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CBSE Class 10 Science Chapter 10 Light: Reflection and Refraction Notes
Introduction to Light – Reflection & Refraction
In Class 10 Science chapter 10 Light, the phenomena of reflection and refraction of light using the
straight-line propagation of light are taught to students
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The reflection of light by spherical mirrors is discussed in the chapter so that their applications
are studied in real-life situations
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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
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This led Maxwell to predict that light itself was carried by electromagnetic waves which
means that light is a form of electromagnetic radiation
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g
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g
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g
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Phenomena like diffraction, polarisation and interference
could be explained by considering light as a wave
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Laws of Reflection
Light incident on a surface separating two media
When light travels from one medium to another medium it either:
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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
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Speed of light is a constant which is 2
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0×108m/s
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Reflection of light by other media
A medium that is polished well without any irregularities on its surface will cause regular reflection of light
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But even then some light gets absorbed by the surface
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Angle of incidence = Angle of
reflection
[∠i=∠r]
To know more about the Law of Reflection, visit here
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Fermat’s Principle
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The principle of least time: Light always takes the quickest path between any two points (which may
not be the shortest path)
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Applications of Fermat’s Principle
We can make several observations as a result of Fermat’s Principle which will prove useful as we explore the
realm of geometric optics:
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In a homogeneous medium, light rays are rectilinear
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The angle of reflection of a surface is equal to the angle of incidence
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Example of Fermat’s Principle
Mirage is an example of this phenomenon
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What we really witness is the light of the sky which is reflected on the road
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Hot air expands more than cool air and
is thinner, this leads to less decrease in the speed of light
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Image formation by a plane mirror
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The image formed by a plane mirror is always virtual and erect
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Characteristics of images
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Images can be real or virtual, erect or inverted, magnified or diminished
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A virtual image is the apparent convergence of diverging light
rays
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If the image
formed is bigger than the object, then it is called magnified
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To know more about the Plane mirror, visit here
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Spherical Mirrors
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
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Relationship between focus and radius of curvature
Focal length is half the distance between the pole and the radius of curvature
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To know more about Spherical Mirrors, visit here
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Centre of curvature (C): The centre of the sphere that the spherical mirror was a part of
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This radius will intersect the mirror at the pole (P)
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Concave Mirror: A spherical mirror with the reflecting surface that bulges inwards
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Focus (F): Take a concave mirror
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This point is called as the focal point or focus
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Rules of ray diagram for representation of images formed
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A ray passing through the centre of curvature hits the concave spherical mirror and retraces its path
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Image formation by spherical mirrors
For objects at various positions, the image formed can be found using the ray diagrams for the special two
rays
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Uses of spherical mirror based on the image formed
Concave and Convex mirrors are used in many daily purposes
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Mirror formula and Magnification
1/v + 1/u = 1/f
where ‘u’ is object distance, ‘v’ is the image distance and ‘f’ is the focal length of spherical mirror, which is
found by similarity of triangles
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It is usually represented as ‘m’
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Sign convention for ray diagram
Distances measured towards positive x and y axes (coordinate system) are positive and towards negative x
and y-axes are negative
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Usually, the height of the object is taken as
positive as it is above the principal axis and height of the image is taken as negative as it is below the
principal axis
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Refraction Through a Glass Slab and Refractive Index
Refraction
The shortest path need not be the quickest path
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This phenomenon of light bending in a different
medium is called refraction
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The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for
the light of a given colour and for the given pair of media
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To know more about Refraction and Law of Refraction, visit here
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When
taken with respect to vacuum, it’s known as an absolute refractive index
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It moves from rarer to denser medium and then again to the rarer medium
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Light bends away from the normal when moving from denser to rarer medium at the surface of
contact of the two media
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This
depends on the ratio of the speeds in the two media
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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
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The ratio of the speed of light in a vacuum to the speed of monochromatic light in the substance of interest
is known as the relative refractive index
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To know more about Refractive Index Formula, visit here
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The
angle at which the incident ray causes the refracted ray to go along the surface of the two
media parallelly is called the critical angle
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This phenomenon is known as Total Internal Reflection
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g mirages, optical fibres
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Spherical Lens
Refraction at curved surfaces
When light is incident on a curved surface and passes through, the laws of refraction still hold true
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Spherical lenses
Spherical lenses are lenses formed by binding two spherical transparent surfaces together
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To know more about Spherical Lenses, visit here
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It is
also called the pole
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Paraxial Ray: A ray close to the principal axis and also parallel to it
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A spherical
lens has two centres of curvatures
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Focal length: Distance between optical centre and focus
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A ray passing through the optical centre undergoes zero deviation
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Lens Formula, Magnification and Power of Lens
Lens formula and magnification
Lens formula: 1/v = 1/u = 1/f, gives the relationship between the object distance (u), image distance (v), and
the focal length (f) of a spherical lens
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Uses
of spherical lens
Applications such as visual aids: spectacles, binoculars, magnifying lenses, telescopes
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e 1/f (in metre)
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To know more about Power of a Lens, visit here
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42 and that of carbon disulphide is 1
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Calculate the refractive index of diamond with respect to carbon disulphide?
The Refractive index of diamonds with respect to carbon disulphide will be 1
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An object kept at 20 cm from a spherical mirror gives rise to an image 15 cm
behind the mirror
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A 3 cm high object is placed at a distance of 80 cm from a concave lens of a
focal length of 20 cm
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The height of the image is 0
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