physics lens 問題

1. Why you think that the image would blurr. Image located at great distance is not necessarily blurred.

For proof of the angular magnification of an astronautical telescope, please read the following web-page:

http://www.saburchill.com/physics/chapters3/0018.html

2. Statement (1) is correct. It is because a diminished real image indicates that the object is beyond a distance of 2 x focal length of the lens, and the image is at distance between the focus and the point of 2 x focal length.

Since it is given that the focal length of the lens is 20 cm, the image must be formed at distance between 20 cm and 40 cm from the lens.

Statement (2) is correct. As said above, the object must be at distance not less than 2x20cm = 40 cm from the lens. The image is at distance longer than between20 and 40 cm from the lens. But given the distance of the object on the diagram that is not far away from the lens, the image distance should be near to 40 cm from the Hence, the separation between the object and image must be not less than (40+40) cm = 80 cm

Statement (3) is correct. This is just the principle of irreversibilty of light. The positions of object and image can be interchanged. Hence, their separation remains the same.