Can Light Bend Around Corners: 9 Important Facts


The answer is “Yes”, light can bend around corners.

When light passes around the edges of an object it tends to bend its path around the corners. This property of light is known as diffraction. The phenomenon of diffraction depends on the propagation of light. For studying this phenomenon, light is treated as a wave.  

Contents:

What is diffraction of light?

DIffraction of light refers to the phenomena of bending of light waves around the corners of an obstructing object having a size comparable to the wavelength of light. The phenomenon of diffraction depends on the propagation of light. For studying this phenomenon, light is treated as a wave.

The degree or extent by which the light rays bend is dependent upon the size of the obstructing object and the wavelength of light. When the size of the object is much larger compared to the wavelength of light then the extent of bending is negligible and cannot be noticed properly. However, when the wavelength of light is comparable to the size of the obstructing object (such as a dust particle) then the extent of diffraction is high i.e. the light waves bend at larger angles. In such cases, we can observe the diffraction of light with the naked eye.

Let us learn more about how light bends around corners:

How can light bend around corners?

According to classical physics, the phenomenon of diffraction is experienced by a light wave because of the way it propagates. The phenomenon was described by Christiaan Huygens and Augustin-Jean Fresnel in the Huygens-Fresnel principle and the principle of superposition of waves. Lightwave propagation can be visually interpreted by taking every single particle in the medium of propagation as a point source that gives rise to the secondary wavefront of a spherical wave.

The displacement of the waves from every point source gets added up to form a secondary wave. Amplitudes and relative phases of every wave play an important role in determining the subsequent spherical wave formed. The amplitude of the resultant wave can take any value lying between 0 and the addition of the individual amplitudes of the point sources.

Therefore, a general diffraction pattern consists of a series of minima and maxima.

According to modern Quantum optics, every Photon that passes through a thin slit gives rise to its own wave function. This wave function depends on several physical factors such as the dimensions of the slit, the distance from the screen, and the initial conditions of the photon generation. 

The diffraction phenomenon can be qualitatively understood by taking into consideration the relative phases of the secondary waves fronts. The superposition of two half circles of waves results in constructive interference. When two half circles of waves cancel each other out, it results in destructive interference.

Diffraction in Atmosphere:

Light gets diffracted in the atmosphere by bending around the atmospheric particles. Usually, the light gets diffracted by the tiny water droplets suspended in the atmosphere. The bending of light can give rise to light fringes light, dark, or colored bands. The silver lining that can be observed around the edges of clouds or the coronas of the moon or the sun is also a result of the diffraction of light. 

Bending of light as seen through hot steam. (can light bend around corners) Image source: Brocken InaglorySolar glory at the steam from hot springCC BY-SA 3.0

Examples of diffraction seen in everyday life

Some examples of diffraction or bending of light can be seen often in our day to day life such as:

CD or DVD: In a CD or DVD disc we can often see the formation of a rainbow-like pattern. This rainbow-like pattern is formed due to the phenomenon of diffraction. Here, the CD or DVD acts as a diffraction grating. 

Hologram: A hologram is designed such as to produce a diffraction pattern. Such holograms are often seen in credit cards or book covers. 

Laser beam propagation: The change in the beam profile of a laser beam as determined by the phenomenon of diffraction that occurs when the laser beam propagates through a medium. The lowest recorded divergence due to the fraction is provided by a planar spatially coherent wavefront with a Gaussian beam profile. Generally, the larger the output beam, the slower is the divergence.

The extent of divergence of a laser beam can be reduced by first diverging the beam with the help of a convex lens and then converging or collimating the beam with the help of a second convex lens having a focal point coinciding with the focal point of the first convex lens. In this way, the resultant beam will have a larger diameter compared to the original beam and hence, the divergence would be reduced.

Diffraction limited imaging: Diffraction limits the resolving power of an imaging system. Due to distraction, the light beam is unable to focus at a single point. Instead, the formation of an error disk takes place which has a central bright spot with a concentric circle surrounding it. It is seen that with a larger aperture the lenses are able to resolve images more finely. 

Single-slit diffraction: The diffraction of a long slit with negligible width is taken. The slit is then illuminated with a point source of light. After passing through the slit the light gets diffracted into a series of circular wavefronts. The slit is wider than the wavelength of light then it can produce interference patterns in the space that lies below the slit.

Diffraction pattern as observed through a single slit. can light bend around corners).Image source: Dicklyon at English WikipediaWave Diffraction 4Lambda Slit, (can light bend around corners) marked as public domain, more details on Wikimedia Commons

The concept of bending of light might induce certain questions in people’s minds. Let us have a look at some of those questions:

Does light travel in a straight line? If so, how?

