Light:-  The medium through which we can see any object with the help of eyes is called            light.                                                                                                                                                 

Or, Light is a form of energy which helps us in seeing objects.

Properties of Light:

1. It is form of energy.
2.  It travels in straight line. This phenomenon is called rectilinear propagation of light.
3.  Light can form shadows.
4. Light is an electromagnetic wave which can travels with medium as well as through vacuum.
5. The main source of light is sun.
6. The range of wavelength of visible light is 400-700 nanometres (nm). Or 4×10^-7to 7×10^-7m
7. The velocity of light is 3×10⁵ Km/sec Or  3×10⁸ m/sec
8. The sun light reach on earth in 8 minutes 20 Seconds.
9. The distance of earth from sun is 1.5×10⁸ Km = 15×10⁷ Km
10. Mass of earth= 5.972 × 10²⁴ Kg

Question: –       Why do we see the object?

When light falls on an object, some of the light gets reflected. The reflected light comes to our eyes and we are able to see an object.

Object:-          Anything which gives out light rays either of its own or due to reflection is called an object.

Point Object:- An object whose dimension are negligibly small is called point object.

Extended object: – An object whose dimension are quite large is called extended object.

Types of Object on the basis of Light emission:

Luminous Object (दीप्त पिण्ड):- An object which produces (emits) light is called a luminous object.

 

Non-luminous object (अदीप्त पिण्ड):- An object which does not produce light is called non-luminous object.    

                                     Ex-moon, wood, table, book, planets etc

Moon is non-luminous body because it shines by reflecting the sunlight falling on it.

Types of Object on the basis of travelling of Light:

Transparent (पारदर्शी)                 Translucent (अल्पदर्शी)                   Opaque (अपारदर्शी)

Transparent Object:-  An object which allows complete passage to light is called a transparent object. We can clearly see through a transparent object.

                                            Ex-    Water, glass

Translucent Object: – An object which allows partial passage to light is called a translucent object. We can see through a translucent object but the vision would be                                                    faint.          Ex-      Butter paper

Opaque Object: –     An object which does not allows passage to light is called an opaque object. We cannot see through an opaque object.

                                   Ex-    Wood, brick, iron, wall etc

When light falls on any surface, either of these phenomenon takes place.

1. Light falling on surface may get absorbed i.e., Absorption
2. Light on  falling on surface may pass through the surface i.e., Transmission
3. Light on falling on surface may bounce back i.e., Reflection

Reflection of Light:- The process of bouncing back of light rays on striking the surfaceis called reflection of light. OR

      The bouncing back of light is called reflection of light.

Characteristics of best Reflector:-

1. It should have shiny surface
2. It should have polished surface
3. It should have smooth surface

Note:- Silver metal is the best reflector

Some basic term related to reflection of light

Ray of light:- A ray of light is the direction in which light travels.

Beam of light:- A bundle of light rays is called beam of light or light beam.

 

 

Incident ray (Apatitkiran):-    The ray of light which falls on the mirror surface iscalled incident ray. (Labelled I in the diagram). 

Reflected ray (paravartitkiran):- The ray of light which send back by the mirror is called reflected ray (Labelled R in the diagram). 

Point of incident (aapatanbindu):- The point on mirror where incident ray falls iscalled point of incident. (labelled O in the diagram)

Normal (abhilamb):- A line perpendicular to the mirror surface drown at point of incident is called normal line (labeled N in the diagram).

The normal line divides the angle between the incident ray and the reflected ray into two equal angles.

Angle of incident:-    The angle between the incident ray and the normal is called angle of incidence. In fig

Angle of reflection:- The angle between the reflected ray and the normal is called the angle of reflection. In fig      

Laws of Reflection

There are two laws of reflection:

First Law of Reflection:-        The incident ray, the reflected ray and normal lie in the same plane.                                                                                                                    Second Law of Reflection:-   Angle of incidence is always equal to the angle of reflection.

Types of Reflection

Regular Reflection The reflectionin which all reflected rays are parallel to each is called regular reflection.     Ex:-  Reflection by a mirror

 

 

 

 

 

 

Irregular (Diffused) Reflection The reflection in which reflected rays are not parallel to each other is called irregular reflection.

 

Regular Reflection	Irregular Reflection
Regular reflection happens from a smooth surface	Irregular reflection happens from a rough surface
Reflected rays are parallel to each other.	Reflected rays are not parallel to each other.
We get to see clear images when image is formed by regular reflection.	We get to see blurred images when image is formed by regular reflection.

Reflection from plane mirrors-                                                                                                                                                              

Mirror:-  A smooth, highly polished reflecting surface is called mirror.it is generally made by depositing a thin layer of silver metal on one side of a plain glass sheet.

Types of Mirror:-               Plane Mirror         (samataldarpan)            Spherical Mirror (Goliydarpan)

Plane Mirror:-  The highly polished plane reflecting surface is called plain mirror.

Use: –      For making periscope, as a reflector in solar

Spherical Mirror: – The part of highly polished sphere/curved is called spherical mirror.

