Destroying a Building Using a Lens

In summary, the question asked for the radius of curvature needed to focus the sun's rays onto the student commons using a giant burning lens. The equation used was (1/s) + (1/s') = 1/f = (n-1)[(1/R1) - (1/R2)], and after some calculations, R2 was found to be 30 m. However, this may not be the correct equation for a plano-convex lens and further research is required.
  • #1
ab200
13
3
Homework Statement
I want to mount a giant burning lens on the new engineering building and melt down the student commons. The lens will sit 60 m away from the commons (laterally) and tower 40 m above its roof. I can only afford to grind one side of the giant lens to be spherical, so the other side will have to be flat. The glass will have an index of 1.5. What radius of curvature in m is needed to focus the suns rays onto the commons?
Relevant Equations
(1/s) + (1/s') = 1/f = (n-1)[(1/R1) - (1/R2)]
(n1/s) + (n2/s') = (n2 - n1)/R
Putting the questionable nature of my professor's problem-writing aside, I found this question pretty confusing. I assumed that 60 m was the focal length, and that R1 was infinity since it is a flat surface. This gives me:
1/60 = (1.5 - 1)[(1/∞) - (1/R2)]
This gave me R2 = 30 m, but that isn't correct.
 
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  • #2
ab200 said:
Homework Statement: I want to mount a giant burning lens on the new engineering building and melt down the student commons. The lens will sit 60 m away from the commons (laterally) and tower 40 m above its roof. I can only afford to grind one side of the giant lens to be spherical, so the other side will have to be flat. The glass will have an index of 1.5. What radius of curvature in m is needed to focus the suns rays onto the commons?
Relevant Equations: (1/s) + (1/s') = 1/f = (n-1)[(1/R1) - (1/R2)]
(n1/s) + (n2/s') = (n2 - n1)/R

Putting the questionable nature of my professor's problem-writing aside, I found this question pretty confusing. I assumed that 60 m was the focal length, and that R1 was infinity since it is a flat surface. This gives me:
1/60 = (1.5 - 1)[(1/∞) - (1/R2)]
This gave me R2 = 30 m, but that isn't correct.
What is distance?
 
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  • #3
@hutchphd's queston is well worth considering but there is an additional issue with your answer.
ab200 said:
1/60 = (1.5 - 1)[(1/∞) - (1/R2)]
In terms of symbols this becomes

1/s=-(n-1)/R

Is this the correct equation for a plano-convex lens? Look it up.
 
  • #4
hutchphd said:
What is distance?
I got it! Thanks for the tip.
 
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  • #5
FWIW, I've read of fires started by sunlight focussed onto adjacent, readily flammable material by the curve of a vase or clear bottle, even the dished base of a drinks can. IIRC, the latter, 'merely' thoughtlessly tossed roadside trash, was determined to be the surprising source of a massive 'bush-fire'...

Let's not forget architectural howlers such as the US and UK skyscrapers whose curved glazed facades did their best to 'Death_Star Zap' their neighbourhood until re-fitted with louvred shades...
 
  • #6
Nik_2213 said:
I've read of fires started by sunlight focussed
Please link to a credible source. You've been here long enough to know that "I've read somewhere" is not a valid source for PF. Thanks.
 
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  • #8
This poor MacGregor owner almost had his boat burn to the pavement from a warped mirror in the head*!



* bathroom, for you landlubbers
 
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  • #9
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FAQ: Destroying a Building Using a Lens

1. How can a lens be used to destroy a building?

A lens can focus sunlight into a small, intense beam that generates enough heat to ignite materials. By concentrating solar energy, the lens can potentially create temperatures high enough to cause structural damage or start fires, which can lead to the destruction of a building.

2. What type of lens is required for such a task?

A large, high-quality Fresnel lens or a parabolic mirror is typically required to focus sunlight to the extent necessary for causing significant damage. These lenses can concentrate sunlight into a very small focal point, generating extreme heat.

3. Is it feasible to destroy a building using only a lens and sunlight?

While it is theoretically possible to cause damage using a lens and concentrated sunlight, practical limitations such as weather conditions, the size of the lens, and the materials of the building make it highly impractical and inefficient for completely destroying a building.

4. What are the safety concerns associated with using a lens to destroy a building?

Using a lens to focus sunlight poses significant safety risks, including accidental fires, eye injuries from intense light, and uncontrolled spread of fire. It is critical to have proper safety measures and controls in place to prevent unintended consequences.

5. Has this method been successfully demonstrated in real-life scenarios?

There are no documented cases of a building being completely destroyed using only a lens and sunlight in real-life scenarios. Most demonstrations of this method are limited to small-scale experiments and controlled environments, primarily for educational or experimental purposes.

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