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

[Dipra Irham]

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?

 

[berkeman]

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.

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

 

[Andy Resnick]

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?

When used in imaging applications, prisms are generally oriented so that total internal reflection occurs- see, for example:

https://www.edmundoptics.com/resources/application-notes/optics/optical-prism-application-examples/

 

[sophiecentaur]

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.

 

[davenn]

When used in imaging applications, prisms are generally oriented so that total internal reflection occurs- see, for example:

https://www.edmundoptics.com/resources/application-notes/optics/optical-prism-application-examples/

cool, just learned something as well

thanks, Andy

 

[Drakkith]

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.

 

[sophiecentaur]

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.