LIGHT BY HASAN JAWED
REFLECTION OF LIGHT (Law of Reflection)
(i) The angle of incidence is equal to the angle of reflection.
(ii) The incident ray, the normal, the point of incidence and
the reflected ray, all lie in the same plane.
Spherical Mirrors & their Uses
Uses of concave mirrors
• Concave mirrors are commonly used in torches,
search-lights and vehicles headlights to get powerful
parallel beams of light.
• They are often used as shaving mirrors to see a larger
image of the face. The dentists use concave mirrors to
see large images of the teeth of patients.
• Large concave mirrors are used to concentrate
sunlight to produce heat in solar furnaces.
Uses of convex mirrors
Convex mirrors are commonly used as rear-view (wing)
mirrors in vehicles, enabling the driver to see traffic behind
him/her to facilitate safe driving. They always give an
erect, though diminished, image. Also, they have a wider
field of view as they are curved outwards. Thus, convex mirrors enable the driver to view much larger area than
would be possible with a plane mirror.
REFRACTION OF LIGHT
The refraction of light when it passes from a fast medium
to a slow medium bends the light ray toward the normal to
the boundary between the two media. When a thick glass
slab is placed over some printed matter, the letters appear
raised when viewed through the glass slab the bottom of a
tank or a pond containing water appears to be raised seen
a pencil partly immersed in water in a glass tumbler. It
appears to be displaced at the interface of air and water.
A lemon kept in water in a glass tumbler appears to be
bigger than its actual size, when viewed from the sides.
The following are the laws of refraction of light :
(i) The incident ray, the refracted ray and the normal to the
interface of two transparent media at the point of
incidence, all lie in the same plane.
(ii) The ratio of sine of angle of incidence to the sine of
angle of refraction is a constant, for the light of a given
colour and for the given pair of media. This law is also
known as Snell’s law of refraction. If i is the angle of
incidence and r is the angle of refraction, then,
Sin i/Sin r = constant.
The one with the larger refractive index is optically denser
medium than the other. The other medium of lower
refractive index is optically rarer. The speed of light is
higher in a rarer medium than a denser medium.
DISPERSION OF WHITE LIGHT BY A GLASS PRISM
The prism has probably split the incident white light into a
band of colors. The sequence of colors VIBGYOR . The
splitting of light into its component colors is called
dispersion.
Different colors of light bend through different angles with
respect to the incident ray, as they pass through a prism.
The red light bends the least while the violetthe most. Thus
the rays of each colour emerge along different paths and
thus become distinct.Itis the band of distinct colors that we
see in a spectrum.
A rainbow is a natural spectrum appearing in the sky after
a rain shower. It is caused by dispersion of sunlight by tiny
water droplets, present in the atmosphere. A rainbow is
always formed in a direction opposite to that of the Sun.
The water droplets act like small prisms. They refract and
disperse the incident sunlight, then reflect itinternally, and
finally refract it again when it comes out of the raindrop Due to the dispersion of light and internal reflection,
different colors reach the observer’s eye.
ATMOSPHERIC REFRACTION
The air just above the fire becomes hotter than the air
further up. The hotter air is lighter (less dens(e) than the
cooler air above it, and has a refractive index slightly less
than that of the cooler air. Since the physical conditions of
the refracting medium (air) are not stationary, the
apparent position of the object, as seen through the hot air,
fluctuate. This wavering is thus an effect of atmospheric
refraction (refraction of light by the earth’s atmospher(e) .
Twinkling of stars
The twinkling of a star is due to atmospheric refraction of
starlight.
Advance sunrise and delayed sunset
The Sun is visible to us about 2 minutes before the actual
sunrise, and about 2 minutes after the actual sunset
because of atmospheric refraction.
SCATTERING OF LIGHT
Scattering of light is the phenomenon by which a beam of
light is redirected in many different directions when it
interacts with a particle of matter.
The blue colour of the sky, colour of water in deep sea, the
reddening of the sun at sunrise and the sunset.
Total Internal Reflection
Total internal reflection is a phenomenon that occurs when
light travels from a more optically dense medium to a less
optically dense one, such as glass to air or water to air.
Examples of Total Internal Reflection -
(a) Mirage – Hotter air is less dense, and has smaller
refractive index than the cooler air. On hot summer days,
the air near the ground becomes hotter than the air at
higher levels noticed that while moving in a bus or a car
during a hot summer day, a distant patch of road, especially
on a highway, appears to be wet. This is also due to mirage.
(b) Diamonds - Their brilliance is mainly due to the total
internal reflection of light inside them.
(c) Optical fibres too make use of the phenomenon of total
internal reflection.Light undergoes repeated total internal
reflections along the length of the fibre there is no
appreciable loss in the intensity of the light signal.
