Physics investigatory project

TO FIND OUT THE REFRACTIVE INDEX OF DIFFERENT SOLUTIONS BY USING TRAVELLING MICROSCOPE

KANICA CHOPRA 2014-2015

STUDENT ‘S INFORMATION

NAME

:

KANICA CHOPRA

CLASS

:

XII –A

SCHOOL

:

SRI SATYA SAI VIDYA VIHAR

ROLL NO

:

………………………………………

YEAR

:

2014-2015

CERTIFICATE OF AUTHENTICITY This is to certify that KANICA CHOPRA a student of class

XII – A has

successfully completed the research project on the topic “TO FIND OUT THE

REFRECTIVE

TRAVELLINNG

INDEX

OF

DIFFERENT

MICROSCOPE”

Under

SOLUTION the

BY

guidance

USING of

Mrs.______________________________subject (teacher) in the academic year 2014-2015.This project is absolutely genuine and does not indulge in plagiarism of any kind. The references taken in making this project have been declared at the end of this report.

(Examiner)

(Subject teacher)

ACKNOWLEDEGEMENT

This project would not have been feasible without the proper and rigorous guidance of my Physics Teacher _____________________________who

guided

me

Mrs. throughout

this

project in every possible way.

My heartiest gratitude to all of them for guiding me on a step by step basis and ensuring that I completed all my experiment with ease. Rigorous hard work has been put in this project will prove to be a breeding ground for the next generation of students and will guide them in every possible way.

INTRODUCTION This test is based on the work done by Wilson [1935] and Milk Regulations 1963. This test is used to check the contamination of bacteria in te sample of milk. It tells us about the viable count of Bacteria that may be present in the milk. So by this particular test we can have a clue about the quality of milk we have that whether it is invaded by bacteria or not and in which quantity these are present.

MILK : GOT THE OXYGEN ?

A compound microscope is a microscope fitted with two or more convex lenses. The high magnification produced by these lenses together enables a detailed study of micro-organisms, cells and tissues. These types of microscopes are therefore widely used in scientific and medical research. Zacharias Janssen, a Dutch spectacle maker, invented the compound microscope in 1590.Galileo unveiled his version in 1610. Several other scientists and inventors later helped refine its design and working capabilities. The basic design of a compound light microscope consists of convex lenses fitted at either end of a hollow tube. This tube is fitted on an adjustable, rotary nosepiece. There is an adjustable stage under the nosepiece; specimen slides are placed or fitted on this stage for observation through the lenses. The stage has a window or hole in it through which a light source can illuminate the specimen under observation.

The light source can be a mirror reflecting natural light or a lamp in the base. The illuminating beam passes through the stage window and through the specimen. The light brightens the area around the specimen, making the specimen stand out in contrast. The level of contrast is controlled by controlling the amount of illumination. A brighter or dimmer effect is achieved by opening or closing an iris diaphragm under the stage, or by adjusting the height of the lamp.

Compound microscope

The upper lenses of the compound microscope, the ones closer to the viewer's eye, are the ocular lenses or the eyepiece. Monocular microscopes have one eyepiece, and binocular ones a double eyepiece. Trinocular versions have a double eyepiece and a camera-fitting arrangement. The objective lenses are the lower lenses closer to the object being viewed. There can be three or four different ones located on the rotary nose-piece of a compound microscope. The nosepiece is rotated to select the objective lenses that offer the magnification most suited for a particular specimen. The four objective lenses are the scanning power objective, the low power objective, the high dry objective and the oil immersion objective. They have magnifications of 4X, 10X, 40X and 100X respectively. The ocular lenses usually have a magnification of 10X. To get the total magnification factor, the eyepiece magnification is multiplied with the objective magnification. So, with 10X ocular lenses and 100X objective lenses, a magnification of 1000X is achieved. This means, a viewed object is magnified 1000 times its actual size. Higher magnifications are also possible. When an object is in focus, the objective lenses form a real, inverted image of the object at a point inside the principle focus of the ocular lenses. The ocular lenses then treat this inverted image as the object and produce an upright image of it. This image is the enlarged one seen by the observer.

Refractive Index Theory What is a refractive index? The refractive index is a ratio of the speed of light in a medium relative to its speed in a vacuum. This change in speed from one medium to another is what causes light rays to bend. This is because as light travels through another medium other than a vacuum, the atoms of that medium constantly absorb and reemit the particles of light, slowing down the speed light travels at. The refractive index ( ) can be calculated using the equation below.

However, it is also important to note that light changes direction when it travels from one medium to another. Therefore, another method to calculate the refractive index of a medium is to apply Snell’s law, which will be very important later in our discussion of refractometers.

The refractive index of any other medium is defined relative to the refractive index of a vacuum, which is assigned a value of 1. Thus, a refractive index of 1.33 for water means that light travels 1.33 times faster in a vacuum than in water.

Factors that affect the refractive index: The two factors which affect the value of the refractive index are:

1. Temperature o

Refractive index values are usually determined at standard temperature.

o

A higher temperature means the liquid becomes less dense and less viscous, causing light to travel faster in the medium. This results in a smaller value for the refractive index due to a smaller ratio.

o

A lower temperature means the liquid becomes denser and has a higher viscosity, causing light to travel slower in the medium. This results in a larger value for the refractive index due to a larger ratio.

o

Refractometers usually have a means of temperature regulation.

2. Wavelength of light o

The refractive index varies with wavelength linearly because different wavelengths interfere to different extents with the atoms of the medium.

o

It is important to use monochromatic light to prevent dispersion of light into different colours.

o

The chosen wavelength should not be absorbed by the medium.

o

The sodium D line at 598 nm is the most frequently used wavelength of light for a refractometer.

Note: These two factors are present in the equation above, where t = temperature in ºC and D = the wavelength of the light used in nm.

How is a refractive index measured? A refractometer is used to measure the refractive index of a medium. There are many different types of refractometers, including the Abbe refractometer, which will be discussed in further detail below. A

refractometer works based on the principle that light bends when it enters a different medium. This instrument measures the angle of refraction of light rays passing through the unknown sample. This measurement combined with the knowledge of the refractive index of the medium directly in contact with the unknown sample, are used to determine the refractive index of the unknown sample by applying Snell’s law described above. The following cross sectional diagram illustrates the inner-workings of a refractometer. A light source shines on the illuminating prism and light rays enter the sample moving in different directions. The largest angle of incidence produced by a light ray (θi) produces the largest possible angle of refraction (θB). The other light rays entering the refracting prism all have a smaller refraction angle and lie to the left of point C. A detector at the back of the refracting prism produces the light and dark regions. In an Abbe refractometer, a detector is not present and there is more optics but the general scheme remains the same. Samples with different refractive indexes produce different angles of refraction which will cause a shift in the borderline between the light and dark regions. The borderline’s position is then used to establish the refractive index of different samples.