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प्रश्न
Define the term 'focus' of a lens.
उत्तर
Principal focus: Rays of light can pass through the lens in any direction and hence there will be two principal foci on either side of the lens and they are referred to as the first principal focus and the second principal focus of a lens.
First Principal Focus (F1)
It is a point on the principal axis of the lens such that the rays of light starting from it (convex lens) or appearing to meet at the point (concave lens) after refraction from the two surfaces of the lens become parallel to the principal axis of the lens.
Second Principal Focus (F2)
It is a point on the principal axis of the lens such that the rays of light parallel to the principal axis of the lens after refraction from both the surfaces of the lens pass through this point (convex lens) or appear to be coming from this point.
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संबंधित प्रश्न
A ray of light incident on a lens parallel to its principal axis, after refraction passes through or appears to come from ______.
In the diagram below, XX’ represents the principal axis, O the optical centre and F the focus of the lens. Complete the path of rays A and B as they emerge out of the lens.
 |
 |
| (a) | (b) |
Study the diagram shown in Fig. 5.56
what are the two other characteristics of the image?
The diagram given below shows the position of an object OA in relation to a converging lens L whose foci are at F1 and F2.
- Draw two rays to locate the position of the image.
- State the position of the image with reference to the lens.
- Describe three characteristics of the image.
- Describe how the distance of the image from the lens and its size change as the object is moved towards F1.
State the condition of the following:
A ray passes undeviated through the lens.
In the following diagram , the object and the image formed by the respective lenses are shown. Complete the ray diagram, and locate the focus. Find the focal length of the lens.
A lens forms the image of an object placed at a distance of 45 cm from it on a screen placed at a distance 90 cm on the other side of it. Name the kind of lens.
How will you differentiate between a convex and a concave lens by looking at a distant object.
When does a ray of light falling on a lens pass through it undeviated?
Complete the following diagram and state what happens to the ray of light after refraction through the lens.
