10th Class Chapter 3 Physics Long Questions

Given Data:

<I =30°

N== 1.52

<r =?

Solution:

Sin<I =0

Sin <r =

1.52in<r = sin 30°

Sin <r = sin30°/1.52

Sin<r =0.33

<r =sin-1(0.33)

<r =19.3° Ans:

Given Data:

F =6cm

M=3

P= ?

Solution:

M =-q/P

3p = - q

Q = -3p

According to lens formula:

I/f = I/p +I/q

I/f = I/p + I/( -3P)

I/f = I/p + I/3P

I/f = 3-1/3p

I/6 = 2/3p

3p =12

P =4cm

Human eye:

Eye is a organ used for vision.

Image formula:

The parts of human eye which play an important role in the image formation are described below.

Retina:

Human eye acts like a camera, in place of the film, the ratina records the picture.The eye has refracting system containing a converging lens.The lens forms an image on the ratina.Ratina is the light sensitive layer at the back of the eye.

Lens:

In the camera the distance of lens from film is adjusted for proper focus but in eye the lens change focal length. The lens of eye is flexible and accomdates object over a wide range of distance.

Cornea:

Light enters the eye through a transparent membrane called the cornea.

Iris:

The iris is the color portion of the eye and controls the amount of light reaching ratina.The iris also controls the size of pupil.

Pupil:

Iris has an opening at its center called pupil.

Total internal reflection:

When the value of angle of incidence becomes greater then the critical angle, then ray does not enter into the second medium,but reflects back in the same medium such reflection of light is called total internal reflection.

Conditions of total internal reflection:

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

Explanation:

When a ray of light passes from danser medium, the refracted ray bends away from normal and the angle of refraction is greater then angle of incidence.If we increase the angle of incidence ,the angle of refraction,also increase till at a particular value of angle of incidence,the corresponding angle of refraction becomes 90° and refraction ray becomes paralled to the surface.The angle of incidence that causes the refracted ray in the rare medium to bend through 90° is called critical angle.

Given data:

Object distance =p=20cm

Image distance =q=20cm

Focal length = f=?

Solution:

According to the mirror formula:

I/f = I/p +I/q

By multiplying pqf on both sides

Pqf/f = pqf/p+pqf/q

Pq=qf+pf

Pq =f (q+p)

Pq/p+q= f

20*20/20+20 =f

400/40 = f

10 =f

F = 10cm

Endoscope is used for viewing and photographing of the internal structure of human body by using light pipes

Types: Grastroscope Bronchoscope Cystoscope

The mirror whose inner curved surface is reflecting is called concave mirror.

Radius of curvature is the radius of the sphere of which spherical mirror is a part.

Real image:

The image that can be obtained on screen is called real image rays of light actually converge to form image, image is inverted

The distance between principal focus and pole of the mirror is call focal length it is denoted by f.

The near point of an eye is the minimum distance of an objects from the eye at which is produce a sharp image on the retina.

Far point:

The far point of the eye is the maximum distance of a distant objects from the eye on which the fully relaxed eye can focus.

Power of lens is defined as the reciprocal of its focal length in meters.

Power of lens = P = I /focal length on meters .

Now a days America and other developed countries use gaint concave mirrors in their huge telescope, the dentists use concave mirror to observe large image of the teeth of the patients.

The focal length of a spherical mirror is one half of the radius of curvature i.e f= r/2 however the focal length of a convex mirror is taken as negative it is because the rays appear to come from the focal point behind the mirror.

We see a page of a book because light reflects from each part of the page in all direction so that some of the light rays from each part of the page enter our eye because almost no light is reflected by the printed words , so we see them as black areas.

Optics:

The study of light behavior is called optics.

Geometrical optics:

The branch of optics that focuses on the creation of image is called geometrical optics .

Compound microscope is used to investigative the structure very small objects ,objects lens has smaller focal length then the eyepiece distance between the objective lens and the eye piece is greater then fo+fe.

A spherical mirror is a part of a sphere the center of this sphere is called center of curvature.

