Why Does a Prism Periscope Focus Light Instead of Scattering It?

In summary, a prism is used in optics to scatter light rays and separate it into rainbow, but in a prism periscope the prism send light rays to a particular direction instead of scattering. This difference in usage is what is explained in the summary.
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Dipra Irham
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a prism is supposed to scatter light rays and separate it into rainbow,then why in a prism periscope the prism send light rays to a particular direction instead of scattering?
 
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  • #2
Dipra Irham said:
a prism is supposed to scatter light rays and separate it into rainbow,then why in a prism periscope the prism send light rays to a particular direction instead of scattering?
Welcome to the PF. :smile:

What is the difference between how a prism is used in optics like a periscope or image erector, versus how you use the prism to form a rainbow. Hint -- do a Google Images search to see how the prism is used in a periscope, and compare it to this image:

https://www.researchgate.net/profile/Mikael_Sundberg2/publication/277798811/figure/fig10/AS:294333388476423@1447185956597/White-light-incident-on-a-triangular-prism-left-side-disperses-and-creates-a-rainbow.png

White-light-incident-on-a-triangular-prism-left-side-disperses-and-creates-a-rainbow.png
 

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  • #3
  • #4
Dipra Irham said:
a prism is supposed to scatter light rays and separate it into rainbow,then why in a prism periscope the prism send light rays to a particular direction instead of scattering?
Prisms can be used in more than one way and their angles can be accordingly different.
You will have seen a diagram of this use of a prism. You will notice that light enters and leaves the prism along a Normal. So there is no dispersion there. The internal reflection is arranged to be at an angle, well different from the Critical Angle so all wavelengths are totally reflected.
 
  • #6
sophiecentaur said:
Prisms can be used in more than one way and their angles can be accordingly different.
You will have seen a diagram of this use of a prism. You will notice that light enters and leaves the prism along a Normal. So there is no dispersion there.

For most prisms in optical systems, the majority of the light doesn't enter at the normal angle. However, the fact that the angle to the normal is generally small and that you're not trying to focus the light with the prism keeps the chromatic aberration low. I believe the dispersion generated by the first surface is counteracted by the exiting surface, perhaps with a small lateral (side-to-side/up-and-down) offset of the image in the different colors. But I admit I haven't worked with prisms in well over a year, and even then I don't think we investigated the chromatic aberration of a prism, so don't take my word as the law on this.
 
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  • #7
Drakkith said:
For most prisms in optical systems, the majority of the light doesn't enter at the normal angle. However, the fact that the angle to the normal is generally small and that you're not trying to focus the light with the prism keeps the chromatic aberration low. I believe the dispersion generated by the first surface is counteracted by the exiting surface, perhaps with a small lateral (side-to-side/up-and-down) offset of the image in the different colors. But I admit I haven't worked with prisms in well over a year, and even then I don't think we investigated the chromatic aberration of a prism, so don't take my word as the law on this.
I agree with that 'second level' of prism knowledge. CA is always a problem in optics but the symmetry of the way prisms tend to be used must help a lot.
The prism is a bit of a poor relative of the lens and people tend to dismiss its problems like I did in my first post. There are several advantages over lenses, They are used to stretch the optics and allow a long focus objective in binoculars so the angle range is small. They are planar so, as you say, the dispersion at one end will be 'ideally' compensated for at the other end of a pair. I have not read about achromatic pairs of glass being used in prisms, as with lenses - perhaps because there is no curvature.
The "100%" reflection is a massive point in favour of using prisms as reflectors in binoculars (at least four reflections are involved) but I notice that dielectric reflectors are very popular for Star Diagonal Reflectors which can be placed on the axis of an astronomical telescope to allow the image to be viewed from the side. Only one prism and one face involved so perhaps CA could be relevant here.
Edit PS, when prisms are used in periscopes (especially long ones) the angular range of the light is very small so they would be the reflector of choice.
 
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1. Why does a prism periscope focus light instead of scattering it?

A prism periscope focuses light by using the principle of refraction. When light passes through the angled surfaces of the prism, it bends or refracts, causing the light rays to converge at a single point, creating a focused image.

2. How does a prism periscope differ from a regular periscope?

A prism periscope uses a prism to bend and focus light, while a regular periscope uses a series of mirrors to reflect light. This difference allows a prism periscope to produce a clearer and more focused image compared to a regular periscope.

3. Can a prism periscope be used to see objects that are farther away?

No, a prism periscope does not magnify objects or make them appear closer. It simply bends and focuses the light that is already present, so the distance of the object being viewed remains the same.

4. Are there different types of prisms used in prism periscopes?

Yes, there are different types of prisms that can be used in prism periscopes, such as equilateral prisms, right-angle prisms, and dove prisms. Each type of prism has a specific shape and angle that affects the way light is refracted and focused.

5. Can a prism periscope be used in low light conditions?

Yes, a prism periscope can be used in low light conditions, but the image produced may not be as clear or bright compared to using it in well-lit environments. This is because the amount of light that enters the periscope affects the quality of the focused image.

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