Why light shows its wave-like properties only when it interacts with objects with dimensions close to the wavelength of light? In Young's Double Slit Experiment, we were taught that light behaves as a wave here because the width of the slits are very close to the wavelength of light itself. But why does light behave like a wave only when it interacts with objects that have dimensions close to the wavelength of light? Even in my book no explanation is given as to why this is true. Can someone please explain as to why this is true? I am so confused.
 A: If you send a water wave (with a fixed wavelength) to an object, a diffraction pattern will form behind the object (assume you send the wave perpendicular to the length of the object). If the wavelength of the waving water is very small compared to the object, the wave will not notably curve around the edges. Almost no diffraction of the wave will be seen (only reflection, approximately). Only at the edges of the object two waves will emerge that can interfere, but because the length of the object is much bigger than the wavelength of the water this effect is small (but it is there). If the wavelength of the water is comparable or larger than the size of the object an interference (diffraction) pattern will emerge. The waves will curve. See here. The same holds for the "inverse": sending a wave to an (infinitely) extended object with an open space in it.


Replace the water waves with electromagnetic radiation, and you will see why waves interfering with objects that have a size comparable with their wavelength will show more wavelike behavior.
A: Actually, in a more general sense it is not the case that light interacts only with features that are at the scale of the wavelength of light.
There is an optical effect referred to as 'highway mirage'. Close to the tarmac the air is heated by the tarmac. Under atmospheric pressure: the warmer the air the lesser its density. So there is a gradient in air density.
The speed of propagation of light in air is not as fast as in vacuum, but the less dense the air is, the faster the speed.
In the case of a mirage: because of the gradient in density the air is acting as a refractive medium. The amount of refraction is small, but under favorable circumstances it is quite noticable. With light that arrives at the road at a sufficiently shallow angle refraction back upwards can occur.
The gradient in air density is an example of a large scale feature that affects the propagation of light.

The goal of a Young's double slit setup is to obtain interference fringes. In order to obtain the fringes anything that would result in things averaging out must be avoided. If the width of the slits is a multiple of the wavelength of light then as light arrives at the screen it can have traveled a range of different distances. But you obtain the fringes only if at every point on the screen the distance that the light has traveled (through either slit 1 or slit 2) is narrowed down. More precisely, the difference in distance traveled must be a fraction of the wavelength of light (modulo the wavelength of light)
A: Consider an optical lens, say a magnifying glass. The light interacts with it, its path bending as it passes through. Yet the lens is far larger than the wavelength, while the individual atoms are much smaller.
It is simply that interference effects are easiest to see when the dimensions are similar, as evidenced in the Young's Slits experiment.
A: It is somewhat misleading to say that light behaves as a wave only when it interacts with objects that have dimensions comparable with its wavelength. It would be closer to reality to say that its wavelike properties are more apparent in such circumstances.
If you want an analogy to help you develop a mental picture of what is happening, imagine a cork and a supertanker floating on a sea that is initially perfectly flat.
If ripples with a wavelength of a few inches travel over the surface of the sea, the cork will ride up and down with them- the motion of the cork will faithfully reflect the wave-like nature of the ripples, but the supertanker will be undisturbed. Being so much longer than the wavelength of the ripples, the supertanker will experience the effects of many ripples averaged out, the effects of their peaks being averaged out by the effects of their troughs.
Conversely, in a gentle swell with a wavelength comparable to length of the supertanker, the ship will seesaw- the bow will sink in a trough while the stern is raised by a peak.
A: Actually it is not true in generality. Take the Poisson or Arago Spot: https://en.wikipedia.org/wiki/Arago_spot
Basically if you have a disk shaped object the shadow will have it its center a bright spot, since all distances from brim of the object are the same and interfere constructively. (This experiment actually settled the particle vs wave debate before it got settled again)
A: The diffraction limit for imaging optics is a phenomenon that may be encountered with objects millions of wavelengths in extent (space telescopes, radio telescope arrays, ...).
A: You are asking "why does light behave like a wave only when it interacts with objects that have dimensions close to the wavelength of light?", and there is a beautiful example of why the sky is blue (and it actually shows you that light can show wave properties when the atoms it scatters off are much smaller then the wavelength, though, the scattering itself is wavelength dependent, making blue light scatter more).

Rayleigh scattering (/ˈreɪli/ RAY-lee), named after the nineteenth-century British physicist Lord Rayleigh (John William Strutt),[1] is the predominantly elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the radiation.

https://en.wikipedia.org/wiki/Rayleigh_scattering
This is caused by Rayleigh scattering, where the atoms in the atmosphere are much smaller then the wavelength of visible light. Still, the blue dominates, because the blue light has higher frequency, that is smaller wavelength, and that wavelength of the blue light is a little bit closer to the size of the atoms in the atmosphere. Thus, blue light scatters more.
This is a phenomenon, because our universe is fundamentally quantum mechanical, and this illustrates how light's wave properties dominate throughout the elastic scattering process.
