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# light(प्रकाश)-mpsc science ## Intro of light(प्रकाश)

➤Light is a form of energy that enables us to see things. Light starts from a source and bounces off objects which are perceived by our eyes and our brain processes this signal, which eventually enables us to see.
➤ Light behaves as a:ray, e.g. reflection wave
➤interference and diffraction particle
➤photoelectric effect
Laws of Reflection
➤ Light incident on another medium When light travels from one medium to another medium it either:gets absorbed (absorption) bounces back (reflection) passes through or bends (refraction) When light is incident on a plane mirror, most of it gets reflected, and some of it gets absorbed in the medium.
.Characteristics of light
➤ Speed of light c=?×µ, where ? is its wavelength and µ is its frequency.
➤Speed of light is a constant which is 2.998×108m/s or approximately 3.0×108m/s.
➤A medium that is polished well without any irregularities on its surface will cause regular reflection of light. For example, a plane mirror. But even then some light gets absorbed by the surface.
Laws of Reflection
➤ The incident ray, reflected ray and the normal all lie in the same plane. Angle of incidence = Angle of reflection [?i=?r]

Propagation of light
➤ Rectilinear propagation of light: Light travels in a straight line between any two points. Fermat’s Theorem The principle of least time: Light always takes the quickest path between any two points (which may not be the shortest path).Rectilinear propagation of light and the law of reflection [?i=?r] can be validated by Fermat’s principle of least time.
Mirror
➤ Any flat and polished surface that has almost no irregularities on its surface that reflect light is called as a plane mirror.
Characteristics of images
➤ Images can be real or virtual, erect or inverted, magnified or diminished. A real image is formed by the actual convergence of light rays. A virtual image is the apparent convergence of diverging light rays.
➤If an image formed is upside down then it is called inverted or else it is an erect image. If the image formed is bigger than the object, then it is called magnified. If the image formed is smaller than the object, then it is diminished.
➤The image formed by a plane mirror is always virtual and erect.
➤Object and image are equidistant from the mirror

## Principle of Reversibility of light

➤ If the direction of a ray of light is reversed due to reflection off a surface, then it will retrace its path."
Spherical mirror
➤Consider a hollow sphere with a very smooth and polished inside surface and an outer surface with a coating of mercury so that no light can come out. Then if we cut a thin slice out of the shell, we get a curved mirror, which is called a spherical mirror.
➤ Relationship between focus and radius of curvature
➤Focal length is half the distance between pole and radius of curvature.
➤F = R/2
Curved Mirror ➤ A mirror (or any polished, reflective surface) with a curvature is known as a curved mirror. Important terms related to spherical mirror
➤Pole (P): The midpoint of a spherical mirror.
➤Centre of curvature (C): The centre of the sphere that the spherical mirror was a part of. ➤The radius of curvature (r): The distance between the centre of curvature and the spherical mirror. This radius will intersect the mirror at the pole (P).
➤Principal Axis: The line passing through the pole and the centre of curvature is the main or principal axis.

## Concave Mirror

➤A spherical mirror with the reflecting surface that bulges inwards.

## Convex Mirror

➤A spherical mirror with the reflecting surface that bulges outwards.
➤Focus (F)
➤Take a concave mirror. All rays parallel to the principal axis converge at a point between the pole and the centre of curvature. This point is called as the focal point or focus.

## Focal length

➤ Distance between pole and focus.

## Snell’s Law:

➤Light bends towards the normal when moving from rarer to denser medium at the surface of the two media.
➤Light bends away from the normal when moving from denser to rarer medium at the surface of contact of the two media.

## Refractive Index

➤The extent to which light bends when moving from one medium to another is called refractive index
➤This depends on the ratio of the speeds in the two media
➤The greater the ratio, more the bending
➤ It is also the ratio of the sine of the angle of incidence and the sine of the angle of refraction, which is a constant for any given pair of media
➤ It is denoted by: n = sin?i/sin?r = speed of light in medium 1/ speed of light in medium 2..

## Total internal reflection

➤ When the light goes from a denser to a rarer medium it bends away from the normal. The angle at which the incident ray causes the refracted ray to go along the surface of the two media parallelly is called critical angle.
➤When the incident angle is greater than the critical angle, it reflects inside the denser medium instead of refracting. This phenomenon is known as Total Internal Reflection
➤E.g mirages, optical fibres.
Spherical lenses
➤ Spherical lenses are the lenses formed by binding two spherical transparent surfaces together.
➤ Spherical lenses formed by binding two spherical surfaces bulging outward are known as convex lenses while the spherical lenses formed by binding two spherical surfaces such that they are curved inward are known as concave lenses.

## Uses of spherical lens

➤Applications such as visual aids: spectacles, binoculars, magnifying lens, telescopes.
➤Power of a lens is the reciprocal of its focal length i.e 1/f (in metre). The SI unit of power of a lens is dioptre (D).

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