HUMAN EYE AND COLOURFUL WORLD-converted.pptx

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HUMAN EYE AND COLOURFUL WORLD-converted.pptx

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HUMAN EYE
AND
COLOURFUL

WORLD


The

ability

of

the

eye

to

focus

both

near

and

distant

objects

by

adjusting

its

focal

length is
called
the
power
of
accommodation
of
the

eye.


The
smallest distance
at
which
the eye can see
objects clearly without strain, is called
the
near point
of
the
eye or the
least distance
of
distinct vision.
It
is
25
cm
for a
normal

eye.


The
farthest point upto which
the eye can see
objects clearly is called
the far
point
of the
eye.
It
is infinity for
a normal

eye.


The
splitting
of
white light into its component colours is called

dispersion.


The
scattering
of
light
by
the colloidal particles
of a
medium
due to
which the path
of the
light becomes visible
is known
as
Tyndall

Effect.

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There
are
three common
refractive defects
of

vision.


Myopia
or
short

sightedness.


Hypermetropia
or
long

sightedness.


Presbyopia.

MYOPIA
OR

SHORT
-
SIGHTEDNESS

A

person

with

myopia

can

see

nearby

objects

clearly

but

cannot

see

distant

objects

distinctly.

Causes
of
Defect:
The
two possible
causes of
this defect

are:

(a)

Excessive

curvature

of

the

eye

lens

or

due

to

the

high

converging

power

of

eye

lens (short
focal

length).

(b)
Elongation
of the

eyeball.

Corrective

Measures:

This

defect

can

be

corrected

by

using

spectacles

with

concave
lens
of
suitable
focal
length
or

power.

HYPERMETROPIA
OR

LONG
-
SIGHTEDNESS


A person
with hypermetropia
can see
distant objects clearly but cannot see
nearby objects
distinctly.

Causes
of
Defect:
The
two possible
causes of
this defect

are:

(i)
Low converging
power
of
eye
lens
because of
large
focal

length.

(ii)
Eye
-
ball being
too

short.

Corrective Measures:
This defect
can be
corrected
by
using spectacles with
convex
lens
of
suitable
focal
length
or

power.

PRESBYOPIA
OR OLD

SIGHT


Presbyopia

is

due

to

decrease

in

power

of

accommodation

with

ageing
.

Cause

of

Defect
:

Presbyopia

arises

due

to

the

gradual

weakening

of

the

ciliary

muscles

and

diminishing

flexibility

of

the

eye

lens

with

age
.

Presbyopia

is

the

hypermetropia

caused

by

the

loss

of

power

of

accommodation

of

the

eye

due

to

old

age
.

Corrective

Measures
:

Presbyopia

defect

is

corrected

in

the

same

way

as

hypermetropia

i
.
e
.

by

using

spectacles

having

convex

lenses
.

Sometimes

when

far

vision

is

also

affected

bifocal

lenses

are

used
.

ATMOSPHERIC REFRACTION

It

can

be

defined

as

bending

of

light

while

going

through

various

layers

of

air

in

the

atmosphere
.

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Twinkling
of
stars:
They
are
point source
of
light
and
atmospheric refraction
causes
bending
of
light. Apparent image is higher than actual image
and
causes twinkling

effect.


Planets do
not twinkle,
as they are
not point source of

light.


Early
sunrise
and
late
sunset
: Sun can be
seen two minutes before actual sunrise
and
two
minutes after sunset
due to
atmospheric

refraction.

SCATTERING
OF

LIGHT


Sun appears
reddish
at
sunrise
and sunset due to,
lower wavelengths (blue) getting
scattered and
higher wavelength (red) reaching

us.


Sky appears
blue
to us
due
to
scattering
of
lower wavelengths
(blue)
.


To an
astronaut
sky
appears dark
as
there
is
no atmosphere,
thus
no

scattering.

Sun

nearly

overhead

Less

blue
scattered

Obser
v
er

Sun

near
horizon

Blue scattered away,
Sun appears

reddish

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