Now this is HOT. A high quality Fresnel lens melts brick.
“Continue reading” for details of this powerful “magnifying glass”.
It measures 31 x 21 inches (48 x 75 cm) with a tight spot size of 0.3 inches. Focal length is 31 inches.
I made up a simple frame with a magnetic slider to allow smooth adjustment.
I then aligned it with the sun and put a brick in the focal point. The spot of concentrated light is extremely bright as it concentrates sunlight 3000 times. I was really surprised to find a melted area on the brick. A few more experiments then some photos were taken.
I loaded the photos, and published to my social network of choice (Google+).
I posted about the general construction and the melting brick.
This is what I love about backyard science. Build something in a morning, post it to the web that afternoon. GoSub Daily Day = Day + 1 Return
Lens was from eBay greenpowerscience (USA) and cost $300 landed in Australia.
Above: The path of the converging beam of light can be outlined with a fog generator.
Above: This is brick shows the red hot area from sunlight concentrated through the Fresnel lens. The linear melt zone is due to the sun’s movement.
Above: This is molten brick bubbling which is probably outgassing rather than vaporising. To take this extremely bright photo I used a neutral density filter lens attachment which goes up to 400:1. To observe it personally, I use arc welder goggles.
Above: I did try melting sand but the results were disappointing. Some of the sand is melted and some is sintered.
Google+ melting some sand
Above: These melting/fracturing marks developed on a glass hemispherical lens while concentrating sunlight from my Fresnel lens. It was close but not touching some wood. Possibly some soot on the outlet side ended up on the glass causing energy absorption and local heating and fracturing of the glass.
The Fresnel lens is capable of melting brick so could come close to local melting of the glass. The Fresnel lens area here is 0.3 m^2 and potentially concentrates up to 300 W of which 52% will be infrared. At this wavelength which glass may not be as transparent as at optical wavelengths.
Glass fracturing is a risk at high powers. I recall this happening to a Nd:YAG slab laser producing 100 W at an Australian uni physics dept costing many tens of thousands of dollars.
Google+ Damaged hemispherical lens
Above: The Fresnel beam cuts through a can easily.
Above: Moonlight, above, was magnified with a Fresnel lens to see if it would turn a Crookes radiometer but it didn’t. In sunlight (100,000 lux = around 1 kW/m2) this set up would put 300 W in to the Radiometer and melt it within moments. Moonlight at 0.25 lux will however result in only 0.75 mW. As the laser experiments above show, it takes about 20 mW to make it run. Hence I would need 30 times more power to make it work.
Related pages
Try something else
External links
Fresnel lens – Wikipedia
Construction Google+ (mine)
Melting brick Google+ (mine)
Melting some sand Google+ (mine)
Damaged hemispherical lens Google+ (mine)
Greenpowerscience (USA)
Photo Date: Nov 21, 2011