## Friday, June 24, 2011

### Class X, PHYSICS, "Refraction of Light and Optical Instruments"

Definitions

1. Emergent Ray

The ray after passing the second medium comes again in the first medium. It is called emergent ray.

2. Emergence Angle

The angle formed by the emergent ray and normal is called emergence angle denoted by < e.

3. Optical Center

The middle point of the lens is called optical center. The ray passing through this point does not bend.

4. Accommodation

The ability of the eye to change the focal length of its lens so as to form a clear image of an object on its retina is called is power of accommodation.

5. Persistence of Vision

When an object is seen by an eye, its image forms on retina. If the object is removed, the impression of image persists in the eye for about 1/10 second. This interval is called Persistence of Vision.

6. Power of Lens

The power of the lens is the reciprocal of the focal length measured in meter. Its unit is Dioptre.

Refraction of Light

Definition

“The change in the direction and velocity of light as it enters from one medium to another is known as Refraction of Light.”

Laws of Refraction

* The incident ray, refracted ray and the normal at the point of incidence all lie in the same plane.
* The ratio of sine of angle of incidence (i) to the sine of angle of refraction (r) is constant for all rays of light from one medium to another. This constant is known as Refractive Index (u). This ratio is also equal to the ratio of the speeds of light in one medium to another.

Refractive Index = sin

Refractive Index

The ratio between the sine of the angle of incidence to the sine of angle of refraction is known as Refractive Index.

Refractive Index = sin

Snell’s Law

The refractive index between two particular mediums is equal to the ratio of speed of light in first medium and speed of light in second medium equal to the ratio between sin

Refractive Index = sin

Prism

Definition

“Prism is a transparent piece of glass. It has three rectangular sides and two triangular sides.

Refraction Through a Prism

where,

*

< i = angle of incidence

*

< i = angle of refraction

*

< e = angle of emergence

*

< d = angle of deviation

Total Internal Reflection

If the value of angle of incidence is increased so much so that it becomes greater than tht of the critical angle then no more refraction occurs but on the other hand refracted ray again comes back in the denser medium. Actually at that time, the surface of denser medium acts as a plane mirror and the incident ray bends in the same medium. This phenomenon is called Total Internal Reflection. It is used in Periscope, Optical Fibers and other instruments.

Total Reflecting Prism

Total internal reflection is used in prism. In prism the angle between two opposite sides is 90 and other two angles are 45 each. If we arrange a ray so that it falls perpendicular to the AB side then it will refract without bending and strike the side AC with angle 45. Then it totally reflects to the side BC.

Conditions for Total Internal Reflection

The ray of light should travel from denser to rarer medium.
The angle of incidence should be greater than the critical angle.

Lenses

Definition

A transparent and smooth glass or any refracting medium surrounded by two spherical surfaces is known as lens.

Types of Lenses

There are two types of lenses:

1. Convex Lens

If the glass is thick at the center and thin at the edges then it is known as convex lens. It is a converging lens.

It has three types:

*

Double Convex Lens

*

Plano Convex Lens

*

Concavo Convex Lens

2. Concave Lens If the lens is thinner in the center and thicker at the edges then it is known as a concave lens. It is a diverging lens.

It has three types:

*

Double Concave Lens

*

Plano Concavo Lens

*

Convex Concave Lens

Formation of Image by Convex Lens

1. Object at Infinity

When object is placed at infinite distance from convex lens the rays coming from the object are parallel to each other and they meet after refraction at the focus.

Details of Image

* Formed at Focus
*

Real

*

Inverted

*

At opposite side

*

Highly diminished

2. Object Beyond 2F

When object is placed at some distance from 2F then image is formed between the focus and center of curvature (2F).

Details of Image

*

Between F and 2F

*

Opposite side of Lens

*

Real

*

Inverted

*

Small in size

3. Object at 2F

When object placed at center of curvature, image is formed at center of curvature at the opposite side.

Details of Image

*

Real

*

Inverted

*

At 2F

*

Same in size

*

At the opposite side of the Lens

4. Object between F and 2F

When object is placed between the focus and center of curvature then the image is formed on opposite side beyond the center of curvature.

Details of Image

*

Real

*

Inverted

*

Large in size

*

Opposite side of lens

*

Beyond 2F

5. Object at F

When object is placed at focus the refracted rays are parallel to each other and meet at infinity.

