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Question
Define the term principal foci of a concave lens and show them with the help of proper diagrams.
Solution
A light ray can pass through a lens from either direction. Therefore, a lens has two principal foci.
For a concave lens, the first focal point is a point F1 on the principal axis of the lens such that the incident rays of light appearing to meet at it, after refraction from the lens become parallel to the principal axis of the lens.
For a concave lens, the second focal point is a point F2 on the lens's main axis. Light rays that hit the lens parallel to the main axis look like they are spreading out from this point after being bent by the lens.
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RELATED QUESTIONS
A ray of light incident at a point on the principal axis of a convex lens passes undeviated through the lens.
- What special name is given to this point on the principal axis?
- Draw a labelled diagram to support your answer in part (a).
Define the term principal foci of a convex lens and illustrate your answer with the aid of proper diagrams.
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- Draw a ray diagram to show it.
- The incident ray passes through a point F on the principal axis. Name the point F.
A beam of light incident on a thin concave lens parallel to its principal axis diverges and appears to come from a point F on the principal axis. Name the point F. Draw a ray diagram to show it.
What do you mean by focal plane of a lens?
If half part of a convex lens is covered, the focal length ______ change, but the intensity of image ______.
What is the path of the ray, after refraction, that passes through the first focus of the lens?
What is optical centre of a thin lens? Draw diagram/diagrams to illustrate it.
The distance between the optical centre and principal focus of a lens is known as ______.
What will happen to the focal length of the lens if a part of the lens is covered with an opaque paper?