Tyndall Effect
The Tyndall effect is the scattering of light as a light beam
passes through a colloid. The individual suspension
particles scatter and reflect light, making the beam visible.
The earth’s atmosphere is a heterogeneous mixture of
minute particles like smoke, tiny water droplets uspended particles of dust and molecules of air.When a
beam of light strikes such fine particles, the path of the
beam becomes visible.
Tyndall effect is seen when a fine beam of sunlight enters a
smoke-filled room through a small hole. Tyndall effect can
also be observed when sunlight passes through a canopy of
a dense forest.
Power of Accomodation of Eye - The ability of the lens to
change its shape to focus near and distant objects is called
accommodation. A normal human eye can see objects
clearly that are between 25 cm and infinity.
Defects of Vision and Their Correction
Nearsightedness: If the eyeball is too long or the lens too
spherical, the image of distant objects is brought to a focus
in front of the retina and is out of focus again before the
light strikes the retina. Nearby objects can be seen more
easily. Eyeglasses with concave lenses correctthis problem
by diverging the light rays before they enter the eye.
Nearsightedness is called myopia.
Farsightedness: If the eyeball is too short or the lens too
flat or inflexible, the light rays entering the eye —
particularly those from nearby objects— will not be
brought to a focus by the time they strike the retina.
Eyeglasses with convex lenses can correct the problem.
Farsightedness is called hypermetropia.
Astigmatism : Astigmatism is the most common refractive
problem responsible for blurry vision. Most of the eyeball’s
focusing power occurs along the front surface of the eye,
involving the tear film and cornea (the clear ‘window’
along the front of the eyeball). The ideal cornea has a
perfectly round surface. Anything other than perfectly
round contributes to abnormal corneal curvature– this is
astigmatism. Cylindrical lens is use to correct astigmatism.
MAGNETISM AND ELECTRICITY
Electricity — Flow of Electrons is called Electricity.
• The electricity produced by friction between two
appropriate bodies, is called static electricity, it is also
called frictional electricity.
Coulomb’s law - The electrostatic force of interaction
acting between two stationary point charges is directly
proportional to the product of magnitude of charges and
inversely proportional to the square of the distance
between them.
F= K(qa-q(b)/r2 .
Electric Field
• The space in the surrounding of any charge in which its
influence can be experience by other charge, is called electric field
REFLECTION OF LIGHT (Law of Reflection)
(i) The angle of incidence is equal to the angle of reflection.
(ii) The incident ray, the normal, the point of incidence and
the reflected ray, all lie in the same plane.
Spherical Mirrors & their Uses
Uses of concave mirrors
• Concave mirrors are commonly used in torches,
search-lights and vehicles headlights to get powerful
parallel beams of light.
• They are often used as shaving mirrors to see a larger
image of the face. The dentists use concave mirrors to
see large images of the teeth of patients.
• Large concave mirrors are used to concentrate
sunlight to produce heat in solar furnaces.
Uses of convex mirrors
Convex mirrors are commonly used as rear-view (wing)
mirrors in vehicles, enabling the driver to see traffic behind
him/her to facilitate safe driving. They always give an
erect, though diminished, image. Also, they have a wider
field of view as they are curved outwards. Thus, convex mirrors enable the driver to view much larger area than
would be possible with a plane mirror.
REFRACTION OF LIGHT
The refraction of light when it passes from a fast medium
to a slow medium bends the light ray toward the normal to
the boundary between the two media. When a thick glass
slab is placed over some printed matter, the letters appear
raised when viewed through the glass slab the bottom of a
tank or a pond containing water appears to be raised seen
a pencil partly immersed in water in a glass tumbler. It
appears to be displaced at the interface of air and water.
A lemon kept in water in a glass tumbler appears to be
bigger than its actual size, when viewed from the sides.
The following are the laws of refraction of light :
(i) The incident ray, the refracted ray and the normal to the
interface of two transparent media at the point of
incidence, all lie in the same plane.
(ii) The ratio of sine of angle of incidence to the sine of
angle of refraction is a constant, for the light of a given
colour and for the given pair of media. This law is also
known as Snell’s law of refraction. If i is the angle of
incidence and r is the angle of refraction, then,
Sin i/Sin r = constant.
The one with the larger refractive index is optically denser
medium than the other. The other medium of lower
refractive index is optically rarer. The speed of light is
higher in a rarer medium than a denser medium.
DISPERSION OF WHITE LIGHT BY A GLASS PRISM
The prism has probably split the incident white light into a
band of colors. The sequence of colors VIBGYOR . The
splitting of light into its component colors is called
dispersion.
Different colors of light bend through different angles with
respect to the incident ray, as they pass through a prism.
