This isn't really a homework, but I am trying to figure out how this would work.
I have an plano-convex lens with an focal length of 500 mm. I have an laser source that I like to focus on, to the infinity. I.e. I like to see the laser lobe.
Case: What happens to image of the lobe if the object is placed closer to the lens than the focal length?
For example 100 mm from the lens?
The curved side is towards the object.
The lens formula gives,
1/f = 1/o + 1/i
The Attempt at a Solution
f = 500 mm
o = 100 mm
i = 1/(1/500-1/100) = -125, which makes no sense.
I tested this in the lab and I have seen the lobe at infinity but when I got back and tested again I couldn´t re-produce the same result. The lobe at infinity wasn´t at the focal length of the lens. I saw the laser lobe but not at the focal point. So I am trying to figure out what I am doing wrong.
For example, if the object is 3 meters in front of the lens, then the image of that object is 600 mm behind the lens.
f = 500 mm
o = 3000 mm
i = 1/(1/500-1/3000) = 600
The lobe at infinity, for the object placed 3 meters in front of the lens, would then be at the focal length of the lens (500 mm behind the lens).
What happens if the object is placed closer than the focal length of the lens? Can i see the image with the lens or does the object distance have to be larger than the focal length of lens?
Its weird, because I know I have seen the lobe when I first did the measurement. - 125 mm should indicate the image is formed in the opposite side of the lens.. but that doesn't make sense to what I have observed.