Have a large convex mirror with variable focus in the geo-sync orbit, beaming down a burning point on the surface of the Earth, like a magnifying glass.

There is not mass loss firing the weapon (which may just be a sheild than moved out of the way), it's just light.

Crazy idea, but comment anyway.

I got the idea from watching an old Gummy Bears' episode on the Disney channel.  

Have a large convex mirror with variable focus in the geo-sync orbit, beaming down a burning point on the surface of the Earth, like a magnifying glass.

Where would the energy come from? Solar radiation or would the mirror be reflecting a ground-based laser?

If it's solar radiation, I don't think it would be feasable. Because of the Earth's atmosphere, you would need to collect an enormus amount of solar energy to destroy something thousands of miles away. Such a weapon would have to be in a very high orbit or else it would have limited military value, a low-fast orbit would pass over a conflict zone rapidly giving short windows of opportunity.

Ditto for ground based lasers, going through the atmosphere twice waste a good deal of the energy.

There's a real possibility that the F-35 could be equippet with a direct energy (i.e. laser) weapon at some point in the future. Ratheon is actively presuing a steady-state laser with the power necessary to destroy a lightly armored target that is compact enough to fit inside the F-35 lift-fan bay. A production version could be fielded by 2010-ish...

The JSF laser is a solid state laser that will be able to fire a distance of ~6 miles. The current prototype has a power of 13 Kw with the final production model having a power output of 100Kw. The Laser will be able to fire two four second bursts four seconds apart and then have to cool for roughly 30 seconds before it is able to fire again.
The penetration power of a 100Kw beam is also not great, certainly it is conceived as a weapon against very soft targets and as a defensive measure.
I believe this is one of the reasons that the USAF is considering a F-35B purchase as it will be easily modified to allow for the laser attachment, since it will have the shaft connected straight to the engine. A 100Kw laser will require at least 1 megawatt of input power and will require serious cooling, so much that the current plan is to pipe the heat through the fuel tank considering the amount of fuel carried on aircraft. This should also only raise the temperature of the fuel tank by a degree or two and enable the stealth to be maintained.
An excellent idea although I am a bit sceptical that they will reach the expected power output level of 100kw anytime before 2015.

As far as a space weapon system is concerned, DfwRevolution is right, sunlight alone is simply not strong enough the penetrate the atmosphere due to refraction. This is why Lasers are tried instead as all the photons in the beam are oriented in the same direction or coherent, the beam having very little divergence over the length of the path, something like 1mm per meter traveled for typical laser designators.
There is a Space based laser concept that continues to attract funding by the US, a large weapon with a power of 5-10 megawatts and a range of 4000K outside atmosphere but would only be able to penetrate a distance to about 3000 meters above sea level before dispersal and interference negated it's effect. It would also weigh about 35 tons (three time Hubble) and hence cost a fortune is place in orbit, let alone the over a billion in construction costs each and require at least twelve to give any sort of coverage.
It really comes down to cost and until a viable interorbit vehicle that can bring costs down is created we will never see very large mirrors or lasers in orbit.

The prototype is expected to be up by 2010, and they say now that they do not expect to be in production before 2015. Which of course means we probably wont see any of this prior to 2020.

http://www.newscientist.com/article.ns?id=dn2585

I guess I meant solar radiation, and yes it could have require an array of mirrors the size of a city block or more -- forget practicality and tell me if it could work. That way someone can make it practical in the future if it was necessary. What's the use of dismissing an idea because our current technology cannot do it? How does one technologically progress that way?

Sorry, I just don't believe in impractical or impossible.  

This is very quick 'cos I've got to go to work... (Work Is The Curse Of The Drinking Classes...)

The ancient Greeks came up with this idea (ground-based, not space-based, obviously) for setting light to attacking ships. An engineering analysis of the plans indicate that it would work.

When I get back, I'll try and dig out the article.

Good point Bsergonomix, if the Greeks thought about it, it can be done, by Toutatis.

Seriously, what would be the efficiency of a system that would be in geosynchronuous orbit (35,850-36,000km), and thereby would only see the sun a max of 12 hours a day ?
Add to this the angles at which it would not work and you might end up with a weapon that can only work (a guess) 6 hours per day, and only in clear atmospheric conditions.

It is like restricting the sheriff's working hours from 2 to 8pm on clear days only.

Wouldn't a hot stream of solar radiation burn through even storm ridden clouds? It should evaporate instantly, i don't know what distance i am thinking of...

I guess I was thinking not of geo-sync, but solar-sync. To orbit the earth such that the reflectors are always in the sun per 24 hrs, granted the world would spin below and I suppose tiny guidance rockets or ratateing mirror surfaces can wave the beam to match the ground motion.

If the reflection area was great enough, it could create enough focused energy to maintain an appropriate burn to the ground..maybe? Unless you wanna bounce it back and forth from a series of land mirrors and create a massive laser effect...

Okay that was kindz nuts.