Isaac Physics - Parabolic Mirror

Please could I have a bit of help with Part C of this question (https://isaacphysics.org/questions/parabolic_mirror_r1?stage=all)? Maybe I’m just missing something obvious, but I’ve had no luck with it so far…
The ray diagram for a newtonian telescope with eyepiece looks like...

https://img-nm.mnimgs.com/img/study_content/content_ck_images/images/vn.png

so in order for the rays leaving the eyepiece to be parallel, the distance from the focal point of the primary mirror to the eyepiece lens is the focal length of the eyepiece lens.
Original post by Joinedup
The ray diagram for a newtonian telescope with eyepiece looks like...

https://img-nm.mnimgs.com/img/study_content/content_ck_images/images/vn.png

so in order for the rays leaving the eyepiece to be parallel, the distance from the focal point of the primary mirror to the eyepiece lens is the focal length of the eyepiece lens.
So it really was that easy! That's what I did at first, but for some reason I read 75.1 as 71.5 lol. Thank you
sillycon is this a similar approach to how you solved part b?
I am unable to solve it.
Original post by mosaurlodon
sillycon is this a similar approach to how you solved part b?
I am unable to solve it.
Yes, quite similar. From what you have there, you can obtain an expression for tan(2*theta). Equate this with your expression from Part A, using appropriate values for x and y. You will need to solve for a and equate. Hope this helps! If you need further assistance, I am happy to help.
(edited 1 month ago)
Thank you! I've managed to get the answer now .
Actually please could I have a little more help with part c?
I must be drawing my diagram wrong because I am a little confused on how the distance from the lens to the focal point could be 1.4cm - could you please point out the flaw:
(edited 1 month ago)
Original post by mosaurlodon
Actually please could I have a little more help with part c?
I must be drawing my diagram wrong because I am a little confused on how the distance from the lens to the focal point could be 1.4cm - could you please point out the flaw:
Take a look at the diagram Joinedup posted - the focal length of the lens is the distance from the lens to where the rays cross
I'll be honest I still am a little confused.

I understand that the distance between the lens and focal point is 'f' so I just need to work out 'x'.

So I thought the only way of working out 'x' would be to use similar triangles since I had no other idea of how to work out 'x'.
EDIT: in the diagram it should be 75.1 - but its still the wrong answer.
(edited 1 month ago)
Original post by mosaurlodon
I'll be honest I still am a little confused.
I understand that the distance between the lens and focal point is 'f' so I just need to work out 'x'.

So I thought the only way of working out 'x' would be to use similar triangles since I had no other idea of how to work out 'x'.
EDIT: in the diagram it should be 75.1 - but its still the wrong answer.
Okay, maybe think about how the light being reflected in the plane mirror would change the distance the light travels from the parabolic mirror before it converges. Would it change at all?
Well, if the light were reflected closer to the top of the plane mirror then it would travel a shorter distance before converging, but it would have travelled a longer distance to reach the plane mirror - so is the distance travelled the same?
ie red line length = orange line length?
Original post by mosaurlodon
Well, if the light were reflected closer to the top of the plane mirror then it would travel a shorter distance before converging, but it would have travelled a longer distance to reach the plane mirror - so is the distance travelled the same?
ie red line length = orange line length?
Not quite what I was getting at... imagine the situation without the plane mirror. How would the total distance a ray travels before it converges (i.e. at the focal point) differ from the situation with the plane mirror, if at all?
Oh I understand what you're hinting at now! Thanks so much! - Im sorry for wasting your time - I really overcomplicated this.
Original post by mosaurlodon
Oh I understand what you're hinting at now! Thanks so much! - Im sorry for wasting your time - I really overcomplicated this.
You're welcome . And please don't be sorry! This wasn't a waste of my time at all