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Lenses PowerPoint PPT Presentation


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Lenses. Type of lenses: Convex lens Convave lens. Refraction Rules for a Converging Lens. Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focus on the opposite side of the lens.

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Lenses

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Lenses l.jpg

Lenses

  • Type of lenses:

    • Convex lens

    • Convave lens


Refraction rules for a converging lens l.jpg

Refraction Rules for a Converging Lens

  • Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focus on the opposite side of the lens.

  • Any incident ray traveling through the focus on the way to the lens will refract through the lens and travel parallel to the principal axis.

  • An incident ray which passes through the optical centre of the lens will in effect continue in the same direction that it had when it entered the lens.

http://micro.magnet.fsu.edu/primer/java/lenses/converginglenses/index.html


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Ray Diagrams


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Refraction Rules for a Diverging Lens

  • Any incident ray traveling parallel to the principal axis of a diverging lens will refract through the lens and travel in line with the focus (i.e., in a direction such that its extension will pass through the focus).

  • Any incident ray traveling towards the focus on the way to the lens will refract through the lens and travel parallel to the principal axis.

  • An incident ray which passes through the optical centre of the lens will in effect continue in the same direction that it had when it entered the lens.

http://micro.magnet.fsu.edu/primer/java/lenses/diverginglenses/index.html


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Image Formation of Lenses

http://www.tutorvista.com/content/physics/physics-ii/light-refraction/convex-lens-formation.php


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2

1

Visual Angle

http://www.microscopy.fsu.edu/primer/java/humanvision/accommodation/index.html

  • How large an object appears, and how much detail we can see on it, depends on the size of the image it makes on the retina.

  • This, in turns, depends on the angle subtended by the object at the eye.


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Magnifying Glass (Simple Microscope)

http://www.microscopy.fsu.edu/primer/java/scienceopticsu/microscopy/simplemagnification/index.html

  • A magnifying glass allows us to place the object closer to our eye so that it subtends a greater angle.

  • The object is placed within the focus of the lens so as to produce a virtual image, which must be at least 25 cm (least distance of distinctvision or near point) from the eye.


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Magnifying Power of a simple microscope(Angular Magnification)

  • Where  is the angle subtended by the object at the near point of the eye and

  •  is the angle subtended by the image to the lens.

  • M is the ratio of the apparent sizes of the image and the object.

D


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Compound microscope

  • A microscope is used to produce an image on the retina larger than that obtainable by placing a small accessible object at the near point.

  • The overall magnification of a microscope is the product of the magnifications produced by the two lenses.


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Eyepiece

Objective

D (25 cm)

Compound Microscope in Normal Adjustment

http://webphysics.davidson.edu/alumni/MiLee/java/Final_Optics/optics.htm

  • In normal adjustment an enlarged virtual image is formed at the near point, 25 cm from the normal eye.


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Magnifying Power of a compound Microscope

  • In normal adjustment, the angular magnification equals the linear magnification

  • Where  is the angle subtended by the object at the near point of the eye and

  •  is the angle subtended by the final image at the eye.


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Resolution of Lens

  • The ability of a lens to produce distinct images of two point objects very close together is called the resolution of the lens.

  • The closer the two images can be and still be seen as distinct, the higher the resolution.

Image of pollen grain with good resolution (left) and poor resolution (right)


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Resolving Power of a Microscope

  • The resolving power of a microscope is its ability to enable detail in the image to be made out.

  • The resolving power depends on

    • The aperture of the objective

      • (The larger the aperture, the better the resolution.)

    • The wavelength of the light

      • (The shorter the wavelength, the better the resolution.)


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The Eye Ring for a Microscope

  • The eye ring is the optimum position for the observer’s eye to gather most light that passing through the objective.

  • The image is then brightest and the field of view greatest.

  • The eye ring is also the image of the objective formed by the eyepiece.

  • An observer should ideally have a pupil diameter equal to the eye ring.


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Optical Aberrations

http://micro.magnet.fsu.edu/primer/lightandcolor/opticalaberrations.html

  • Chromatic Aberration

  • Spherical Aberration

  • Coma

  • Astigmatism

  • Curvature of Field

  • Distortion

http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/basics/g13/


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Modern Microscope Component Configuration


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Refracting Telescope

  • A telescope is used to produce an enlarged retinal image of a distant inaccessible object.

  • The job of a telescope

    • Light gathering power

    • Magnifying power

    • Resolving power


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Magnifying Power of a Refracting Telescope

  • Where  is the angle subtended at the eye by the object without the telescope,

  •  is the angle subtended by the final image at the eye.


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Refracting Telescope in Normal Adjustment

  • In normal adjustment the final image seen through the eyepiece is adjusted to line at infinity so that the eye is the most relaxed.

  • In normal adjustment,

  • The length of a telescope in normal adjustment = fo+fe


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Resolving Power of a Telescope (1)

  • Resolving power of a telescope is the ability to separate two closely positioned stars.

  • Diffraction by the objective is a factor that limits the resolving power of a telescope.


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Resolving Power of a Telescope (2)

  • The resolving power of a telescope

    • depends on the quality of the optical surfaces,

    • depends on the wavelength observed,

    • increases as the diameter of the objective increases.

  • Large lenses are difficult to make and they tend to sag under their own weight.


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The Eye Ring for a Telescope

  • In normal adjustment, it can be shown that


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Reflecting Astronomical Telescope

  • Advantages of reflecting telescope:

    • No chromatic aberration

    • A mirror can have a much larger diameter than a lens

    • No spherical aberration if paraboloidal mirror is used


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Hubble Space Telescope

Eskimo nebula

Eagle nebula

HST’s primary mirror


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Terrestrial Telescope

  • An erecting lens is inserted between the objective and the eye piece to erect the inverted image formed by the objective.

  • This system has the disadvantage of increasing the length of the telescope.

  • An advantage is that it makes it possible to vary the magnification of the telescope.


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Galilean Telescope

  • Advantages:

    • The final image is erect so it is useful for terrestrial purposes.

    • It is shorter than the terrestrial telescope

  • Disadvantages:

    • Small field of view


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Spectrometer

  • The spectrometer is an instrument used for

    • Producing, viewing and taking measurements on a pure spectrum using either a prism or a diffraction grating.

    • Measuring accurately the refractive index of a material in the form of a prism.


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Construction of a spectrometer

  • The essential parts are

    • The collimator which is fixed to the base of the instrument, consisting of a slit of variable width, and an achromatic lens.

    • The turntable, which can be rotated, and to which a prism or grating can be attached. The circular edge of the table has a scale graduated in degrees.

    • The telescope, which can also be rotated. A vernier scale is fitted to the telescope where it adjoins the table, enabling their relative orientation to be measured to 0.1o, or less.


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Turntable

Diffraction grating

Collimator C

θ

Light

source

Telescope T

Eyepiece

Achromatic lenses

Cross-wire

Eye

Functions of the collimator and the telescope (1)

  • Spectrometer used to measure wavelength of light


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Functions of the collimator and the telescope (2)

  • The collimator is set to produce a parallel beam of light from the light source near the slit.

  • The telescope is set to receive parallel beam of light and hence measures the angle of deviation of light through the diffraction grating or the prism.


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Adjustments of the spectrometer

  • The eyepiece is focussed on the crosswires.

  • The objective lens of the telescope is focussed so that the crosswires are in its focal plane.

  • Using a slit of width appropriate to the source brightness, the collimator lens is moved so that the slit is in its focal plane.

  • Using the table levelling screws, the axis of the table is made perpendicular to the plane containing the principal axes of the collimator and telescope lenses.


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