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Ray Diagrams for Lenses: Examples
A ray diagram can be used to find details of the image formed by a lens (or mirror).
It can tell us the size, position and type of image (real or virtual) formed.
Two rays are needed to give this information as described here.
 
For simplicity, we usually assume that one end of the object is placed on the principal axis of the lens.
This  simplifies things for the following reason.
A ray through the centre of the lens is undeviated.
Therefore, the image of any point on the principal axis must also be somewhere on the principal axis.
This means that, all we have to do is find the position of the image of other end of the object and then we will have our answers.
 
N.B
These diagrams are (usually) drawn to scale horizontally but not vertically.
This becomes apparent if you think about he size of the pupil of the eye through which the light enters.
All the light entering the eye (for example, in the first and third diagrams below) could, in practice, be represented by rays forming a very thin cone very near the axis of the lens.
 
Convex (Converging) Lens with Object Between the Lens and the Focal Point
If the object distance is less than or equal to the focal length the image is virtual.
 
When object is on F, the image is described as being at infinity.
It may sound surprising that you can see something, perfectly clearly, which is at infinity!
Saying that the image is at infinity simply means that the image of each point on the object is formed by a set of rays which are parallel to each other, making them appear to be coming from a great distance (as described here).
Try re-drawing the above diagram with the object placed exactly on F to get the idea.
 
 Convex (Converging) Lens with Object Between F and 2F
If the object distance is greater than the focal length the image is real.
 
When object distance is equal to twice the focal length, the image distance is also equal to twice the focal length.
In this case, the image is the same size as the object.
 
When object is at infinity (ok, a long way away... in practice, 25m or more), the image is in the focal plane.
 
 Concave (Diverging) Lens
Only one example is given for the diverging lens because these lenses always form virtual, diminished images of (real) objects.
 
In optics there also exists the concept of a virtual object
In multi lens instruments like microscopes, telescopes etc we come across situations like the one shown below
 
 
The light was converging towards a point and the lens makes the beam less convergent so that it forms an image further at a different place.
The point towards which the light was converging is described as a virtual object of which this diverging lens is forming a real image.
 
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