Answers

1. a) The focus is the position of the image of a very distant object.
   b) The object is very far away (in fact, it’s at infinity).
   c) The focal length is the distance between the focus and the lens.
   d) See For You To Do, Steps 2 and 3.
2. a) Yes.
   b) Upside-down.
   c) Yes. Light spreads out as it moves from the object to the lens (to see this, imagine the object is a tiny light). If the light were not bent, it would continue spreading after it went through the lens, so there could not be an image.
   d) Move the screen back from the lens.
3. a) Move the object away from the lens (more than twice the focal length away).
   b) Bring the object close to the lens (closer than twice the focal length).
4. a) The image of the closer light is blurred.
   b) The image of the more distant light is blurred.
   c) Not if one object is very distant and the other is very close. But if you find a sharp image and then move the object a small distance, the image may remain sharp. That means there is a range of object distances that will produce a sharp image. (In photography, this range of object distances is called depth of field.)
5. The image is located on the film. Part or all of the lens moves in and out to change the focus. Notes: As the aperture—the opening—of the lens decreases (this corresponds to increasing the f-number), the range of sharp focus increases. For some cameras, the aperture is kept small to maintain a large range of object distances that produce a reasonably sharp image. For these cameras, the focus of the lens is fixed.