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#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?

0
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.
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#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.

Are all the equations above (in my first post -highlighted and not highlighted) for focal length?
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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.
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#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?
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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.
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#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
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