Why can't we see objects smaller than wavelength of visible light?

In summary: What you "see" is usually scattered light, not reflected light. Reflection is not even a well defined concept for very small particles like atoms or molecules.Nevertheless, there still is some scattered light, but that does not allow you to resolve the scatterer. The optical resolution of a microscope depends on the wavelength w used and the numerical aperture (NA) available. The resolution is around 0.61 w/NA. Using standard visible light microscopes, this gives you a resolution of roughly 200 nm in the best case.
  • #1
iVenky
212
12
I read in Feynman's book that it is not possible to see objects less that wavelength of visible light with a microscope. That's the reason why we go for electron microscope. Why can't we objects which is less than wavelength of the information carrying medium (which in this case is visible light)?
 
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  • #2
objects smaller than the highest frequency of visible light can't be seen because they won't reflect back any photons for your lights to respond to. If its visible in an ultraviolet frequency then our eyes won't detect it.

But researcher have done some work in this area to improve resolution:

http://news.sciencemag.org/sciencenow/2011/04/scattering-microscope-peers-into.html [Broken]
 
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  • #3
jedishrfu said:
objects smaller than the highest frequency of visible light can't be seen because they won't reflect back any photons for your lights to respond to. If its visible in an ultraviolet frequency then our eyes won't detect it.

But researcher have done some work in this area to improve resolution:

http://news.sciencemag.org/sciencenow/2011/04/scattering-microscope-peers-into.html [Broken]

Thanks. But once again one doubt. Why don't they reflect back the photons? Do they get scattered?
 
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  • #4
its like the object isn't there the wave just goes through it until reflected by something matching the wave length.
 
  • #5
iVenky said:
Thanks. But once again one doubt. Why don't they reflect back the photons? Do they get scattered?

What you "see" is usually scattered light, not reflected light. Reflection is not even a well defined concept for very small particles like atoms or molecules.

You may be able to see the effects of light scattering off such small particles (like the sky being blue), but you will not be able to resolve these particles using light of large wavelength. Whether light gets scattered or not depends strongly on the size of the scatterer. In the limit of tiny particles, the scattering process is called Rayleigh scattering. In the simple case of a round particle of size and light of wavelength w, the scattering probability behaves proportional to d^6/w^4. So the scattered intensity becomes small quickly as the particle size becomes small compared to the wavelength.

Nevertheless, there still is some scattered light, but that does not allow you to resolve the scatterer. The optical resolution of a microscope depends on the wavelength w used and the numerical aperture (NA) available. The resolution is around 0.61 w/NA. Using standard visible light microscopes, this gives you a resolution of roughly 200 nm in the best case.

The underlying concept limiting your optical resolution is in this case diffraction. If you look up the diffraction pattern of a circular aperture, you will find that it will be a so-called Airy disk. So the light coming from the aperture will not be imaged to a spot, but to such an Airy disk with the central spot having a certain diameter depending on the wavelength used and the size of the aperture. If you use two apertures, you get two such disks. If their central spots overlap strongly, you cannot distinguish whether you had one or two apertures to start from and your resolution is insufficient to resolve the object.
 
  • #6
Diffraction is the basis of the explanation. Everything you see is basically a diffraction image. When the object is close to the size of the wavelength it is not easy to identify a 'physicsl' shape.
 

1. Why is the wavelength of visible light important for our ability to see objects?

The wavelength of visible light is important for our ability to see objects because it determines the size of the smallest details that our eyes can perceive. Objects that are smaller than the wavelength of visible light cannot be seen because the light waves cannot interact with them in a way that our eyes can detect.

2. Can we use technology to see objects smaller than the wavelength of visible light?

Yes, we can use technology such as microscopes to see objects smaller than the wavelength of visible light. Microscopes use lenses to magnify the image of the object, making it appear larger and allowing us to see smaller details that are normally invisible to the naked eye.

3. What determines the wavelength of visible light?

The wavelength of visible light is determined by the frequency of the light waves. In the electromagnetic spectrum, visible light falls between infrared radiation (with longer wavelengths) and ultraviolet radiation (with shorter wavelengths). Each color of visible light has a different wavelength, with red having the longest wavelength and violet having the shortest.

4. Is it possible for animals to see objects smaller than the wavelength of visible light?

Yes, some animals have the ability to see objects that are smaller than the wavelength of visible light. For example, insects have compound eyes that allow them to see ultraviolet light, which has a shorter wavelength than visible light. This enables them to see details on flowers that are invisible to humans.

5. Can we see objects smaller than the wavelength of visible light if we change the color of the light?

No, changing the color of the light does not change the size of the smallest objects we can see. The wavelength of visible light is still the determining factor, regardless of its color. However, using different wavelengths of light can help us see different details on the same object, as some objects may reflect or absorb certain wavelengths more than others.

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