Microscopes- basics of optics
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describes basics of light, and working of a light microscope.
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Microscopes- basics of optics
- 2. MICROSCOPE Presented by: Dr. SONAL AGGARWAL MDS II Year
- 3. CONTENTS • The basics properties of light. • Image formation. • The light microscope and it’s components. • Advantages and limitations of light microscope • Other microscopes
- 4. Light
- 6. • Natural light is a mixture of lights of different wavelengths. • Monochromatic light means light of one color. In scientific terms, it means light of a single wavelength.
- 7. Retardation •DEFINITION. Monochromatic ray, of plane polarized light, upon entering an anisotropic mineral is split into two rays, the FAST and SLOW rays, which vibrate at right angles to each other.
- 9. Refraction DEFINITION. Refraction is the bending of a wave when it enters a medium where its speed is different.
- 11. Refractive index DEFINITION. Refractive Index (Index of Refraction) is a value calculated from the ratio of the speed of light in a vacuum to that in a second medium of greater density.
- 12. • DEFINITION. Total internal reflection is the phenomenon which occurs when a propagated wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface.
- 13. Image formation •Focal point. •Focal length. •Conjugate foci. •Real image. •Virtual image.
- 14. Focal point & Focal length
- 15. Conjugate foci
- 17. Object is at a finite distance of more than 2f (two focal lengths) from the lens.
- 18. Object at a distance of 2f from the lens.
- 19. Object is between one and two f from the lens.
- 20. Object is less than f from the lens.
- 22. Virtual image
- 24. DEFINITION. The effect produced by the refraction of different wavelengths of light through slightly different angles, resulting in a failure to focus. Chromatic aberration
- 27. References • Bancroft’s Theory and Practice of Histological Techniques 7th Edition • Basics In Light Microscopy- Dr. Arne Seitz • Fundamentals Of Optics 4th Edition- Francis A. Jenkins • Modern Optical Engineering 3rd Edition- Warren J. Smith
- 30. MICROSCOPE- II Presented by: Dr. SONAL AGGARWAL MDS II Year
- 31. CONTENTS • The basics properties of light. • Image formation. • The light microscope and it’s components. • Advantages and limitations of light microscope • Other microscopes
- 32. DEFINITION
Editor's Notes
- Light can be described as an electromagnetic wave. Light can be described as a particle (photon). Visible light occupies a very narrow portion of 400 – 700nm between UV and Infrared radiation in the electromagnetic spectrum. Electromagnetic energy is complex, which is both wave like and particle like. The natural light we see is a complex mixture of lights with different wavelengths.
- Shorter wavelengths have higher energy for a given brightness of light. Energy content of light is expressed as energy level or amplitude based electron volts per photon. Visible light has an energy of one electron volt per photon and the energy increases as one moves towards the violet and ultraviolet range of the spectrum. It is this higher energy in the shorter wavelengths that is exploited to elicit fluorescence in some materials. The longer wavelengths can travel longer distances while shorter wavelengths can penetrate through thicker objects.
- Some of the light is absorbed by the medium through which it passes. This is seen as reduction in amplitude and energy level. The medium through which the light passes can also have an effect in the speed of light which is known as Retardation. Isotropic material: same properties throughout e.g. glass Anisotropic: varying properties: Young’s modulus; wood, composite
- ***All the electromagnetic radiations transport energy and all have a common velocity in vacuum of c 2.998 1010 cm/s.*** 1. The media through which the light passes will be able to slow down or retard the speed of the light in proportion to the density of the medium. Higher the density, greater the retardation. 2. Light rays entering a sheet of glass at right angle are retarded but their direction is unchanged.
- If the light enters the glass at any angle other than right angle, a deviation in the direction will occur in addition to retardation, known as Refraction.
- The refraction of light when it passes from a fast medium to a slow medium bends the light ray toward the normal to the boundary between the two media. A curved lens will exhibit both refraction and retardation. The extent of which is determined by angle of incidence, refractive index and curvature of the lens.
- Snell’s law- the formula Higher the density of the medium, greater the RI. The RI of most transparent substances is known and is of great value in the computation and design of lenses, microscope slides and coverslips, and mounting media. Air has a refractive index of 1.00, water 1.30, and glass a range of values depending on type but averaging 1.50.
- At a specific critical angle (θcritical) the beam of light is totally reflected from the glass/water interface, rather than passing through and refracting. The critical angle is the angle of incidence above which the total internal reflection occurs. for reflection, Snell’s law takes on the form I incident = I reflected
- Parallel rays of light entering a simple lens are brought together by refraction to a single point, the ‘principal focus’ or focal point, where a clear image will be formed of an object. The distance between the optical center of the lens and the principal focus is the focal length.
- There are conjugate foci on either side of the lens such that the object placed at one focus will produce a clear image on the screen placed at the other focus. if any object is placed at the position previously occupied by its image, it will be imaged at the position previously occupied by the object. The object and image are thus interchangeable, or conjugate.
- The conjugate foci vary in position. As the object is moved closer to the lens, image will be formed further away, at a greater magnification and inverted. This is real image. This is formed by the objective lens of the microscope. This kind of image can be projected onto a screen. The type of image produced by a converging lens depends on the distance of the object from the lens.
- The image is real, inverted, reduced, and located at a distance of between f and 2f on the other side of the lens. Applications: Lenses of the eye, camera, telescope.
- The image is real, inverted, the same size as the object, and located at a distance of 2f from the opposite side of the lens. Applications: Used to invert an image without changing its size, as in a field telescope.
- The image is real, inverted, enlarged, and located more than 2f from the opposite side of the lens. Applications: Compound microscope, slide projector, motion picture projector.
- Image is virtual, erect, enlarged, and located on the same side of the lens as the object. Applications: Magnifying glass; eyepiece lens of microscopes, binoculars, and telescopes.
- Note. (1) If the object is at infinity, the image is a point at the principal focus. (2) If the object is at the principal focus, no image is formed.
- If the object is placed yet nearer the lens, within the principal focus, the image is formed on the same side as the object, is enlarged, the right way up, and cannot be projected onto a screen. This is the ‘virtual image’. It is that formed by the eyepiece of the microscope of the real image projected from the objective.
- The image formed by the eyepiece of the microscope of the real image projected from the objective.
- White light is composed of all spectral colors and, on passing through a simple lens, each wavelength will be refracted to a different extent, with blue being brought to a shorter focus than red. This lens defect is chromatic aberration. It is possible to construct compound lenses of different glass elements to correct this fault.
- An achromatic lens usually consists of two elements. The different wavelengths of light do not meet at the same focus point, causing chromatic aberration. An achromat is corrected for two colors, blue and red, producing a secondary spectrum of yellow/green. Specifically, the difference between the green focus and the red/blue focus is called secondary color. Minimizing secondary color is the purpose of fancier refractor designs. , which in turn can be corrected by adding more lens components – the more expensive apochromat. An apochromatic lens normally has 3 or more elements. The different wavelengths meet at the same point of focus, effectively eliminating chromatic aberration. And they are designed such that the difference between the primary focus point and the focus point of the remaining colors (such as violet) is extremely small.
- Spherical aberration is caused when light rays entering a curved lens at its periphery are refracted more than those rays entering the center of the lens and are thus not brought to a common focus.