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# Help - maths watch

1. Two plane mirrors are placed perpendicular to each other
A light ray with an angle of incidence of 60 degrees reflects off both mirrors.

1. Measure the angle A between the final reflected ray and the incident ray.

2. Show angle A is always equal to 180 degrees whatever the angle of incidence is. (Hint: angles of a triangle always add up to 180).

I understand that the final reflected ray is 120 degrees but I don't get how to find A or how to prove it.
2. have you drawn a sketch?
3. (Original post by BTAnonymous)
have you drawn a sketch?
Yes, I know 120+60 is 180 but I don't get how to prove it
4. (Original post by hp4300)
Yes, I know 120+60 is 180 but I don't get how to prove it
say that the angle is X this time, and show no matter what value X is, A must always be 180 degrees. have a go
5. 180-60 = 120
6. (Original post by BTAnonymous)
say that the angle is X this time, and show no matter what value X is, A must always be 180 degrees. have a go

I've drawn the diagram like this, where would A be?

Is the diagram correct
7. (Original post by BTAnonymous)
say that the angle is X this time, and show no matter what value X is, A must always be 180 degrees. have a go

I'm a bit confused, I got angle A as 120 degrees (60 add 60 is 120)
8. (Original post by hp4300)

I've drawn the diagram like this, where would A be?

Is the diagram correct
yeah, a is the whole angle between the red and blue line so two times theta.
9. (Original post by BTAnonymous)
yeah, a is the whole angle between the red and blue line so two times theta.
Then A would be 120 degrees and not 180
10. (Original post by hp4300)

I've drawn the diagram like this, where would A be?

Is the diagram correct
wait no it's says final reflected ray. the ray will reflect again on mirror two right
11. (Original post by BTAnonymous)
wait no it's says final reflected ray. the ray will reflect again on mirror two right
It would be another line parallel to the blue one on mirror 2? Where would A be then?
12. (Original post by hp4300)
It would be another line parallel to the blue one on mirror 2? Where would A be then?
I don't know. I'm confused. the question implies angle A is a straight line as it is 180 degrees? yet i get angle A as 120? sorry, I'm either not functioning today or the question is wrong.
13. (Original post by BTAnonymous)
I don't know. I'm confused. the question implies angle A is a straight line as it is 180 degrees? yet i get angle A as 120? sorry, I'm either not functioning today or the question is wrong.
Thanks for your help, I'll try and figure it out later.
14. (Original post by hp4300)
Thanks for your help, I'll try and figure it out later.
please let me know when you do; it is frustrating me lol.

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