Exploring a periscope made with prisms

This is a periscope made by students with two right angled prisms in a course on Optics taught by Physics faculty, Rema Krishnaswamy.
(These kind of setups are often used in optics research to manipulate or displace a beam of light. Also look up ‘Dove prism’)

What phenomena/concepts is this setup based on? We wish to pose some questions/ideas for us to explore various Physics aspects related to this setup.

(The students have used chalk dust from a duster to make the path of the laser light more clearly visible)

Image credits: Atula, Akheel and Prajwal (Students of the BSc Physics programme)
question mark on chalk board
(Photo by Pixabay on Pexels.com)

What other ideas in Physics could we think of while exploring this setup?

(click to expand each point)

What if the refractive index of glass was not 1.5?

Will this setup of two right angled prisms work as a periscope for any medium other than glass? If we had hollow right angled prisms and we filled them with water, will this still work?

What happens if we immerse the setup in a water tank?

Does the periscope still work?

What refractive index should the material of the right angled prisms have to make a periscope in water?

Lastly, what should be the refractive index of the liquid so that these right angled glass prisms would still work as a periscope?

Could you still try to make it work in water?

You could try to change the orientations of the prism. This may not work perfectly; could you still try to make the rays travel like the ones in the photo, as far as possible? Why did this arrangement work?

What else did you observe in this experiment?

What if you had three right angled prisms? Will it be possible to make the incoming and outgoing rays to better resemble the ones in air?

What if you had equilateral prisms? We could try these things with this type of prisms.

A hollow prism

Now let us take a hollow prism made of glass sheets (One way of doing this is to use microscope slides and a glass base and stick them with clear/transparent araldite). We can fill this with water, repeat the experiments and observe what happens at the different interfaces for different conditions. Could you think about the different phenomena and concepts involved?

Further resources

A suggested teaching sequence & activities on total internal reflection
  • Some safety instructions need to be given to students while handling lasers. Even though most lasers used in labs are low power lasers, if the light enters someone’s eyes, it can cause damage. Please make sure that stray laser light is not pointed at the level of people’s faces. Perhaps it is a good practice to keep the light in a horizontal plane near about the height of the table top.
  • Preferably students should be given the following instructions one after the other, as and when they finish a given task and writing about it. They could perhaps work in pairs.
  • Take a laser and a right angled prism. Make the beam fall on different faces of the prism. Can you try to make the beam fall on the prism such that we get to see total internal reflection? What else do you notice about the prisms and the laser beam?
  • Now take two such right angled prisms and arrange them to make a periscope (although a real periscope is used vertically – like ones used in submarines, here we will set it up in a horizontal plane). Basically a light beam should emerge parallel to the incoming beam but shifted by a certain distance. This technique happens to be used in optics research labs. You can use chalk dust or something similar to make the path of the laser beam more easily visible.
  • Why did this setup work? Suppose the refractive index of glass was say, 1.2 instead of 1.5. Could we have made this kind of periscope with two right angled prisms then?
  • Now take the two prisms that make up your periscope and place them in a glass aquarium such that you can still see your periscope work like earlier.
Setup placed in an aquarium
  • Now pour water in the aquarium. Does the periscope still work? Why do you see what you see? Please also reason quantitatively with specific number. the refractive index of water (with respect to vacuum/air) is 1.33, while that of glass is 1.5
  • Now try to rearrange the prisms so that you still make it work almost like the periscope in air, i.e. the outgoing ray is almost parallel to the incoming ray. Please sketch your setup and measure the angles by which you had to tilt the two prisms.
Trying to make a periscope work in water
A hollow prism

We can use clear/transparent araldite to stick together microscope slides to make a hollow equilateral prism as well as a hollow right angled prism. Of course, another glass plate will have to be stuck at the bottom. We can now fill different liquids in this prism a perform experiments on light with these fluids.

Reflection can give us polarised light (this property is often used in photography)
An advanced example related to total internal reflection and polarisation – Fresnel rhomb

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