You are lost in the woods with a friend. He's nearsighted and you're farsighted. You decide to start a fire to attract attention. Thankfully, it's a bright sunny day and you're both wearing your glasses.
Whose glasses do you use to focus the sun's light to start the fire?
To focus the sun's light and start a fire, you should use your friend's nearsighted glasses. The reason for this is that nearsighted glasses have concave lenses, which spread out the incoming light rays. When you hold the glasses between the sunlight and a flammable object (such as dry leaves or a piece of paper), the spread-out light rays will converge and form a focused spot. This spot can generate enough heat to ignite the flammable object and start a fire.
On the other hand, your farsighted glasses have convex lenses, which converge the light rays. While these lenses are helpful for correcting your vision, they are not suitable for starting a fire because they will not spread out the light rays enough to create a focused spot.
Therefore, in this situation, you would use your friend's nearsighted glasses to focus the sun's light and start a fire.