In summary
Images formed by lenses has different features based on where the object is positioned with respect to the lens.
There are different region along the principal axis where object can be positioned and each region will determine kind of image obtained.
The regions considered includes:
- Beyond Center of curvature C
- Object exactly at C
- Object between C and F
- Object exactly at F
- Object between F and the optical center of the lense
- Object at infinity
We will now consider each of the positions and examine the kind of images formed.
Object at infinity
Object at infinity is object that is at large distance in respect to the focal length of the lens.
When you focus a distance object, there is that distance between the screen and the lens where the sharp clear image is formed, the distance between the screen and the lens is the focal length and the point where the image is formed is the principal focus of the lens. See the diagram below.
when you focus on distance object such that a sharp image is formed on a white screen as in figure above, then the distance between the screen and the lens is the approximate focal length for the lens.
The rays diagram for the convex lens focusing distance object is shown.
The figure below shows two rays from infinity coming from opposite sides of the principal axis.
The characteristics of image formed when object is at infinity.
- Real (formed on the screen)
- Inverted
- Diminished (smaller than worship
- Formed at F
The setup of lenses is used in the objective lens of a telescope.
Object beyond 2F
The characteristics of image formed is
- Real
- Inverted
- Diminished
- Formed between F and C on the side of the lens
Object beyond C is a useful setup for cameras and in human eyes
Object at 2F
The figure below shows the image formed by concave lens when an object is exactly at 2F, that is, image at center of curvature of the lens.
The characteristics of images formed includes:
- Real
- Inverted( upside down)
- Same size as the object
- Formed at F on the opposite side of the lens
Object at F is a useful setup in terrestrial telescope
Object between F and 2F
It is the only position we have a magnified real image . The image formation is as shown in figure below.
The characteristics of images formed is:
- Real
- inverted( upside down)
- Magnified (bigger than object)
- Formed beyond 2F on the other side of the lens.
Object between F and 2F is a useful setup for microscope objective and photographic enlarger.
Object at F
When object is placed at F, The image formation is as shown in the diagram below
The rays emerge parallel after refraction by the lens and is formed at infinity.
A good example of rays of light moving to infinity is rays of light coming from a spotlight, Therefore the object at infinity setup is common in searchlight and spotlights.
Object between F and the lens p
when object is between F and the lens, the rays of light don’t converge but diverge, but if extended backwards, they seems to meet behind the object.
The between F and lens is the only position where the convex lens produces virtual and upright image. The figure below shows image formed when object is between F and the lens
The characteristics of image formed is
- virtual
- erect
- magnified
- on the same side as object
Object between F and P is useful in magnifying glasses and the microscope.
Image formed by Diverging lenses
Unlike Convex lens, the image formation by concave lenses does not depend on position of the object but it is always virtual, erect and diminished and the image is always formed on the same side as the object.
The figure below shows formation of an image by concave lens when the object is between 2F and F.
The following shows an image for concave lens when an object is between F and the lens.
conclusion
Images formed by concave lenses depends on the position of the object but. The image by concave lenses are virtual only when it is between F and the lenses.
The image by concave lens does not depend on position of the image. Regardless of the position of the object from the lens, the image is always virtual, diminished and upright.