The inability to directly photograph an atom or objects smaller than the wavelength of visible light is primarily due to a phenomenon known as diffraction. Diffraction occurs when a wave encounters an obstacle or passes through a narrow opening, causing it to spread out and interfere with itself.
Visible light consists of electromagnetic waves with wavelengths ranging from about 400 to 700 nanometers (nm). Atoms and objects on an atomic scale are significantly smaller than the wavelength of visible light. This size difference leads to a fundamental limitation in directly imaging such small objects using traditional optical microscopes.
To understand why, let's consider the concept of resolution. The resolution of an imaging system is the ability to distinguish between two closely spaced objects. In the case of optical microscopes, the resolution is limited by the diffraction of light. According to the Rayleigh criterion, which describes the diffraction limit of an optical system, the minimum resolvable distance (δ) is approximately equal to half the wavelength of light (λ) divided by the numerical aperture (NA) of the imaging system:
δ ≈ λ / (2 × NA)
As the wavelength of light decreases, the resolution improves. However, even with the smallest visible light wavelength (about 400 nm), the resolution of a traditional optical microscope is limited to several hundred nanometers.
Atoms, on the other hand, have sizes on the order of picometers (10^-12 meters). Since the size of an atom is far smaller than the wavelength of visible light, it is impossible to directly "see" an atom using conventional optical techniques. The diffraction of light prevents the formation of a focused image of an object smaller than the wavelength.
To overcome this limitation, scientists have developed various indirect imaging techniques that utilize methods other than visible light, such as electron microscopy or scanning probe microscopy. These techniques involve interactions between electrons or probes and the sample, allowing for higher-resolution imaging of atomic and subatomic structures.
In summary, the inability to photograph atoms or objects smaller than the wavelength of visible light is due to the diffraction limit imposed by the wave nature of light. This limitation can be overcome by employing alternative imaging techniques that exploit different interactions or waves with shorter wavelengths, enabling the visualization of atomic-scale objects.