Producing heat(fire) using magnifying Glass

[Rho_Mi_la]

Hello! :D I wish to know how is Magnification related to Focal Length or Focal point in terms of magnifying lenses? We are researching how magnifications affect the heat produced by magnifying lenses, unfortunately, I found out that it's mostly the size. does this mean that the magnification really has no effect on the heat produced? or What other factors affect it?

 

[Nessdude14]

When you hold a magnifying glass in the sun and place something at its focal point, it creates a high temperature because it takes all of the radiation that hits the large area of the magnifying glass, and focuses it into a tiny point, making a high concentration of heat. You can focus light at the focal point of any magnifying glass, but the larger the area of radiation you can focus onto the point (the larger the magnifying glass), the higher the concentration of heat at the focal point.

The maximum angular magnification (MA) of a magnifying glass is given (approximately) by:
$MA=\frac{25 cm}{f}$
f is the focal length, meaning that higher magnification is achieved by a shorter focal length.
A magnifying glass with a high magnification will mean that the focal point is closer to the glass, but this does not effect the concentration of heat at the focal point.

 

[HallsofIvy]

The point is that there is NO "magnification" involved here at all. It is simply a matter of focusing all the light that passes through the glass on a single point. And how much that is depends on the area (and so radius) of the glass.

 

[sophiecentaur]

The 'magnification factor' of a convex lens relates to how close it can put a virtual image to your eye that you can still focus on. It relies on your own individual powers of accommodation. (That will be where the "25cm" in the above post comes from) This is not really relevant for a burning glass. The main factor in a burning glass is the actual area of the lens (the energy gathering potential) The temperature that you can obtain at 'the spot' will depend, among other things, on the size of spot you can produce.

If you look at the size of the Sun's image on a camera sensor, for a wide angle lens, you can see it will be a lot smaller than for a telephoto lens. The 'f' number for the aperture of a lens gives you a clue about the suitablilty for burning because any lens with a given f setting (e.g. f4) will have the same intensity of light per unit area of the sensor. Now, the f number is the ratio of focal length to aperture - so a long lens will need a wider aperture (in mm) for the same intensity BUT, of course, the Sun's image will be bigger so more total energy will hit the sensor.
Which brings us to the "among other things". If you have a highly conductive target then the maximum temperature reached will be limited by the total energy flux (lens area). But if you are trying to heat a really good insulator, the maximum local temperature reached will be limited by the f number (i.e. the intensity of the spot - whatever size it is).

So the answer must be ---- It Depends! Sorry (But a good big-un will always beat a good little-un)

 

[sardar]

Hello! That's a great question. In terms of magnifying lenses, the magnification does play a role in the heat produced, but it is not the only factor. The focal length and focal point also play a significant role in the amount of heat produced. The focal length is the distance between the lens and the focal point, where the light rays converge to form a focused image. The focal point is the point where the light rays converge and intersect.

When using a magnifying glass to produce heat, the magnification increases the intensity of the light rays passing through the lens, making the light more focused and concentrated. This, in turn, increases the temperature at the focal point, resulting in the production of heat. However, if the focal length is too short, the light rays will not converge enough to create a significant amount of heat.

Other factors that can affect the heat produced by a magnifying glass include the type of lens material and the intensity of the sunlight. A larger lens surface area will also result in a larger concentration of heat. Additionally, the angle at which the lens is held in relation to the sun's rays can also impact the heat produced.

In conclusion, while magnification does play a role in the heat produced by magnifying lenses, it is not the only factor. The focal length, focal point, lens material, and sunlight intensity all play crucial roles in the amount of heat produced. I hope this helps with your research!