Human eye class 10

MADE BY :- SHRISH OJHA MADE BY :- SHRISH OJHA andand ARYAMAN SINGH MARJARA ARYAMAN SINGH MARJARA

1. The Human Eye.

2. The Parts and Functions of a Human Eye.

3. Power of Accommodation and LDDV

4. Defects of Vision and their Correction -

Myopia, Hypermetropia and Presbyopia.

5. Refraction through a Prism.

6. Expression for Refractive Index of Prism.

7. Dispersion .

8. Rainbow.

9. Atmospheric Refraction – Tyndall Effect, Apparent position & Twinkling of Stars, Delayed Sunrise & Sunset.

10.Scattering of Light - Blue Colour of the Sky and Red Colour of the Sun.

Internal structure of Human Eye

CRYSTALLINE LENCE AQUEOUS HUMOURAQUEOUS HUMOUR PUPILPUPIL IRISIRIS CORNEACORNEA VITREOUS HUMOURVITREOUS HUMOUR OPTIC NERVEOPTIC NERVE RATINARATINA CILIARY MUSCLESCILIARY MUSCLES

•The crystalline lens is a lens which merely provides the finer adjustment of focal length required to focus objects at different distances on the ratina.

It is a watery fluid between cornea It is a watery fluid between cornea and lensand lens

It is a gap between iris which It is a gap between iris which ensure the amount of light entering ensure the amount of light entering the eyethe eye

•Iris is a dark muscular diaphragm behind the cornea and it controls the

size of the pupil.

• The pupil regulates and controls the amount of light entering the eye.

Cornea is a thin membrane which forms the transparent bulge on the front surface of the eye ball.

Light enters in the eye through it. Most of the refraction of light rays entering the eye occurs at the outer

surface of the cornea.

It is a oily fluid lens and retinaIt is a oily fluid lens and retina

The nerve which carry the The nerve which carry the information regarding the image to information regarding the image to the brainthe brain

Image is formed on a light-sensitive screen called the retina. The eye lens forms an inverted real image of the object on the retina.

These are the muscles which contract or expand to make These are the muscles which contract or expand to make the focal length of the lens to increase or decrease.the focal length of the lens to increase or decrease.

Power of Accommodation

The ability of the eye lens to adjust its focal length is called accommodation.

The eye lens is composed of a fibrous, jelly-like material and its curvature can be modified by the ciliary muscles. Hence, the focal length can be changed as per the requirement. This enables us to see the distant or near by objects clearly.

Least Distance of Distinct Vision (LDDV):

The minimum distance, at which objects can be seen most distinctly without strain, is called Least Distance of Distinct Vision(LDDV). For a normal eye, LDDV is 25 cm.

Myopia or Short-sightedness or Near-sightedness

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

Such a person may clearly see upto a distance of a few metres.

In myopic eye, the image of a distant object is formed in front of the retina and not on the retinal itself.

This defect may arise due to

(i) excessive curvature of the eye lens (short focal length of the eye lens)

or

(ii) Elongation of the eyeball.

Myopia can be corrected by using a concave lens of suitable power(focalpower(focal length).

LDDV = 25 cm

O

LDDV = 25 cm

I

O

LDDV = 25 cm

Myopic Eye

OI

I

O

LDDV = 25 cm

IO

LDDV = 25 cm

II

Near Point

Myopic Eye corrected with Concave Lens

Normal Eye

Hypermetropia or Long-sightedness or Far-sightedness

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

Such a person may has to keep a reading material much beyond 25 cm from the eye for comfortable reading.

In hypermetropic eye, the image of a nearby object is formed behind the retina and not on the retinal itself.

This defect may arise due to

(i) long focal length of the eye lens or

(ii) Very small size of the eyeball.

Hypermetropia can be corrected by using a convex lens of suitable power (focal length).

LDDV = 25 cmHypermetropic Eye

O

Near Point

LDDV = 25 cm

IO

O

LDDV = 25 cm

I

I

LDDV = 25 cm

IO

LDDV = 25 cm

OII

Hypermetropic Eye corrected with Convex Lens

Normal Eye

Presbyopia

The power of accommodation of the eye usually decreases with ageing.