Spherical mirrors are of two types.          Concave mirror अवतल दर्पण   and          Convex mirror उतल दर्पण 

Concave mirror:-That spherical mirror which reflection occurs through inner surface of mirror are called concave mirror. I.e. its reflective surface is depressed. Convex mirror:-That spherical mirror whichreflection occurs through outer surfaceof mirror are called convex mirror. I.e. its reflective surface is bulged.

Image: –  An image of an object is formed when light rays coming from the object meet or appear to meet at a point after reflection from the mirror or lens.                                            There are two types of image.

Real image: – The image is that which formed when reflected rays actually meet at a point is called real image.

It is always inverted.

It can be obtained on screen

Virtual image: – The image is that which formed when reflected rays do not meet actually but appear to meet at certain point is called virtual image.

It is always erect.

It cannot be obtained on screen.

Image formation in Concave Mirror:

The image is usually inverted, real and smaller than the object.

When the object is kept too close to the concave mirror, the image is erect, virtual and larger than the object.

Uses of Concave Mirror:

Used by dentists and ENT specialists to focus a beam of light to see inside a patient’s mouth or ears.

Used as barber’s mirror, because it shows a larger image when object is too close.

Image formation in Convex Mirror:

  Image is erect, virtual and smaller than the object.

Uses of Convex Mirror:

Used in rear view mirrors, as it shows smaller images from a bigger field of view.

Used on hairpin bends, to see the vehicles coming from other side of the bend.

Lateral Inversion in Plane Mirror: – It is the phenomenon when LHS becomes RHS or vice-versa when an object is exposed to plain mirror.                                                            The word ‘AMBULANCE’ is written in laterally inverted form on the front of the ambulance.                                                                                                                               This is deliberately done so that the driver in a vehicle ahead can easily spot the ambulance and can give way.

Formation of Image in plane Mirror

In figure, PQ is a plain mirror and O is an object. Incident rays OA and OC are coming from the object. These incident rays are reflected from the mirror and reflected rays emerge as AB and CD. If reflected rays AB and CD are extended behind the mirror, they appear to meet at point ‘I’. This is that point where image is formed.

Some features of image formed by a plain mirror:
(I) Obtained Image is always formed behind the mirror.
(II) Obtained image is virtual, which means it cannot be obtained on screen.
(III) Image is erect and laterally inverted.
(IV) Image size is equal to object size.  
(V) Distance of object from the mirror = Distance of image from the mirror.
(VI) Distance of the object and image is the same from the mirror.

Multiple Images (Reflected light can be reflected again) :-
(i) When two or more mirrors are placed at an angle to one another, we can see multiple images of an object.
(ii) For example in the hair dresser shop two mirrors are placed opposite to each other. If a man is sit between them, His image is formed in both the mirrors. The image in one mirror would act as an object for another mirror and this sequence would continue. This will give result in the formation of multiple images.

 

 

 

 

 

 

NOTE:-The number of images formed depends on the angle between the two mirrors.  

* ​​​​​​If the given mirrors are at a right angle to each other, 3 images will be formed.

* If the given mirrors are at 30° angle, we shall get 11 images.

* When the mirrors are kept opposite and parallel to each other, there would be infinite number of images formed.

  Periscope: –The instrument by which we can see an object lying above or below our viewing plane is called periscope.

Kaleidoscope(Bahurupdarshi):-The instrument that applies multiple reflections to form number of images is called kaleidoscope.

The toy kaleidoscope was invented around 1817 by Sir David Brewster.      

Construction-Three plain mirrors are fixed at 60⁰to form a prism and some coloured glass pieces are placed in the middle. When it is rotated, the glass pieces move and form unique symmetrical designs.The designs formed are never the same. Due to this property designers of fabrics, wall paper and artists use kaleidoscope to get ideas for new patters.

Prism:-A solid transparent object with two identical ends and flat sides are called prism.

The shape of the ends gives the prism a name, such as the Triangular prism, pentagonal prism, hexagonal prism etc.

Dispersion of light प्रकाश का वर्ण विक्षेपण :-  The splitting of light into it’s constituent colours is called dispersion.

The splitting of light into its component colours is called as dispersion.

Example: Rainbow

Components of White Light

The white light, or visible spectrum, is composed of seven colours. These colours are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red).

The phenomenon of splitting up of white light into its constituent colours is called dispersion oflight. When white light passes through a prism, it breaks down into its component colours. This is the reason rainbows show all the colours of the visible spectrum.

Newton’s Disc: Since Newton was the first to make this it is called Newton’s Disc. It is a circular disc on which seven colours of the rainbow are painted on seven different sectors. When the disc is rotated at certain speeds, it appears to be white. This shows, how the seven colours make the white light.

HUMAN EYE

The important sense organs which give us the sense of vision is called eye.

* The human eye is almost spherical in shape. Following are the main structures in the human eye.​​​​

Cornea:- The transparent protective membrane that bulges outwardand located at the front portion of the eye is called cornea.