Radius of curvature:

Radius of curvature is the radius of the sphere of which spherical mirror is a part.

The ratio of the sine of the angle of incidence ‘’I’’ to the sine of the angle of refraction ‘’r’’ is always equal to a constant i.e,

Sini/sinr = constant

Where the ratio sini/sinr is equal to the refractive index of the second medium with respect to the first medium.

Sini/sinr=n

A smooth surface of silver reflect rays of light in one direction only the reflection by this smooth surface is called regular , reflection the rough surfaces of these objects reflect the rays of light in many directions such a type of reflection is called irregular reflection.

The angle of incidence must be greater than critical angle in order to get total internal reflection.

Principal focus is a point on the principal axis of lens where a bean of light parallel to the principal axis converges to or diverges after refraction from the lens.

Lenses of many different types are used in optical devices such as cameras, eye glasses etc, they also enable millions of people to use clearly and read comfortably.

Power of lens is defined as the reciprocal of its focal length in meters.

Power of lens = p = I / focal length in meters

The S.I unit of power of lens is ‘’Dioptre’’ denoted by a symbol D. If f is expressed in meters so that 1D = 1m-1.

The angle of incidence that causes the refracted ray in the rare medium to bend through 90° is called critical angle.

When object is placed at 2F from converging lens the image is shown in figure below.

If the opticians room is small then for testing the patient’s eye sight original words are placed at the back side of patient and mirror is placed in front of the patient , so that image of words is formed at the distance doubled then the size of room.

State law of refraction:

There are two laws of refraction:

First law of refraction:

The incident ray, the refracted ray and the normal at the point of incidence all lie in the some plane.

2^nd law of refraction:

The ratio of the sin of the angle o incidence I to the in of the angle of refraction r is always equal to a constant i.e

Sini/sinr= constant= n

When the value of angle of incidence becomes greater than the critical angle , then the ray does not enter in to the second medium but reflects back in the same medium such reflection of light is called total internal reflection.

Reflection of light:

We see objects if light from it enters our eyes. Some objects such as the Sun, electric lamps and candles make their own light. We call these luminous sources. Most things that we see do not make their own light but reflect it from a luminous source they are non-luminous objects for example this page, teaching board and the Moon.

Reflection of light is same as for other types waves, such as reflection of water waves in ripple tank. Most object reflect a certain portion of the light falling on them, mirrors reflect more light and in a regular manner. An ordinary mirror is made by depositing a thin layer of silver on oner side of a piece of glass and protecting it with paint. The silver at the back of the glass - acts as the reflecting surface.

Suppose that a ray of light is incident on a flat, shiny surface, The following terms are used in describing the reflection of light

1- Incident ray: The approaching ray of light

2 Reflected ray: The ray of light reflected from a reflecting surface.

3 Point f incidence: The point at which the incident ray strikes the reflecting surface

4 Normal: The line drawn at right angles to the reflecting surface at the point of incidence.

5 Angle of incidence (0): The angle between the incident ray and the normal.

6 Angle of reflection(0) : The angle between the reflected ray and the normal

To determine the magnification of the image, and its position with precision, it is necessary to draw very accurate scaled diagrams. The mirror equation on the other hand is a precise mathematical relationship between the object distance and the image distance for a given mirror. The following quantities are included in mirror formula:

F = the focal length of the mirror

H_o = the height of the object

H₁ = the height of the image

D_o = the distance of the object form the mirror

D₁ = the distance of the image from the mirror

M = the magnification of the image

To obtain this relation, we use the ray in normal to the mirror. As a result, the ray reflects at an angle below the principal axis that is equal to its incident angleabove the axis. Therefore, the two green triangles in this diagram are similar, form which it follows that ratios of equivalent sides of similar triangles are equal.

_{Ho/-hi=do/di eq (1)}

The image height is negative because it is inverted. Its is clear that the two yellow triangles in this diagram are also similar, since they are both right triangles and share the common angle ϕ. Thus , by using the same rule of similar triangles we get.

