# Trying to understand diffraction

• Jigyasa
In summary, light and sound can diffract around obstacles regardless of their size, but the intensity on the other side is low if the obstacle is much larger than the wavelength. For visible light, this intensity is not enough to see, while for sound waves the intensity is often high enough for you to hear the source.
Jigyasa
I have a problem understanding diffraction and its dependence on the wavelength of the source. We know that light cannot travel through obstacles of the size of everyday objects while sound can. That is why we can hear sound across the corner of a building but we can't see a light source across the cornerBut the light and sound source both have a series of spherical wavefronts emanating from them. The spacing between these wavefronts is of the order of their respective wavelengths but whatever the spacing, we will observe some part of some wavefront on the other side of the corner right?Why can light/sound only emanate out of obstacles of the order of their wavelength?

Jigyasa said:
We know that light cannot travel through obstacles of the size of everyday objects while sound can. That is why we can hear sound across the corner of a building but we can't see a light source across the corner

Both light and sound will diffract around an obstacle, regardless of how big the obstacle is compared to the wavelength. The problem is that if the obstacle is MUCH larger than the wavelength then the wave will barely diffract around the object and the intensity directly on the opposite side is very small. For visible light this intensity isn't enough to see, while for sound waves the intensity is often high enough for you to hear the source because the wavelegths are much closer to the size of the obstacle.

As an example of when you can see light waves diffracted around a large object, see the Arago spot.

sophiecentaur
Jigyasa said:
Why can light/sound only emanate out of obstacles of the order of their wavelength? etc . . .
You are entering into a very complicated topic here and you can easily think there are contradictions.
If you think in terms of a small object, placed in the way of a wave, it will intercept only a small amount of wave power and so its pattern will be very low level - not really affecting what you see / hear. Size normally does matter. The resulting pattern of a single slit will be a broad pattern with very low level (not much energy gets deviated). You really need to look at the Maths of the situation, if at all possible.
The information in This link is the best way into Diffraction patterns because it is the simplest situation and is often described as Interference. between two 'point' sources The closer the slits, the wider the fringes in the pattern and the wider the separation, the closer together and narrower are the fringes. A wide slit can be considered (Calculus comes into this) as an infinite set of point sources and you can add up the contributions of each point source. This link shows the pattern of a single, wide. aperture. I suggest you do a google search and choose one of the hundreds of links to suit you best.

## 1. What is diffraction?

Diffraction is a phenomenon that occurs when waves, such as light or sound waves, encounter an obstacle or opening and bend around it, creating an interference pattern. This can be observed when light passes through a narrow slit or when sound travels around a corner.

## 2. How does diffraction relate to the concept of interference?

Diffraction is a type of interference, as it involves the bending and overlapping of waves to create an interference pattern. However, diffraction differs from other types of interference, such as constructive and destructive interference, as it does not require multiple sources or waves to interact with each other.

## 3. What is the difference between Fraunhofer diffraction and Fresnel diffraction?

Fraunhofer diffraction occurs when waves pass through a small opening or slit that is far away from the source, so that the waves are approximately parallel. Fresnel diffraction, on the other hand, occurs when waves pass through a small opening or slit that is close to the source, causing the waves to diverge.

## 4. How does diffraction affect the quality of images?

Diffraction can cause blurring and distortion in images, as it causes light waves to bend and interfere with each other. This can be observed in photographs or in the diffraction patterns produced by objects with fine details, such as a CD or a DVD.

## 5. What are some practical applications of diffraction?

Diffraction has many practical applications, including in the fields of optics, acoustics, and spectroscopy. It is used in technologies such as optical gratings, which are used to separate and analyze different wavelengths of light, and in ultrasound imaging, which uses sound waves and their diffraction patterns to create images of internal structures in the body.

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