CBSE CLASS 11, Human Eye and Colourful World, PHYSICS NOTES-(PART II)

Human Eye and Colourful World

According to the CBSE Syllabus 2025-26

CBSE Class 10 Science Chapter 10th Human Eye and Colourful World Notes

Study Material and Notes of Ch 11 Human Eye and Colourful World Class 10th Science

Remaining Topics of this Chapter

(5) Hypermetropia
a. Causes of Hypermetropia
b. Correction of Hypermetropia
(6) Presbyopia (Old age Hypermetropia)
a. Causes of Presbyopia
b. Correction of Presbyopia
(7) Advantages of the eyes in front of the face
(8) Refraction through a glass prism.
a. Prism
b. Angle of Prism
c. Angle of deviation (d)
d. Spectrum
e. Acronym
(9) Total internal reflection
a. Conditions necessary for Internal Reflection
b. Critical Angle
c. Rainbow
d. Atmospheric Rainbow
e. Advanced Sunrise
f. Twinkling of stars
(10) Scattering Effect
a. Tyndall effect
(11) Dependence of colour of scattered light.
a. Anger signs are made in red colour.
b. The colour of the sky appears blue on a clear day.
c. Appearance of the sky to an astronaut in space
d. Clouds appear white
e. The colour of the sun appears red during sunrise and sunset

HYPERMETROPIA (Farsightedness)

This is also called farsightedness. A person with this eye defect can only see distant objects clearly compared to near objects. The near point of the eye moves away. The image is formed behind the retina. This condition can be corrected using a convex lens.

CAUSES OF HYPERMETROPIA

a. Focal length of the eye lens becomes too long.

b. Eye ball becomes too small.

CORRECTION OF HYPERMETROPIA

The use of a convex lens of suitable power can correct the defect.

PRESBYOPIA (Old age Hypermetropia)

This is an age-related condition caused due to the weakening of ciliary muscles, hardening of the lens, and reduced lens flexibility. A person with this defect usually finds difficulties focusing on nearby objects and is unable to read or write. The near-point of the old person having presbyopia gradually recedes and becomes much more than 25 cm away.

CAUSES OF PRESBYOPIA

a. Gradual weakening of ciliary muscles.

b. Diminishing flexibility of the eye lens.

CORRECTION OF PRESBYOPIA

a. Use of a convex lens of suitable power.

b. Sometimes a person may suffer from both myopia and hypermetropia.

c. Such people require a bifocal lens for correction.

CATARACT

This is an age-related condition caused due to the loss of transparency of the lens by the erosion of lens proteins. It usually results in blurry vision and cloudy lenses and can be corrected by replacing the old lens with an artificial lens.

ADVANTAGE OF THE EYES IN FRONT OF THE FACE

a. It gives a wider field of view.

b. It enhances the ability to detect faint objects.

c. It provides three three-dimensional view.

DISPERSION OF WHITE LIGHT BY A GLASS PRISM

A prism splits the incident white light into a band of seven colours. The band of the coloured components of a light beam is called its spectrum. Isaac Newton was the first to use a glass prism to obtain the sunlight spectrum. Different colours 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 violet the most. Thus the rays of each colour emerge along different paths and thus become distinct.

REFRACTION THROUGH A GLASS PRISM

Prism:

It is a pyramidal piece of glass with two triangular bases and three rectangular lateral surfaces.

Angle of Prism:

The angle between two adjoining lateral surfaces. Refraction through a glass prism.

Angle of deviation (d):

It is the angle between incident ray and emergent ray.

When white light is passed through a glass prism, it splits into its seven constituent colours to form a band of seven colours. This phenomenon is called dispersion.

Spectrum:

The band of seven colours formed due to dispersion of white light is called spectrum.

Acronym:

It is a group of alphabets that represent sequential colours in spectrum.

V I B G Y O R

a. Angle of deviation ∝ 1/wavelength

b. Red is the least deviated colour as it has largest/longest wavelength.

c. Violet is the most deviated colour as it has smallest wavelength in visible spectrum.

d. Issac Newton was the first person who proved that sunlight is made up of seven colours :

(i) He passed sunlight through a glass prism to form a band of seven colours.

(ii) He tried to split the colours further by putting another prism ahead of the prism forming spectrum but he failed to obtain more colours.

(iii) He formed a spectrum from sunlight and placed an identical but inverted prism in front of prism forming the spectrum. All the seven colours combined by the inverted prism and emerged as white light.

RAINBOW:

It is a natural spectrum appearing in the sky after rain showers. Rainbow is observed in the direction opposite to the sun.

Three phenomenon which are involved in rainbow formation are :

(i) Dispersion

(ii) Refraction

(iii) Internal reflection

> Some water droplets remain suspended in air after rain. These droplets behave as glass prism.

> When light enters the rain drop, it first refracts and disperses.

> Then it reflects internally and again refracts as it come out of the drop and the seven colours reach the eye of observer in form of rainbow.

