# Purpose of knife-edge in Schlieren (optics)

• Replusz
In summary, the knife-edge in Schlieren optics is used to block the boring light that passes through places with small density variations, allowing for the enhancement of the interesting light that has been perturbed by these variations. It is a more practical alternative to using a small absorbing dot at the focus.
Replusz
I wanted to know what the purpose of the knife-edge in Schlieren optics is, but can't think of a suitable answer.
Is it only to reduce the intensity of light equally?
Thanks!

Replusz said:
Is it only to reduce the intensity of light equally?
Not equally but selectively.

Replusz
Replusz said:
I wanted to know what the purpose of the knife-edge in Schlieren optics is, but can't think of a suitable answer.
Is it only to reduce the intensity of light equally?
Thanks!

Most of the light will pass through places where the density variations are small and will focus to a tiny diffraction limited spot at the focus. You are not interested in this light. It is boring. The light that passed through interesting density variations is perturbed. It no longer has a nice flat wave front and it won't make a tight little spot at the focus. You can greatly enhance the contribution of the interesting light if you block the boring light. What you need is a little absorbing dot about the size of the diffraction limited spot placed at the focus. However that is hard to make and hard to place in three dimensions. Instead, for the cost of about half of the interesting light you can much more easily block the boring light by inserting a knife edge at the focus. It only has to be placed correctly in two dimensions, and one of those, the focus dimension, is fairly forgiving.

Replusz
Yes, your answers are logical, well-thought and I think correct. Thank you very much, guys!
Replusz

## 1. What is the purpose of the knife-edge in Schlieren (optics)?

The knife-edge in Schlieren (optics) is used to block out any direct light from entering the system, allowing only refracted light to pass through. This creates a contrast between regions of different densities, making it possible to visualize changes in density within a transparent medium.

## 2. How does the knife-edge in Schlieren (optics) work?

The knife-edge works by creating a sharp shadow on the viewing screen, which is caused by the refracted light passing through regions of different densities. This shadow is then captured by a camera or observed by the human eye, allowing for visualization of density changes.

## 3. What are the advantages of using a knife-edge in Schlieren (optics)?

One advantage of using a knife-edge in Schlieren (optics) is that it is a simple and inexpensive method for visualizing density changes in a transparent medium. It also allows for the detection of small changes in density and can be used to visualize both static and dynamic processes.

## 4. Are there any limitations to using a knife-edge in Schlieren (optics)?

One limitation of using a knife-edge in Schlieren (optics) is that it only works with transparent media. It also requires precise alignment and careful control of the light source and background to produce accurate results. Additionally, it is not suitable for visualizing changes in density within opaque materials.

## 5. What are some practical applications of the knife-edge technique in Schlieren (optics)?

The knife-edge technique in Schlieren (optics) has a variety of applications in different fields. It is commonly used in aerodynamics to visualize air flow around objects, in combustion research to study flame propagation, and in biomedical imaging to visualize changes in refractive index within tissues. It can also be used in quality control and non-destructive testing in industries such as manufacturing and materials science.

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