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National Bird Day: Why Birds Matter to Us

National Bird Day is celebrated every year on 5 January . This day is observed to spread awareness about birds, their importance in nature, and the need to protect them. Birds are beautiful living beings and play a very important role in keeping our environment healthy. Origin of National Bird Day National Bird Day was first celebrated in the year 2002 . It was started by bird lovers and environmental groups to protect birds from dangers like deforestation, pollution, and illegal hunting. The main aim of this day is to teach people, especially students, why birds are important and how we can help save them. Why Is National Bird Day Celebrated Every Year? National Bird Day is celebrated every year because many bird species are disappearing due to human activities. Cutting trees, using plastic, pollution, and climate change are harming birds and their homes. This day reminds us that: Birds need protection Nature should be respected Everyone has a responsibil...
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Cardinal points of a lens system | Optics | General theory of image formation

Cardinal points of a lens system

  • There are total six cardinal points of a lens system, which are first and second focal points, first and second principal points, and first and second nodal points.

First and second focal points (First and second focal planes)

  • A pair of points lying on the principal axis and conjugate to points at infinity are known as focal points.

First and Second focal points
  • A point on the principal axis in the object space so that the rays starting (or appear to start) from it become parallel to the principal axis after refraction from the lens system is known as first focal point (F1).
  • A point on the principal axis in the image space so that the rays parallel to the principal axis in the image space focus (or appear to focus) at this point after refraction from the lens system is known as second focal point (F2).

First and Second focal planes
  • The plane passing through the first focal point, and perpendicular to the optic axis is first focal plane.
  • The plane passing through the second focal point, and perpendicular to the optic axis is second focal plane.

First and second principal points (Principal points and Principal planes)

  • Two conjugate points characterized by unit positive transverse magnification are principal planes or unit planes, and the intersection of these points on the principal axis give two points known as principal points.

First and Second principal points
  • If we extend AB in forward direction and C2F2 in backward direction, then they intersect at M2. On drawing a perpendicular from M2 on optic axis, we get an intersection point on optic axis, this intersection point H2 is second principal point.
  • Similarly H1 is first principal point.
  • M1H1 is first principal plane, and M2H2 is second principal plane.

Note

  • The distance of first focal point is measured from first principal point, and the distance of second focal point is measured from second principal point.
  • If there is same medium on either side of lens system, then both the focal lengths will be equal, H1F1 = H2F2.
  • If an object is placed on first principal plane, then its image will be equal to the size of the object, and straight and will lie on the second principal plane.
  • The image of M1 will be M2.
  • M1H1 = M2H2.
  • The lateral magnification of these planes will be unit and positive.

First and second nodal points (Nodal points and Nodal planes)

  • Two conjugate points on the principal axis of the optical system characterized by unit angular magnification are nodal points.
  • The planes perpendicular to the principal axis and passing through the nodal points are known as nodal planes.
  • The distance of nodal points are measured from focal points.

First and Second nodal points
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