is made of seven colors. Violet [lowest wavelength] bends
most when passing through a prism and red [highest
wavelength] least. This phenomenon is called dispersion.
It is a transverse wave not a mechanical wave.
of reflection says that Angle of incidence is equal
to the angle of reflection. Also the angle of incidence,
angle of reflection and normal lie on the same plane.
is caused when the reflection surface is irregular
so reflected rays are not parallel.
Human Eye and its Constituents
white part of the eye is tough to protect interior from
transparent coating is called cornea. Behind the cornea is
the iris, which gives color to the eye.
iris has an opening called pupil. Iris controls the
amount of light entering the eye. Behind the pupil
is a convex lens which focuses light on the retina.
retina has two cells: rod shaped for dim light and
cone shaped for bright light and colors.
of these cells go to the brain via the optic nerve.
the crystalline lens of eye becomes milky or translucent,
this is called cataract.
myopia, shortsightedness the person can’t see far
away objects. This is corrected by using a concave lens.
metropia or farsightedness is corrected using convex
affects accommodation of eye i.e. ability to adjust
its focal length to see objects nearby or far. Such a
person uses bi-focal i.e. upper part concave for far sight
and lower part convex for near sight.
This occurs when the cornea is not spherical in shape. For
example, the cornea could have a larger curvature in the
vertical plane than in the horizontal plane or vice-versa.
If a person with such a defect in eye-lens looks at a wire
mesh or a grid of lines, focusing in either the vertical
or the horizontal plane may not be as sharp as in the
other plane. Astigmatism results in lines in one direction
being well focused while those in a perpendicular
direction may appear distorted. Astigmatism can be
corrected by using a cylindrical lens of desired radius of
curvature with an appropriately directed axis. This defect
can occur along with myopia or hypermetropia.
Fig 2: Eye defects
formed by a plane mirror is virtual, erect, laterally
mirrors are concave or convex.
mirrors are used in dental equipment's, by doctors to
check ear, nose, and throats. Reflectors of torches,
headlights of vehicles, shaving mirrors to get enlarged
image of face.
of concave mirrors can be real or virtual, inverted or
erect and same size, larger or smaller, depending on
position of object with respect to mirror.
Fig 1: Concave mirror
N.C.E.R.T Class XI Science
= center of the reflecting surface. Pole
Center of curvature. Lies outside in case of concave,
inside in case of convex.
Principal focus. Distance between P and F is focal length
mirror form image of objects spread over a large area.
They are used in rear view or side mirrors of vehicles.
Images are virtual, erect and diminished.
Fig 2: Convex Mirror
N.C.E.R.T Class XI Science
are transparent. They form images by refraction of light.
lenses [magnifying glass] are thicker at middle and
thinner at edges.
of convex lens can be real or virtual, inverted or erect
and same size, larger or smaller, depending on position of
object with respect to mirror [same as concave mirrors].
of concave lens is always virtual, erect and diminished
[same as convex mirror].
lens is used in microscope, refracting telescopes,
reflecting telescopes [also have concave lens].
lenses are other way round. Convex lens are converging and
concave lenses are diverging. Power of convexlens is
positive and concave lens is negative.
of light: Bending of light when it enters a medium
is called refraction. Due to refraction pencil appears to
bend in water, coin in water appears raised. The object
inside water appears enlarged. Twinkling of stars,
advanced sunrise or delayed sunset and apparent flattening
of sun at sunrise and sunset into oval shape is also
caused due to atmospheric refraction.
and twilight are also due to refraction. they are maximum
at the poles and decrease towards the equator.
internal reflection occurs when a ray of light
traveling from denser to rarer medium is incident at an
angle more than the critical angle, it is reflected inside
and no refraction takes place.
Mirage, diamonds can be cut in such a manner to facilitate
total internal reflections to make it shiny, Optical
light ray entered from rarer [optically rarer] to denser
[optically denser] medium it bends towards the normal
[also speed decreases] and when it enters rarer medium
from denser it bends away from the normal [speed
increases]. Optical density is different from mass
travels fastest in vacuum, then air, then liquids and
effect - Scattering of light causes blue sky
phenomenon and reddening of sun at sunrise and sunset.
Blue wavelength is easily scattered by dust particles,
hence sky appears blue. Violet is scattered more easily
than blue but our eyes are more sensitive to blue color.
However at a great height effect of scattering are
negligible so sky appears dark.
lights are Red as it has higher wavelength and won’t be
scattered by dust or fog or smoke.
at sunrise or sunset, the sunlight travels greater
distance to reach us; the smaller wavelengths are
already scattered so only longer wavelength i.e. Red
birds have more cones than rod cells and in night birds
reverse is the case.
pattern founded in 1821 has 63 characters.
of vision: Image on the retina persists for one
sixteenth of a second after it’s removed. Hence in
cinemas the images are flashed at a rate faster than 16
that give out light are luminous objects.
need a source of light and an opaque object.
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