Regarding the proposed duplicate:
Why does the image lose its clarity when it is formed in front of the retina?
Does the above question answer mine? Short answer: No
Thank you for sharing that question. The only answer there was informative. However, I find my question to be an extension of that answer. I understood that we observe blurred images as point sources do not focus at a single point on the screen. But I don't understand how real image is formed when light rays diverge from a point in front of the screen as explained in this question using the second diagram. Here, I'm looking for an answer with rigorous explanation of physics behind blurred images.
In our practical session, in order to determine the approximate focal length of a converging lens we were asked to focus a distant object on a screen and measure the distance between the lens and the screen. We had to adjust the distance of the screen from the lens until we obtained a sharp image. When the screen was not at the focal plane we obtained a blurred image. This is how we had to find the image location for all other positions of the object.
I don't understand "Why do we see a blurred image when the screen is not placed at the position where the image is formed by a convex lens?". I tried to reason my observations using the following information.
I've learnt that a real image forms when the refracted light rays actually converge or appear to converge behind the screen, and a virtual image is formed when refracted light rays actually diverge or appear to diverge from a point in front of the screen. I've also verified this fact using ray diagrams and from experiments.
Let us consider the following diagram:
The red dotted lines represent different positions of the screen at different points of time. When the screen is placed at A, real image is formed on its surface as the rays appear to converge behind the screen. As the rays from one point on the object fall on the screen at different points, we get a blurred image. When the screen is at B, we obtain a sharp image as all rays from a particular point on the object converge into a single point at the screen's surface.
But, I was unable to explain why I get a blurred image on a screen when it's placed at C. The real image is formed in front of the screen. The real image formed can be considered to be a real object which emits light in only certain direction away from the lens. Having said that, it's obvious we couldn't obtain a real image on a screen placed at C. This is further supported by the ray diagram itself.
Let us consider the following diagram which consists of only the image and screen C from the previous one:
It can be seen that the rays emitted from a particular point on the object do not meet at all. They diverge from that point on the object and for the screen, it would finally result in a virtual image. And since, virtual image couldn't be obtained on a screen, I don't think we could see even a blurred image. But this contradicts the observations. And so, my reasoning is incorrect. I don't find any inconsistencies with my reasoning. No real image would be formed on a screen if an object if placed in front of it without any optical devices between.
In short, it would be helpful if you could explain why we observe blurred real images on the screen when the screen is not placed where image is formed.
Image Courtesy: My own work :)