Light is an electromagnetic wave and therefore it travels in the form of a wave. However, the wavelength of light is very small. Hence, a light wave is approximately taken as a ray that travels in a straight line. The wave property of light can be observed only when it interacts with objects having a size comparable to the wavelength of light. For the objects in our day-to-day life, the interaction with light is taken as rays that travel in a straight line. For smaller objects, light bends around corners due to diffraction.

How is interference related to water waves?

can light bend around corners
Water waves.
 Image source: (can light bend around corners)VerbcatcherWave diffraction at the Blue Lagoon, AbereiddyCC BY-SA 4.0

The interference of light waves causes the optical effects resulting from the bending of light. We can visualize this fact by imagining the waves of light as water waves. Supposed you keep a wooden plank on a water surface to float, you will notice that the water waves would make the wooden plank bounce up and down in accordance with the incident water waves. These water waves further spread out in every direction and interfere with the neighboring water waves.

When the crests of two water waves merge it leads to the formation of an amplified wave i.e. constructive interference takes place. However, when the trough of a wave interferes with the crest of another wave, they cancel each other out resulting in a null amplitude that has no vertical displacement i.e. destructive interference. When the troughs of two separate waves interfere they form a more depressed trough.

This same pattern is observed in the case of light waves. When the light from the sun encounters droplets of water suspended in the atmosphere, the light waves interact with each other in a manner similar to that mentioned above in the case of water waves. In the case of light waves, constructive interference takes place when the peak amplitude of two light waves interact to produce a more amplified wave.

In other words, when two crests of light waves interact or interfere they form a brighter pattern. Destructive interference occurs when the trough of a light wave interferes with the crest of another wave. This destructive interference is observed by the formation of a darker pattern. 

Diffraction of light causing the display of colors in a spider web. (can light bend around corners)
Image source: Brocken InagloryDiffraction pattern in spiderwebCC BY-SA 3.0

How can light bend around corners inside an optical fiber?


Light rays get refracted after entering the optical fiber material.

The light waves propagate through The optical fiber core by getting refracted back and forth from the boundary or the interface between the core and the cladding. Light propagates through the optical fiber without passing or transmitting through the fiber by a phenomenon of total internal refraction. 

Total internal reflection can take place only when the angle of the incident light on the boundary of the optical fiber is greater than the critical angle of the fiber. When the angle is greater than the critical angle the light gets refracted into the optical fiber instead of leaking out through the cladding.

What is the condition of maximum deviation of light in prism?

The maximum deviation of light in a prism can be possible due to the following two conditions:

1. The maximum deviation of light can take place only if the angle incident on the prism is a right angle i.e. 90 degrees. This property is also known as grazing incidence due to the fact that the light rays almost “graze” along the surface of the prism.

2. The second condition for maximum deviation of light in a prism is that when an emerging ray gets reflected at 90 degrees or we can say that it grazes along the surface of the prism. This condition is similar to the condition mentioned above for the second surface.

Note: we should not confuse the maximum deviation angle with the angle of minimum deviation of a prism.

What is the difference between scattering and diffraction?

Scattering of light: Scattering of light occurs when light strikes small objects such as dust particles or gaseous molecules of water vapor, it tends to get deviated from its straight path of propagation. This phenomenon is termed the scattering of light. Scattering of light can be noticed or observed in several environmental phenomena. The blue color of the sky, the white color of clouds, the red color of the sky during sunset and sunrise, the Tyndall effect, etc. are some examples of scattering of light.

Traffic lights or danger signals are usually red in color because red scatters the least out of all wavelengths. The extent of scattering is inversely proportional to the fourth power of wavelength of light. The phenomenon of scattering can be observed as wave interactions and particle interactions both. The property of scattering is linked with wave interactions.

Diffraction of light: diffraction of light refers to the phenomenon by which light rays tend to bend around the corners of an object having a size comparable to the wavelength of light. Diffraction is observed only by treating light as a wave only. The property of diffraction is linked with wave propagation. The pattern interference pattern observed during single slit experiment, gratings, holograms excreta occur due to diffraction.

Is it possible for an incident ray to have an angle of more than 90 degrees?

The angle of incidence to a surface is defined as the angle made by the light ray from the normal to the point it touches. Therefore, the maximum angle that can be made with the normal to the surface is 90 degrees on either side.

We hope this post answered your queries regaing the phenomenon of diffraction.

Sanchari Chakraborty

I am an eager learner, currently invested in the field of Applied Optics and Photonics. I am also an active member of SPIE (International society for optics and photonics) and OSI(Optical Society of India). My articles are aimed towards bringing quality science research topics to light in a simple yet informative way. Science has been evolving since time immemorial. So, I try my bit to tap into the evolution and present it to the readers. Let's connect through https://www.linkedin.com/in/sanchari-chakraborty-7b33b416a/

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