects whose dimensions are much greater than the wavelength of the light"

The way light is observed by our eyes and our laboratory instruments is by its interaction with materials. We observe the wave nature by interference phenomena,but a single beam of light does not show peaks and troughs, as the velocity c, and the small wavelengths do not allow such a perception : ideally a light beam can be represented by a straight line in space.

When light falls on an object if its wavelength is very much smaller than the size of the object the straight line can be a good description and it is called a "ray". The "through" must be describing slits, where a straight line can also describe the path of the light beam except if the object (or slits) has a very small size, of the order of the wavelength, (for visible light microns to nanometers,) then the wave nature becomes apparent through interference and diffraction patterns.

Otherwise the ray optics are adequate to describe the behavior visible light.