Convex Lens
A convex lens forms images of different sizes, nature, and at different positions, for objects kept at different distances from its optical centre.
We consider the following six cases that cover all possible types of images formed by this lens. The ray diagrams have been drawn using (up to) two of the three (special) rays mentioned earlier. The characteristics, of the image formed, have been written along with the corresponding ray diagram.
1. Object at infinity
The image of a very far off object (object at infinity) is a real, diminished and almost point-like image. It is formed at the focus of the lens.
2. Object (somewhat) beyond the '2F' point of the lens
The image formed here is a real, diminished, inverted image. It is formed between the 'F' and '2F' points of the lens, on its other side (the side opposite to the side on which the object has been put).
3. Object at the '2F' point of the lens
The image formed here is a real, inverted image that has the same size as the object. It is formed at the '2F' point of the lens, on the other side of the lens.
4. Object between the '2F' and 'F' points of the lens
The image formed here is a real, inverted and magnified image. It is formed beyond the '2F' point of the lens, on its other side.
5. Object kept at the (first) principal focus or the 'F' point of the lens
The image formed here is regarded as a real, inverted and magnified image. It is regarded as formed 'very far off' or at infinity.
6. Object between the optical centre and the 'F' point of the lens
The image formed here is a virtual, erect and magnified image. It appears to be formed beyond the 'F' point of the lens, on the same side as the object is.
The characteristics of the images formed by a convex lens, have been summarised below in the form of a table.
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S. No.
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Position of the object
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Details of the Image formed
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Nature
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Size
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Position
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1.
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Very far off (at infinity)
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Real and Inverted
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Very small (almost a point)
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At the focus F of the lens
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2.
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Beyond the 2F point
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Real and Inverted
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Smaller than the object
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Between the F and 2F points, on the other side of the lens
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3.
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At the 2F point
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Real and Inverted
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Same as that of the object
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At the 2F point, on the other side of the lens
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4.
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Between F and 2F points
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Real and Inverted
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Larger than the object
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Beyond the 2F point on the other side of the lens.
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5.
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At the F point
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Real and Inverted
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(Very much)
Larger than the object
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Very far off or at infinity on the other side of the lens
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6.
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Between the optical centre and the F point
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Virtual and erect
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Larger than the object
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Beyond the F point on the same side of the lens.
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Activity 8
- Keep a book vertically on the bench at least 1.5 m away from your eyes.
- Hold a convex lens 'at arm's length' and look at the image of the book seen through the lens.
- Record what happens if you move the lens and your eyes closer to the book?
- Observe any change in:
(i) the size of the image of the text and
(ii) in the nature of the image (upright or inverted).
Now, replace the convex lens by a concave lens and repeat the above steps. What do you observe now?
You will see that when the lenses are kept very near to the book the text gets magnified through the convex lens; it gets considerably diminished through the concave lens. The image of letters and diagrams, through the convex lens can appear inverted, or erect, depending on the distance of the lens from them.
We can now understand why we often use a convex lens (of small focal length and provided with a frame and a handle) as a simple magnifying glass or as a reading lens. This is because a convex lens produces a virtual, erect and magnified image of an object, when the object is kept close to it.