TOTAL INTERNAL REFLECTION

When light enters obliquely from a denser medium to a rarer medium and the angle of incidence exceeds critical angle, the light reflects in the denser medium. This is called internal reflection.

CONDITIONS NECESSARY FOR INTERNAL REFLECTION

a. Light should enter obliquely from a denser to a rarer medium.

b. The angle of incidence should exceed critical angle, the light reflects in the denser medium.

Critical angle:

The angle of incidence for which the angle of refraction is 90º.

ATMOSPHERIC REFRACTION

The refraction of light by the Earth’s atmosphere is known as atmospheric refraction. Atmospheric refraction is caused by the bending of light rays when they pass through the layers of the earth’s atmosphere, which are of different optical densities.

ATMOSPHERIC REFRACTION:

The refraction by different layers of atmosphere is called atmospheric refraction.

(i) Apparent flickering of objects placed behind a hot object or fire.

(ii) Stars near the horizon appear slightly higher than their actual position.

(iii) Advanced sunrise and delayed sunset.

(iv) Apparent flattering of sun’s disc.

(v) Twinkling of stars.

(i) AN OBJECT PLACED BEHIND THE FIRE OR A HOT SURFACE APPEARS TO FLICKER WHEN SEEN THROUGH THE AIR.

a. The air above hot surface becomes hot and rises. The space is occupied by cool air. The refractive index of hot air is less than that of cool air.

b. So, the physical condition of the medium are not constant.

c. Due to changing Refractive Index (RI) of medium, the light appears to come from different directions.

d. It results in fluctuation in apparent position of object.

(ii) STARS WHEN SEEN NEAR THE HORIZON APPEAR SLIGHTLY HIGHER THAN THEIR ACTUAL POSITION DUE TO ATMOSPHERIC REFRACTION.

a. The refractive index of earth’s atmosphere in general increases from top to bottom.

b. So, the light coming from a star near the horizon has to travel from rarer to denser medium and it bends towards the normal.

c. As a result the star appears higher.

(iii) ADVANCED SUNRISE

a. The sun appears about two minutes earlier than actual sunrise and the sun remains visible for about two minutes after actual sunset.

b. When the sun is below horizon, the rays have to pass from rarer to denser medium.

c. So rays bend towards the normal. As a result the sun appears higher than its actual position.

(iv) TWINKLING OF STARS

The twinkling effect of stars is due to the atmospheric refraction of starlight. Stars are very far from us, so they behave as point source of light. Since the physical conditions of the earth’s atmosphere are not constant the light from stars appears to come from different directions. The starlight undergoes continuous refraction as it passes through the atmosphere before it reaches Earth. 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 starlight entering the eye flickers.

SCATTERING OF LIGHT

When a light beam goes through a medium, it hits the particles existing in them. Due to this phenomenon, some of the light rays get absorbed while a few get scattered in various directions. The intensity of the scattered light rays depends on the particles’ size and wavelength. Spreading of light in various directions by colloid particles.

Scattering ∝ 1/wavelength

Tyndall effect:

When light passes through a colloid its path becomes visible. This is called Tyndall effect.

Example:

(i) Path of light becomes visible when light enters a dark and dusty room through a slit or ventilator.

(ii) Path of light becomes visible when light passes through dense canopy of trees in a forest.

Dependence of colour of scattered light

(i) If particles are very fine, they scatter mainly the blue colour of light (shorter wavelength).

(ii) Medium sized particles scatter mainly the red colour (longer wavelength).

(iii) Even larger particles scatter all the colours of light that is why it appears white.

Wavelength of red light is about 1.8 times to that of blue light.

DANGER SIGNS ARE MADE IN RED COLOUR

Red is the least scattered colour. It is least scattered by fog and smoke and can be seen in the same colour over a long distance. So, danger signs are made in red colour.

COLOUR OF SKY APPEARS BLUE ON A CLEAR DAY

The upper layer of atmosphere contains very fine particles of water vapours and gases. These particles are more effective in scattering of light of shorter wavelength mainly blue than larger wavelength. So, the sky appears blue.

APPEARANCE OF SKY TO AN ASTRONAUT IN THE SPACE

The sky would appear dark to an astronaut in the space as scattering is not very prominent at such high altitude due to absence of particles.

CLOUDS APPEAR WHITE

Clouds are formed by water vapours. Water vapours condense to form water droplets due to larger size of droplets, all colours of light are scattered and clouds appear white.

COLOUR OF SUN APPEAR RED DURING SUNRISE AND SUNSET

→ At sunset and sunrise, the colour of the sun and its surroundings appear red. During sunset and sunrise, the sun is near the horizon and therefore the sunlight has to travel larger distance in atmosphere.

→ Due to this most of the blue light (shorter wavelength) are scattered away by the particles. The light of longer wavelength (red colour) will reach our eye. This is why sun appear red in colour.