Kalabamboo
Badges: 14
Rep:
?
#1
Report Thread starter 1 year ago
#1
Just for the bits I have highlighted:

so the focal length is the radius divided by 2 for a convex mirror but what about a concave mirror? Why does that formula f=r/2 only apply to a convex mirror?

the next bit where it talks about the focal length of a thin lens:
what does r1 and r2 represent? Why are there 2 radii? Surely, a sphere can only have one radius?

Please help me - thank you in advance to those who help!

Name:  4.PNG
Views: 100
Size:  423.7 KB
0
reply
username3442196
Badges: 8
Rep:
?
#2
Report 1 year ago
#2
I think r = 2 f applies to convex and concave mirrors, it comes from angle i = angle r in reflection.
Lenses can have differering radii for each surface - for example a plano - convex lens has r = infinity on the flat side, and most spectacle lenses have convex on one side, concave on the other.
0
reply
Kalabamboo
Badges: 14
Rep:
?
#3
Report Thread starter 1 year ago
#3
(Original post by old_teach)
I think r = 2 f applies to convex and concave mirrors, it comes from angle i = angle r in reflection.
Lenses can have differering radii for each surface - for example a plano - convex lens has r = infinity on the flat side, and most spectacle lenses have convex on one side, concave on the other.
Hi! Thanks for replying.

Are all the equations above (in my first post -highlighted and not highlighted) for focal length?
0
reply
username3442196
Badges: 8
Rep:
?
#4
Report 1 year ago
#4
Only the ones with f in!
Are you thinking of the bottom two? That's saying for converging lenses, (1/r+1/r) works out positive (not obvious if one surface is convex, so r is positive, and one is concave so that r is negative) putting that into your highlighted equation gets a positive value for f, meaning overall a converging lens, and vice versa -> negative which would be a diverging lens.
0
reply
Kalabamboo
Badges: 14
Rep:
?
#5
Report Thread starter 1 year ago
#5
(Original post by old_teach)
Only the ones with f in!
Are you thinking of the bottom two? That's saying for converging lenses, (1/r+1/r) works out positive (not obvious if one surface is convex, so r is positive, and one is concave so that r is negative) putting that into your highlighted equation gets a positive value for f, meaning overall a converging lens, and vice versa -> negative which would be a diverging lens.
Thank you And sorry one more thing:

the angle of refraction is in the air/vaccum for the bit they talk about n2 =1?
Name:  5.PNG
Views: 19
Size:  55.9 KB
0
reply
username3442196
Badges: 8
Rep:
?
#6
Report 1 year ago
#6
The general equation is sin(angle1)/sin(angle2) = n2 / n1 (better expressed as n x sin(angle) = constant)
The equation at end is a special case, where light is going from (optically) dense medium to less dense, and the angle in the less dense is 90 degrees. Then the angle in the dense = 'critical angle' (they call limit angle). If the less dense medium is a vacuum (~air) then n2 = 1 which gives their equation.
0
reply
Kalabamboo
Badges: 14
Rep:
?
#7
Report Thread starter 1 year ago
#7
(Original post by old_teach)
The general equation is sin(angle1)/sin(angle2) = n2 / n1 (better expressed as n x sin(angle) = constant)
The equation at end is a special case, where light is going from (optically) dense medium to less dense, and the angle in the less dense is 90 degrees. Then the angle in the dense = 'critical angle' (they call limit angle). If the less dense medium is a vacuum (~air) then n2 = 1 which gives their equation.
Thank you so much for helping me with my question
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

Have you experienced financial difficulties as a student due to Covid-19?

Yes, I have really struggled financially (53)
17.55%
I have experienced some financial difficulties (84)
27.81%
I haven't experienced any financial difficulties and things have stayed the same (115)
38.08%
I have had better financial opportunities as a result of the pandemic (40)
13.25%
I've had another experience (let us know in the thread!) (10)
3.31%

Watched Threads

View All