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Re: [ATM] (no subject)
Patrick
>Another thing I'm wondering relates to finding its focal length. We
>took the mirror outside and tried focussing sunlight with it (carefully
>of course!) and while it did start paper on fire at the focus, we could
>not get the spot any smaller than about 2 cm. I was expecting it to be
>a _much_ smaller spot, give that I am assuming it to be parabolic.
The angular radius of Sun is known.
If you know the size of the smallest image than you can calculate
focal length. If image was 2cm across than f=1/tan(alfa/2)
alfa is the angular size of the sun. It is 31.9 ' (0.53 degrees)
f=2.16 m.
If it is 17.5", f/4.5 mirror than from this f=17.5*4.5= 2 meters
The image of the Sun will be 18 mm dia.
There is only small difference. Your measurement was OK.
And you had a sunny day.
Regards
Vladimir.
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