ASSESSMENT SHEET

NAME Shashank Jain

CLASS XII-B

SUBJECT - Physics

TOPIC Ray Optics

SUBJECT TEACHER Mukta Maam

SCHOOL City Montessori School

BRANCH Aliganj, Campus-I

INTERNAL EXTERNAL ASSESSMENT ASSESSMENT

ACKNOWLEDGEMENT

Thank you would be an understatement for those people who

have helped me through this project but since I dont have any other word in my vocabulary, so for them its a big Thank You. I dont know how much I can thank them but I can just put some of my words for them.

Starting with my institution, I would like to thank my school, City Montessori School, which provides us with not only facilities but also an environment and inspiring guides, helping us to be a better human and a better pursuer of science. Through my years in this school I have not only been enlightened to the world of science and discoveries but also to their applications in our daily life making it easier.

Of course, our Physics teacher Mukta maam has been an ever helping and inspiring figure for us, leading to an increment in the hunger of our knowledge. Dynamic during the classes, she has been actively involved in the clearance of our concepts and understanding of the topic. The best fact being that she never lets us lose the interest that keeps us bound to this subject, in fact she takes care that it is our first priority to keep ourselves interested and very much involved in it.

In any of my tasks, my familys support has to be compulsive condition as without them I am hardly anything, so forgetting them would be a sin. I dont need to say anything more.

I once again thank everyone who has made a contribution to my project.

Yours Sincerely,

Shashank Jain

INDEX

Lenses and there types 1

Lenses 2 Types of Simple Lenses 3 Focal Length and Focus 5

Finding focal length using various method 8

Lens Displacement and formula method 9 Boys Method 10

Optical Benches 13

Parallax 14

LENSES AND THERE TYPES

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Lenses: A lens is an optical device which transmits and refracts light,

converging or diverging the beam. A simple lens consists of a single optical element. A compound lens is an array of simple lenses (elements) with a common axis; the use of multiple elements allows more optical aberrations to be corrected glass or transparent plastic. Elements which refract electromagnetic radiation outside the visual spectrum are also called lenses: for instance, a microwave lens cap be made from paraffin wax.

A biconvex lens

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Types of Simple Lenses: Lenses are classified by the curvature of the two optical surfaces.

A lens is biconvex (or double convex, or just convex) if both surfaces are convex. If both surfaces have the same radius of curvature, the lens is equiconvex. A lens with two concave surfaces is biconcave (or just concave). If one of the surfaces is flat, the lens is plano-convex or plano-concave depending on the curvature of the other surface. A lens with one convex and one concave side is convex-concave or meniscus. It is this type of lens that is most commonly used in corrective lenses.

If the lens is biconvex or plano-convex, a collimated beam of

light passing through the lens converges to a spot (a focus) behind the lens. In this case, the lens is called a positive or converging lens. The distance from the lens to the spot is the focal length of the lens, which is commonly abbreviated f in diagrams and equations.

If the lens is biconcave or plano-concave, a collimated beam of light passing through the lens is diverged (spread); the lens is thus

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called a negative or diverging lens. The beam, after passing through the lens, appears to emanate from a particular point on the axis in front of the lens. The distance from this point to the lens is also known as the focal length, though it is negative with respect to the focal length of a converging lens.

Convex-concave (meniscus) lenses can be either positive or negative, depending on the relative curvatures of the two surfaces. A negative meniscus lens has a steeper concave surface and is thinner at the centre than at the periphery. Conversely, a positive meniscus lens has a steeper convex surface and is thicker at the centre than at the periphery. An ideal thin lens with two surfaces of equal curvature would have zero optical power, meaning that it would neither converge nor diverge light. All real lenses have nonzero thickness, however, which makes a real lens with identical curved surfaces slightly positive. To obtain exactly zero optical power, a meniscus lens must have slightly unequal curvatures to account for the effect of the lens' thickness.

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Focal Length and Focus: The focal length of an optical system is a measure of how

strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance.

In most photography and all telescopies, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view; conversely, shorter focal length or higher optical power is associated with a wider angle of view. On the other hand, in applications such as microscopy in which magnification is achieved by bringing the object close to the lens, a shorter focal length (higher optical power) leads to higher magnification because the subject can be brought closer to the center of projection.

The focal point F and focal length f of a positive (convex) lens and a negative (concave) lens

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge.[1]Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by aberrations of the imaging optics. In the absence of significant aberrations, the smallest possible blur circle is the Airy disc, which is caused by diffraction from the optical system's aperture. Aberrations tend to get worse as the aperture

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diameter increases, while the Airy circle is smallest for large apertures.

An image, or image point or region, is in focus if light from object points is converged almost as much as possible in the image, and out of focus if light is not well converged. The border between these is sometimes defined using a circle of confusion criterion.

A principal focus or focal point is a special focus:

For a lens, or a spherical or parabolic mirror, it is a point onto which collimated parallel to the axis is focused. Since light can pass through a lens in either direction, a lens has two focal pointsone on each side. The distance in air from the lens or mirror's plane to the focus is called the focal length.

Elliptical mirrors have two focal points: light that passes through one of these before striking the mirror is reflected such that it passes through the other.

The focus of a hyperbolic mirror is either of two points which have the property that light from one is reflected as if it came from the other.

Focal blur is simulated in this computer generated image of glasses, which was rendered in POV-Ray.

Diverging (negative) lenses and convex mirrors do not focus a collimated beam to a point. Instead, the focus is the point from which the light appears to be emanating, after it travels through

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the lens or reflects from the mirror. A convex parabolic mirror will reflect a beam of collimated light to make it appear as if it were radiating from the focal point, or conversely, reflect rays directed toward the focus as a collimated beam. A convex elliptical mirror will reflect light directed towards one focus as if it were radiating from the other focus, both of which are behind the mirror. A convex hyperbolic mirror will reflect rays emanating from the focal point in front of the mirror as if they were emanating from the focal point behind the mirror. Conversely, it can focus rays directed at the focal point that is behind the mirror towards the focal point that is in front of the mirror as in a Cass grain telescope.

