Tuesday, 27 November 2007 - 4:26 PM EST

The converging lens has a shorter focal length because when the light from it passes through the diverging lens the normal lines upon exiting force the light to spread apart, meaning they meet at a point farther away then before.

Tuesday, 27 November 2007 - 7:58 PM EST

If the converging lens had a focal length that was longer than the diverging lens, the light would never focus.  A diverging lens naturally causes the light to spread out, so the purpose of putting a converging lens in front is so that the light will be bent enough after passing through the lens that it will be able to focus after passing through the diverging lens.  If the focal length of the converging lens is longer than that of the diverging lens, the light will not be "bent" sufficiently enough to ever focus after passing through the second lens and thus it would be impossible to do this experiment without a focal point for the light.

Tuesday, 27 November 2007 - 9:25 PM EST

The diverging lens has a longer focal point because by definition it spreads light rays out. Also, it has to do with sign conventions. As the image point for the first lens is behind the second lens, the 16 cm object distance is negative and the image distance is positive 28.5. However, when you're dealing with reciprocals, the smaller the number in the denominator, the larger the number overall. Therefore...the diverging lens is guaranteed to have a longer focal distance by simple mathematics.

Gah...I hate blogs cuz if you're wrong not only do you get a bad grade but everyone else reads your mistakes and stuff. 

Tuesday, 27 November 2007 - 11:59 PM EST

I agree with Matt, if the the converging lense had a farther focal point then the diverging lense, then the diverging lense wouldnt even have a focal point therefore producing an out of focus image. This is the case because the rays would be spread out. Thus image would defeat the purpose of having an image in focus.

Wednesday, 28 November 2007 - 12:31 AM EST

When you think about it does make sense. To understand why the focal length of the converging lens has to be shorter we should take a look at how each of the light beams would bend if they were separated before we can understand how they would interact when together. When light goes through a converging lens the beams of light will be focused on one point and on a diverging lens it will focus the beams outward. In this example the converging lens is before the diverging lens. So when the light goes through the converging lens it will focus it a point but, when it comes into contact with the diverging lens the light beams will start to bend out and focus at a point that is further away. That is what will happen if the focal length is shorter then the diverging lens but if the converging lens’s focal point is longer then the diverging lens’s focal point. Then the lens will not be able to focus the light waves because the diverging lens will cause the light beams to the point that they will never focus.

Wednesday, 28 November 2007 - 7:17 AM EST

We know that a diverging lens causes light rays passing through it to spread out, so when one places the converging lens in front the light rays will bend because of that lens so that the image will be able to focus.  If the converging lens had a longer focal length than the diverging one, the light would not bend enough to focus the image after the light rays pass thorugh the second lens and one would not be able to accomplish this experiment without a focal point for the light.  Therefore, when you have the converging lens with a longer focal lenght than the diverging lens, the light rays wouldn't be able to focus.

Wednesday, 28 November 2007 - 8:21 AM EST

As most everyone already explained, the converging lens has to have a shorter focal length, otherwise the image would never focus.  This is because a diverging lens naturally causes light rays to spread out after they pass through the lens.  Therefore, if the focal point of the converging lens were beyond the diverging lens, then the image would never be able to focus.

Wednesday, 28 November 2007 - 9:16 PM EST

If i'm interpreting this right, a longer focal length makes the "bend" of the light less severe, so for the light to actually focus after passing through the two lenses, the light must bend inwards after passing through the converging lens more than it is bent outwards by the diverging lens. Therefore, the focal length of the converging lens must be smaller than that of the diverging lens.