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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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Law of refraction by Fermat's principle | Optics

Law of refraction by Fermat's principle

Fermat's principle

  • According to Fermat’s principle of extremum path, a ray of light in passing from one point to another through a set of media by any number of reflections or refractions selects a path for which the time taken is either minimum or maximum.

Law of refraction by Fermat's principle


  • Optical path between A and B is
  • l = µ1 (AO) + µ2 (OB)
  • l = µ1 √{a2 + x2} + µ2 √{b2 + (c - x)2)}
  • From Fermat’s principle, the position of point O is such the optical path l should be either maximum or minimum.
  • dl/dx = 0

  • This is mathematical form of Snell's law.
  • Thus when a light ray refracted from one medium to another then the ratio of sine of angle of incidence to the sine of angle of refraction is always remain constant, and is equal to the refractive index of the second medium with respect to first medium. This is also known as Snell's law
  • Also the incident ray, normal to the refractive surface and refracted ray all lie in the same plane.
  • These are the laws of refraction.
  • If light ray travels from rarer to denser medium then it bends towards the normal and if it travels from denser to rarer medium then it bends away from the normal.
To know about this lecture in more detail please visit on https://youtu.be/1r6c2o52pio
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