Details of Image

*

Real

*

Inverted

*

Extremely Large

*

Opposite side of Lens

*

At infinity

6. Object between F and O

When object is placed between the lens and principal focus, then the refracted rays does not meet at opposite side but image is formed at the same side where the object is placed.

Details of Image

*

Virtual

*

Erect

*

Large

*

Same side of lens

*

Beyond the object

Optical Instruments

1. THE EYE

Functions of the Parts of Eye

1. Sclera Scelortic

It is a layer enclosed in cavity filled with a fluid called Vitrous Humour. It is the outer coating of eye.

2. Choroid

It is a dark membranous coating. This is coated with black pigments. It keeps the inner parts of the eye ball light proof.

3. Retina

It is semi-transparent membranes of nerve fibers forming the innermost coating of the eye and sensitive to light. It is a screen on which image is formed.

4. Cornea

It allows light into the eyes. It is transparent and bulging in shape.

5. Iris

It is like diaphragm of a camera. It has a tiny opening at its center called pupil, which regulates the quantity of light entering the eye.

6. Crystalline Lens

This is a lens that automatically contracts and expands, alters the focal length of eye lens.

7. Ciliary Body

It holds crystalline lens in the proper position.

8. Aqueous Humour and Vitrous Humour

The place between cornea and the lens is filled by a transparent liquid called Aqueous Humour. The large chamber of the eye between the lens and the back of eye is filled with a jelly like substance called Vitreous Humour. These liquids serve mainly to keep the spherical shape of the eye.

Main Defects of Eye

1. Short Sightedness (Myopia)

If a person can see object placed near, but cannot see distant object, this defect is known as short sightedness.

Causes

This defect appears due to increase in thickness of eyeball. The focal length decreases making the image to form before retina.

Removal of Defect

It is removed by using a concave lens of suitable focal length.

2. Long Sightedness (Hypermetropia)

If a person can see distant objects, but not near objects, this defect is called long sightedness.

Causes

This defect appears due to decrease in thickness of ball. The focal length increases so that the image is formed beyond the retina.

Removal of Defect

It is removed by sing a convex lens of suitable focal length.

3. Astigmatism

It is the defect in which the clear image of an object does not form on the retina.

Causes

This defect appears due to non-sphericity of the cornea.

Removal

This defect can be removed by using lenses of different focal length.

4. Presbyopia

The accommodation power of eye loses by which a person suffers a long sightedness. This defect is called Presbyopia or Lack of Accommodation.

Causes

This defect appears due to loss of accommodation power of the lens of the eye.

Removal

This defect can be removed by using convex lens.

2. CAMERA

Definition

A camera is an optical device for obtaining still photographs or for exposing cinematic films.

Construction

It consists of a light proof box with a lens at one end and a photographic plate or film at other end and a shutter to control the light rays.

Working

To make an exposure, the shutter is opened and an image is formed by lens on the photographic plate or film, small in size. Photographic plate or film saves this image. In this way an image is obtained.

3. COMPOUND MICROSCOPE

Construction

It consist of two convex lenses at the end of two tubes. One tube can slide into other so that the distance between them can be change. The lens near the object is the small convex lens of short focal length is called objective. The lens near the eye is the larger convex of longer focal length is called eyepiece.

Working

The object is placed between F and @F and its real, inverted and magnified image A’B’ is formed. The eyepiece is brought close to it so that it comes within its focal length. The first image A’B’ acts as an object and a virtual, erect and magnified final image A”B” is formed. The magnification of a microscope can be varied by using different objectives.

4. ASTRONOMICAL TELESCOPE

It is used to see heavenly bodies.

Construction

It consists of two convex lenses at the end of the two metallic tubes. One tube can slide into other so that the distance between can be changed. The lens near the object is a convex lens of longer focal length called the objective, while the lens near the eye is a small convex lens of shorter focal length called the eyepiece.

Working
The rays from distant object entering the objective and form a real, inverted and diminished image A’B’ near the principal focus. The eyepiece is adjusted so that the image formed by the objective comes within its focal length. Thus the eyepiece acts as a magnifying glass and a virtual, erect and magnified image A”B” is formed by the first image.

Difference between Real Image and Virtual Image

Real Image

1. Real image is formed when rays after reflection actually meet at a point.
2. Real image is inverted and can be seen on a screen.
3. It has a physical existence.

Virtual Image

1. Virtual image is formed when rays do not actually meet but appear to diverge from a point.
2. Virtual image is erect and cannot be seen on a screen.
3. It does not have a physical existence.