Using Prisms to See Around Corners and Reflections - GCSE Physics P3

[Molly1235]

Hello,

I would really appreciate some help with this question as my understanding of the topic is not that great, so trying to decipher the answer from the notes in my textbook is not easy. This is Edexcel GCSE Physics P3.

"Draw and label a diagram to show how you could use prisms...
A) to see round the corner of a building
B) to see a reflection of yourself"

I know this has something to do with critical angle but I really don't understand what. The only thing I can think is that you need a reflection to change the light so it goes around the corner, but then another for it to come back for you to see it? And I have no clue for part b!

Would be appreciative of some guidance.
Thanks :-)

 

[haruspex]

Have you heard of total internal reflection? Do you understand what that looks like in terms of light rays through a prism, and how it relates to critical angle?

 

[Molly1235]

Yes, I know of total internal reflection, but can't quite connect this to critical angle and the prism in my head?

 

[haruspex]

Total internal reflection means a surface of the prism can act as a mirror, but only for light coming from within the prism. So the trick is to have the light from the source enter the prism square on, then reflect of a surface at an angle, and exit the prism square on to the third surface.
Total internal reflection will occur if the angle of incidence exceeds the critical angle (which is why it's called critical). This will depend on the medium.
Having discovered how to make a prism behave like a mirror, you can then arrange several to achieve the objectives.

 

[Nickie]

Hi there,

I understand that this topic may be confusing, so let me try to explain it to you. Prisms are optical devices that use refraction to bend light. This bending of light can be used to see objects that are not in direct line of sight, such as around corners or through reflections.

To see around a corner of a building, you can use two prisms, one on each side of the corner. The first prism will bend the light from the object around the corner, and the second prism will bend the light back towards your eyes, allowing you to see the object. The critical angle comes into play here, as the angle at which the light hits the prism must be at a certain angle in order for it to bend enough to see the object. This angle is called the critical angle and can be calculated using Snell's law.

For part B, to see a reflection of yourself, you can use a single prism. Place the prism at an angle in front of you, and the light from your reflection will be bent towards your eyes, allowing you to see yourself. This is because the angle of incidence (the angle at which the light hits the prism) is greater than the critical angle, causing the light to reflect back towards your eyes.

I hope this helps to clarify things for you. Remember, prisms use refraction to bend light, and the critical angle is the angle at which the light must hit the prism in order to be bent enough to see objects around corners or through reflections. Let me know if you have any other questions. Good luck with your studies!