People can not see nearby objects comfortably and distinctly without corrective eye-glasses.

This defect is called presbyopia.

It arises due to

(i) gradual weakening of the ciliary muscles and

(ii) diminishing flexibility of the eye lens.

Sometimes, a person may suffer from both myopia and hypermetropia. Such

people require bi-focal lenses which consists of both concave and convex

lenses. The upper portion is concave for distant vision and the lower portion

is convex for near vision.

N1

N2

Eye

A

Refracting Surfaces

Prism

ie

REFRACTION OF LIGHT BY A TRIANGULAR PRISM

QP

S

R

Refraction of Light through Prism:

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Refracting Surfaces

Prism

i

A

B C

e

Or1 r2

N1 N2

μ

Q

P SR

DISPERSION OF WHITE LIGHT THROUGH A PRISM

The phenomenon of splitting a ray of white light into its constituent colours (wavelengths) is called dispersion and the band of colours from violet to red is called spectrum (VIBGYOR).

A

B C

D

White light

RO

YG

B I

V

Screen

N

A

B C

White light

N

A

B’ C’ White light

Recombination of spectrum of white light:

A rainbow is a natural spectrum which is caused by dispersion of sunlight by tiny water droplets present in the atmosphere after a rain shower.

The incident sunlight with suitable angle of incidence is refracted, dispersed, and finally refracted out by the rain drops.

Due to the dispersion and internal reflection, different colours reach the eye of the observer.

RAINBOW

Eye

41º43º

Sunlight

Rain drop

Formation of Rainbow

A line parallel to Sun’s ray

ATMOSPHERIC REFRACTION

Flickering of objects above a fire:

The apparent random wavering or flickering of objects can be seen through a turbulent stream of hot air rising above a fire.

The air just above the fire becomes hotter than the further up. The hotter air is lighter 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, fluctuates. This wavering is therefore due to an effect of atmospheric refraction on a small scale in the local environment.

Refraction of light by earth’s atmosphere is called atmospheric refraction.

Twinkling of Stars:

The twinkling of a star is due to atmospheric refraction of starlight.

The atmospheric refraction occurs in a medium of gradually changing refractive index.

Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position.

The star appears slightly higher (above) than its actual position when viewed near the horizon.

This apparent position is not stationary, but keeps on changing slightly, since the physical conditions of the earth’s atmosphere are not stationary.

Since the stars are very distant, they approximate point-sized sources of light.

As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of light entering the eye flickers- the star sometimes appear brighter, and at some other time, fainter which gives the twinkling effect.

Eye

Den

sity of A

tmo

sph

ere &

R

efracti ve ind

ex increa se

Apparent position of the Star

Real position of the Star

SCATTERING OF LIGHTTyndall Effect:The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air.

When a beam of light strikes such fine particles, the path of the beam becomes visible.

The light reaches us, after being reflected diffusedly by these particles.

The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall Effect.

Tyndall Effect can be seen when a fine beam of sunlight enters a smoke-filled room through a small hole. In this, scattering of light makes the particles visible.

It can also be seen when sunlight passes through a canopy of a dense forest. In this, tiny water droplets in the mist scatter light.

The colour of the scattered light depends on the size of the scattering particles.

Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths. If the size of the scattering particles is large enough, then, the scattered light may even appear white.

SCATTERING OF LIGHT - Activity

S

L1 ScreenL2

I

Sodium thio sulphate solution (hypo)

Conc. Sulphuric acid

ii) Why is the colour of the clear sky blue ? The fine particles in the atmosphere have size smaller than the wave length of visible light. They can scatter blue light which has a shorter wave length than red light which has a longer wave length. When sunlight passes through the atmosphere, the fine particles in the atmosphere scatter the blue colour more strongly than the red and so the sky appears blue.

If the earth had no atmosphere there would not be any scattering of light and the sky would appear dark. The sky appears dark at very high altitudes.