Dave's ESL Cafe's Student Discussion Forums

[iloveyou]

Hello, I was reading a textbook, and got confused
between forward and backward.

the picture and text is below.

In my opinion, forward should be backward and back should be forward...
because as the light is moving from left to right, in minus lens the focal point is backward and in plus lens the focal point is forward...
seems to me so

Is it that the sense of direction is different between my country and English speaking coutries?


Picture link
http://www.howstuffworks.com/lens.htm/printable

Text
A lens can be thought of as two rounded prisms joined together. Light passing through the lens is always bent toward the thickest part of the prisms. To make a minus lens (above on the left), the thickest part, the base, of the prisms is on the outer edges and the thinnest part, the apex, is in the middle. This spreads the light away from the center of the lens and moves the focal point forward. The stronger the lens, the farther the focal point is from the lens.

To make a plus lens (above on the right), the thickest part of the lens is in the middle and the thinnest part on the outer edges. The light is bent toward the center and the focal point moves back. The stronger the lens, the closer the focal point is to the lens.

[redset]

The explanation isn't exactly clear, but it is correct. The front of the lens is the left side, where the light is coming from, and the back of the lens is on the right of the image.

With the plus lens the light is focused behind the lens (at the back), and with the minus lens the light is bent outwards, which puts the focal point at the front of the lens. It might help to think of it as the lens in your eye - the light comes in at the front, and because our eyes have plus lenses the image is focused inside at the back (on the retina).

The reason I don't like the description is that it says the focal point is 'moved back' and 'moved forward' - from where? A flat lens has no focal point, and if you make it thicker and thicker as a plus lens (which puts the focal point at the back) then the focal point moves forwards from an infinite distance, getting closer and closer to the lens. So yeah, confusing description!