H_o/-h_i=d_o-f/f eq (2)

Comparing equation no (1) and eq (2) we get d_o/d_i = d_o- f/f

To isolate `f’ , cross multiply d_o f= d_i (d_o – f) eq (3)

Divide both sides of equation 3 by d_o d_i f, we get d_o f/d_i d_o f = d_i d_o/d_i d_o f – d_i f / d_i d_o f

Or 1/d_i=1/f-1/d_o hence it is know as mirror formula. It relates the distances of the object and the image to a concave mirror , in terms of the mirror, in terms of the mirror’s focal length. The ratio of the height of the image to the height of the object is define as the magnification, M;

M =h_i/h_o =d_i/d_o

We derived the magnification and mirror equations for a concave mirror forming a real image, but the equations apply as well to convex mirrors and to virtual images if we use the sign conventions described below.

Light travels in straight lines in a transparent material, such as air, if it passes into a different material, such as water, it changes direction at the boundary of the two material i.e it is bent.

Refraction of light is the change of direction of light as it moves from one material (called medium) to another.

Refraction of light is same as for other types such as refraction of water waves, in ripple tank.

The following terms are used describing the refraction of light

1. Incident ray:

The ray of light approaching the boundary ( refracting surface ) between two media.

2. Refracted ray:

The ray of light moving away from he boundary between the two media.

3. Point incidence:

The point at which the incident ray strikes the refracting surface

4. Normal:

The line drawn at right angles to the refracting surface at the point of incidence .

5. Angle of incidence ():

The angle between the incident ray and the normal

6. Angle of refraction ():

The angel of refraction ’ is the angle between the refracted ray and the normal.

7. Optically denser medium :

The angle of refraction is larger than the angle of incidence when light passes into a medium of a lower index of refraction, This leads to an interesting phenomenon. As the angle of incidence increases, the angle of refraction also increases.

At a certain angle of incidence known as the critical angle, the refracted light ray lies along the boundary of the two media. When light strikes a transparent boundary, even though much of the light is transparent boundary, even though much of the light is transmitted some is reflected. Total internal reflection occurs when light traveling form a region of a higher index of refraction strikes the boundary at an angle greater than all light reflects back into the region of the higher index of refraction.

To construct an equation for the critical angle of any boundary, you can use

Snell’s Law and substitute ₁ = _c and ₂=90°.

N₁ sin_c = n₂ sin90°

Since sin90° = 1

Therefore n₁ sin_c= n₂

Hence sin_c= n₂/n ₁

Thus we can define critical angle as ` the sine of the critical angle is equal to the index of refraction of the refraction of the refracting medium divided by the index of refraction of the incident medium

NO transmitted ray ≥

The two condition s required for total internal reflection to occur are as follows.

The light must travel from an optically more dense medium into an optically less dense medium

Optical fibers: light can be trapped by total internal reflection inside a bent glass rod and `piped’ along a curved path called optical fiber.

Various glasses and plastics can be used to make optical fibers, Optical fiber transmits a beam of light by means of total internal reflection. Total internal reflection can occur in any transparent medium that has a higher index of refraction than the surrounding medium.

An optical fiber cable has a cylindrical shape and consists of three concentric sections: the core, the cladding and the jacket.

The core is the innermost section and consists of one or more very thin strands, made of glass or plastic. Each strand is surrounded by its own cladding, a glass or plastic coating that has optical properties different form the core

Power of lens:

The degree of convergence or divergence of light rays falling on lens is called power of lens. Instrument makers often quote the power of a lens rather than its focal length. The power of a lens D, in diopters, a dimensionless number, is given by the equation

D = 1/f

Where f is the focal length of the lens expressed in meters. Eyeglass lenses are typically characterized in terms of diopters. The power of a lens in diopters should not be confused with the familiar concept of power in watts. It is an unfortunate fact that the word “power” is used for two completely different concepts. If you examine a prescription for eyeglasses, you will note lens powers given in diopters.

If you examine the label on a motor, you will note energy consumption rate given as a power in watts.