Question

1.Think about what "focusing" a camera really means, how far away is the lens from the detector in a camera when the camera is focused on a distant mountain? On a friend close by? How does your eye accomplish this?

2. How far would you have to place the screen to see an image of the target placed at the focal point of a converging lens?​

Answer 1

Q1. to capture an object, it should be projected to the photo sensitive material in the camera.

the lens in the camera does the image creation of the object which is being captured.

object distance , image distance and focal length of a lens are related by:

(1/image distance)-(1/object distance)=1/focal length

so when we "focus" the camera, we change the distance between the lens and the sensor so that depending upon the object, the image is captured correctly by the sensor.

when object distance is high, for a given focal length, image distance will nearly equal to focal length.

on a friend close by, it will be very close to the lens.

the eye accomplishes this by the curve of the cornea of the eye.

Q2.

let focal length =f

object distance u=-f (using gaussian sign convention)

using lens equation, if image distance is v,

then (1/v)-(1/u)=1/f

==>(1/v)+(1/f)=1/f

==>1/v=0

==>v=infinity

otherwise, as we also know, when object is placed at the focus of a conveging lens, image is produced at the infinity

hence the screen need to be placed theoretically at infinity.