PR Hyperlearning: Physics Passage 56, Q2

[4563728]

Hello,
The question in the text says, "When the eye is focused on a distant object, such as a star, the image will appear:
A. On surface of lens
B. between lens and focal point
C. at the focal point
D. beyond the focal point

I though that since we are looking at a very distant object, the eye would relax (not contract so much) to increase the focal point of the eye. And since the image must focus on the retina so we can see the object, would the image converge at a point between the focal point and the retina, since the focal point will be something huge?

The correct answer was C, but I really don't see their logic if, even in the passage, it says that image must be produced on the retina of the eye.

Help!!

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[SpaceBuff12]

If the light rays are coming from a distant object then they are hitting your eye parallel to the axis of the lens which will focus the rays at the focal point.

You're focusing too much on the retina and assuming that the focal point has to be between the retina and the lens. Focusing on light rays from distant objects will put the focal point of the lens on the retina.

 

[brood910]

Or maybe they are assuming that "distant" = infinity.
The distance of star = you can say it's "infinity" because 1/VERY LARGE NUMBER ~ 0
1/f = 1/inf + 1/img
1/f = 1/i
i = f

Edit: I just checked their answer. I was right.

 

[4563728]

Their explanation makes sense mathematically, but I thought the image gets produced on the retina. How can we see the image if it is equal to the focal point (hence, not on the retina)?

 

[Cmdr_Shepard]

Their explanation makes sense mathematically, but I thought the image gets produced on the retina. How can we see the image if it is equal to the focal point (hence, not on the retina)?

As long as the object is far enough away, the image is always going to be produced at the focal point (the eye-cornea-lens system bends the light to converge on the retina). The focal point in the eye is at the retina.

 

[4563728]

I though for a lens the focal point was the same distance on both sides of the lens? So if we look at something really far away, the focal point is large, so on the real side of the lens, the focal length would be very large too. Which means the light rays would not converge at the retina.

 

[Cmdr_Shepard]

But if you look at something far away, that's not what you're supposed to define as the focal point or focal length. That's the object distance, which is approximately infinite in that case. The focal length is the same on both sides of a converging lens true, but that's not relevant to your Q.

 

[4563728]

Oh, you're right! My mistake, thank you!

 

[Cmdr_Shepard]

You're welcome.