Aqueous humour:– There is a liquid behind cornea is called aqueous humour.

Iris: – The thin circular muscle, colour part structure behind the cornea is called iris.                                                                                                                                     It works like the shutter of a camera. It controls the amount of light entering the eye.                                                                                                                         The colour of iris imparts distinct colour to the eyes of an individual. A person with blue iris has blue eyes.

Pupil: – The circular opening that is located at the centre of the iris is called pupil.                                                                                                                                           Light enters into eye through the pupil. When the light amount is high, iris contracts and allows less light into the eye. When the light amount is low, iris             dilates and allows more light into the eye.

 

Lens: –  A jelly transparent biconvex structure which is present behind the pupil is called lens. The size and thickness of the lens change as per the distance of                   an object by the ciliary muscle.

Retina:- The light – sensitive inner lining of the back of the eye (eyeball) is called retina.
       Images are formed on the retina and thus it works like a screen. The light ray comes through the pupil and passes through the lens and converges on a screen           called retina. Retina contains several nerve cells. There are two kinds photosensitive cells found in retina.

     (i)  Cone cells, which are sensitive to bright light and sense colour.
     (ii) Rods cells, which are sensitive to dim light

Optic Nerve: It is located at the back of the eye. Optic nerves are the bundle of over one million nerves fibres that carry visual messages from the retina to the                               brain.

Blind Spot: It is the small region where the optic nerve and the retina meet. It has no sensory cells. Hence no image is formed at this spot on the retina.

Persistence of Vision दृष्टि का स्थायित्व :- The feature of eye by which imagestays on the retina for about 1/16 of a secondis called persistence of vision. Due to              this, when many still images are shown in a sequence; they give the illusion of moving images. Movies and animation are made by exploiting this property of          the human eye.

Care of Eyes

Eyes are very important organ and they are sensitive too. So, proper care of eyes is very important. Some tips for care of eyes are as follows:

Do not read in too bright or too dim light.

Do not look directly at a bright object or at the sun.

Do not keep the book too close to your eyes while reading. Don’t keep the book too far either.

If something gets into the eye, do not rub the eye. Wash it with cold water.

In case of any problem like itching or burning sensation consult an ophthalmologist. A doctor who specializes in the disease of eyes is called an ophthalmologist.

Vision Defects दृष्टि दोष:

Myopia निटक दृष्टि दोष:-The vision defect in which we cannot see a distant objectand see nearby object is called myopia.

This disease is also called short sightedness or near sightedness.

Spectacles with concave lenses can improve the vision in a myopic person. 

Hypermetropia दूर दृष्टि दोष:-The vision defect in which we cannot see a nearby object and see far object is calledhypermetropia.

This disease is also called long sightedness.

Spectacles withConvex lens can improve the visionhypermetropic person.

Cataract (Motiyabind):-The vision defect during old age, in which the crystalline lens becomes milky and cloudyis called cataract.

The patient of cataract cannot see clearly.

Cataract is treated by surgery. In cataract surgery, the cloudy lens is removed and an artificial lens is transplanted in the eye.

Visually Challenged Person:

      The person that is suffering from disability of vision(partial /complete)is called visually challenged persons.

For a visually challenged person life can be very difficult. These people usually show a marked development of other senses like the sense of hearing and sense of touch. However, additional resources can enable them to develop their capabilities further. So that make their life easy.

Aids for visually challenged person

Optical aids                          Non-optical aids

Tactual aids (using sense of touch)

Auditory aids ( using sense of hearing)

Optical Aids:- The optical instruments that are used to improving the vision is called optical aids.  Ex-TV monitors, Magnifying devices, Telescopic etc.

 Optical aids enlarge an image or a text so that they could be visible.

Optical aids can help a person who is partially visually challenged.                 

Non-optical Aids:- The instrument that are used for a completely visually challenged person is called non-optical aids.

Non-optical aids rely on the senses of hearing and touch.

Tactual aids:-Aids which rely on the sense of touch are called tactual aids.

Ex-  Tactile buttons on the pedestrian light and in public transport

Tactile strips at the edge of the platforms are also meant for visually challenged persons.

Even the currency notes have tactile markings so that a visually challenged person can recognize notes of different denominations.

In some countries; specially trained guide dogs are pressed into the service of a visually challenged person.

Auditory aids:-Aids which rely on the sense of hearing are called auditory aids.

               Ex-Tape recorder, Gramophone, Linguaphone etc.

Louis Braille:
The most popular resource for visually challenged persons is known as Braille. Braille code is used for common languages, mathematics and scientific notation. Louis Braille was born in 1809 in France. He became blind due to an accident in childhood. He developed the Braille script in 1824.

Louis Braille                                                 

Braille Script:-The Braille scripts are written in the form of raised dots. This system is composed of 63 characters. Each character is written on a grid of 6 cells. This can be written with the help of Braille slate and stylus. A reader needs to touch the Braille script to read it.