What is the focal length for a 10-inch reflector?

[Surasit Jum]

What is the focal length for a 10-inch reflector? And I need a 7.8-inch reflector for my lamphouse.

[Jim Cassedy]

There isn't quite enough information in your question to give an accurate answer.
But there is a mathematical formula that can be used to determine the focal length.
Fortunately, it's not too difficult, even for someone like me who was never very good
at math. I once had to do this myself. You can find the formula online or look here:

FocalLengthCalculator.jpg


If you search a bit online, there might even be an online calculator that you
can just put your numbers in to, and it will do everything else. Good Luck

---> UPDATE/ EDIT: I found an online calculator you can just put numbers into
and it will do all the math. I tried it with the figures above, (10in diameter and .5in
depth) and the answer matches 12.5 as in the first example. Be aware that the
assignments of the "d"and "D" values are swapped from the way they are in
the first example formula.

You can find the online calculator here:
https://www.analyzemath.com/parabola...alculator.html

Here's a screenshot:
OnlineFocalCalculator.jpg