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FINDING FOCAL LENGTH USING

VARIOUS METHODS

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LENS DISPLACEMT AND FORMULA METHOD:

In this experiment, we have chosen two methods to find out the focal length of a spherical lens. They are Lens formula method and Lens replacement method.

Firstly, we talk about the theory of the first method. It is

known that there is a formula called lens formula

+

object

distance u, image distance v and the focal length of a spherical lens. Therefore we record a few pairs of data of object and image distance in order to make use the formula to determine the focal length of the lens given.

Secondly, we talk about the "lens replacement method. If we separate a lamp housing and a screen to s cm, there must be two position between s which can form the image on the screen by the lens according to the reversibility of the light.

Therefore we can find out the focal length by the formula

given

where d is the distance between the two lens

positions.

Symbols Definition

u Object distance

v Image distance

f/F Focal length of lens

s Separation of lamp housing and the screen

d1 1st lens position

d2 2nd lens position

d Distance between 1st and 2nd lens positions

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BOYS METHOD: AIM:

The aim of this experiment is to determine the focal length (f) of a convex lens and its refractive index.

YOU WILL NEED:

A 10 cm focal length bi-convex lens, an optical pin (or long (8 cm) thin nail), ruler, plane mirror, sheet of matt black paper, light source for illuminating the apparatus (an adjustable desk lamp is ideal), retort stand, boss, clamp, cork.

rA

d

L A

B M

P

(a) (b)

f

Note: Diagrams not to scale relative to each other

L

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(a)

WHAT TO DO:

Setup the apparatus as shown in diagram (a) with the lens on the mirror and with the pinhead vertically above the center of the lens. Move the pin up and down until there is no parallax between the pin and its inverted virtual image formed by reflection from the plane mirror and refraction by the lens. Measure the distance between the pin and the center of the lens. This is the focal length of the lens (f).

Note: traditionally the point of the pin has been used to locate the image but this means that it could be near the experimenters eye. For safety stick the point of the pin in the cork and locate the image using the head. It is still possible to obtain an accurate reading using this method.

ANALYSIS AND CONCLUSION:

Record the value of the focal length. Repeat the experiment three times, moving the pin form the final position between each reading.

(b)

WHAT TO DO:

Set up the apparatus as shown in diagram (b) with the lens resting on the piece of matt black paper. Place the pin so that it is roughly half the focal length from the lens. Use the lamp to make sure that the pin is well illuminated.

Move the pin up and down until there is no parallax between the pin and its inverted virtual image formed by reflection from the lower face of the lens (A).

Record the distance (d) of the pin from the centre of the lens. Repeat the readings.

Turn the lens over and make a similar set of measurements using face B as the lower face.

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ANALYSIS AND CONCLUSION:

Use your results to calculate the refractive index of the glass of the lens.

When the pin has no parallax with its image the virtual object distance is -rA and the image distance is d.

These quantities are related by the equation for a lens:

+

=

+

=

From this equation rA may be found since d has been measured and f is known from part (a).

The equation for the focal length (f) of the lens involving its refractive index (n) and the radii of curvature of its two faces (rA and rB) is:

= ( )

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Optical Benches

The optical bench is commonly used in physics labs today, and consists of a long, rigid member with a linear scale applied to it. Holders for light sources, lenses and screens are placed on the apparatus so that image formation can be observed. A typical nineteenth century optical bench is shown below. This was made by the Geneva Society for the Construction of Instruments of Physics and was in the collection of Bowdoin College in Brunswick, Maine when I photographed it in 1979.

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Parallax

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek word (parallaxis), meaning "alteration". Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances.

Astronomers use the principle of parallax to measure distances to the closer stars. Here, the term "parallax" is the semi-angle of inclination between two sight-lines to the star, as observed when the Earth is on opposite sides of the Sun in its orbit. These distances form the lowest rung of what is called "the cosmic distance ladder", the first in a succession of methods by which astronomers determine the distances to celestial objects, serving as a basis for other distance measurements in astronomy forming the higher rungs of the ladder.

Parallax also affects optical instruments such as rifle scopes, binoculars, microscopes, and twin-lens reflex cameras that view objects from slightly different angles. Many animals, including humans, have two eyes with overlapping visual fields that use parallax to gain depth perception; this process is known as stereopsis. In computer vision the effect is used for computer stereo vision, and there is a device called a parallax rangefinder that uses it to find range, and in some variations also altitude to a target.

A simple everyday example of parallax can be seen in the dashboard of motor vehicles that use a needle-style speedometer gauge. When viewed from directly in front, the speed may show exactly 60; but when viewed from the passenger seat the needle may appear to show a slightly different speed, due to the angle of viewing.

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BIBLIOGRAPHY

BOOKS NCERT Physics (part I & II) Textbook for Class XII Encyclopaedias

WEBSITES

https://en.wikipedia.org/wiki/Lens_Optics#/ http://www.astronomycast.com/2014/02/ep-335-lenses/ http://www.colorado.edu/physics/2000/quantumzone/opticsp

hysics.html/

IMAGE COURTESY https://www.google.co.in/imghp?hl=en&tab=wi&ei=reLpVb-

NCsTvugSVjL5Y&ved=0CBIQqi4oAQ https://en.wikipedia.org/

SOURCES AND OTHER INFORMATION

https://www.google.co.in/ https://en.wikipedia.org/ http://www.ncert.nic.in/

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BIBLIOGRAPHY