The red light bends the least while the violetthe most. Thus
the rays of each colour emerge along different paths and
thus become distinct.Itis the band of distinct colors that we
see in a spectrum.
A rainbow is a natural spectrum appearing in the sky after
a rain shower. It is caused by dispersion of sunlight by tiny
water droplets, present in the atmosphere. A rainbow is
always formed in a direction opposite to that of the Sun.
The water droplets act like small prisms. They refract and
disperse the incident sunlight, then reflect itinternally, and
finally refract it again when it comes out of the raindrop Due to the dispersion of light and internal reflection,
different colors reach the observer’s eye.
ATMOSPHERIC REFRACTION
The air just above the fire becomes hotter than the air
further up. The hotter air is lighter (less dens(e) than the
cooler air above it, and has a refractive index slightly less
than that of the cooler air. Since the physical conditions of
the refracting medium (air) are not stationary, the
apparent position of the object, as seen through the hot air,
fluctuate. This wavering is thus an effect of atmospheric
refraction (refraction of light by the earth’s atmospher(e) .
Twinkling of stars
The twinkling of a star is due to atmospheric refraction of
starlight.
Advance sunrise and delayed sunset
The Sun is visible to us about 2 minutes before the actual
sunrise, and about 2 minutes after the actual sunset
because of atmospheric refraction.
SCATTERING OF LIGHT
Scattering of light is the phenomenon by which a beam of
light is redirected in many different directions when it
interacts with a particle of matter.
The blue colour of the sky, colour of water in deep sea, the
reddening of the sun at sunrise and the sunset.
Total Internal Reflection
Total internal reflection is a phenomenon that occurs when
light travels from a more optically dense medium to a less
optically dense one, such as glass to air or water to air.
Examples of Total Internal Reflection -
(a) Mirage – Hotter air is less dense, and has smaller
refractive index than the cooler air. On hot summer days,
the air near the ground becomes hotter than the air at
higher levels noticed that while moving in a bus or a car
during a hot summer day, a distant patch of road, especially
on a highway, appears to be wet. This is also due to mirage.
(b) Diamonds - Their brilliance is mainly due to the total
internal reflection of light inside them.
(c) Optical fibres too make use of the phenomenon of total
internal reflection.Light undergoes repeated total internal
reflections along the length of the fibre there is no
appreciable loss in the intensity of the light signal.
Tyndall Effect
The Tyndall effect is the scattering of light as a light beam
passes through a colloid. The individual suspension
particles scatter and reflect light, making the beam visible.
The earth’s atmosphere is a heterogeneous mixture of
minute particles like smoke, tiny water droplets uspended particles of dust and molecules of air.When a
beam of light strikes such fine particles, the path of the
beam becomes visible.
Tyndall effect is seen when a fine beam of sunlight enters a
smoke-filled room through a small hole. Tyndall effect can
also be observed when sunlight passes through a canopy of
a dense forest.
Power of Accomodation of Eye - The ability of the lens to
change its shape to focus near and distant objects is called
accommodation. A normal human eye can see objects
clearly that are between 25 cm and infinity.
Defects of Vision and Their Correction
Nearsightedness: If the eyeball is too long or the lens too
spherical, the image of distant objects is brought to a focus
in front of the retina and is out of focus again before the
light strikes the retina. Nearby objects can be seen more
easily. Eyeglasses with concave lenses correctthis problem
by diverging the light rays before they enter the eye.
Nearsightedness is called myopia.
Farsightedness: If the eyeball is too short or the lens too
flat or inflexible, the light rays entering the eye —
particularly those from nearby objects— will not be
brought to a focus by the time they strike the retina.
Eyeglasses with convex lenses can correct the problem.
Farsightedness is called hypermetropia.
Astigmatism : Astigmatism is the most common refractive
problem responsible for blurry vision. Most of the eyeball’s
focusing power occurs along the front surface of the eye,
involving the tear film and cornea (the clear ‘window’
along the front of the eyeball). The ideal cornea has a
perfectly round surface. Anything other than perfectly
round contributes to abnormal corneal curvature– this is
astigmatism. Cylindrical lens is use to correct astigmatism.
MAGNETISM AND ELECTRICITY
Electricity — Flow of Electrons is called Electricity.
• The electricity produced by friction between two
appropriate bodies, is called static electricity, it is also
called frictional electricity.
Coulomb’s law - The electrostatic force of interaction
acting between two stationary point charges is directly
proportional to the product of magnitude of charges and
inversely proportional to the square of the distance
between them.
F= K(qa-q(b)/r2 .
Electric Field
• The space in the surrounding of any charge in which its
influence can be experience by other charge, is called electric field
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