Optical Instruments
Magnifying Glass
A converging lens functions as a simple microscope, also known as a magnifying glass. When an object is placed between the optical center and the principal focus of the lens, the image formed is:
- Magnified
- Erect
- Virtual
Magnifying glasses are commonly used for reading small text and examining specimens in laboratories.
The Compound Microscope
The compound microscope provides greater magnification than a simple microscope. It has two lenses:
- Objective lens: Has a short focal length and forms an image.
- Eyepiece: Acts as a magnifying glass to view the image formed by the objective lens.
Credit: PhysicsMax
The image formed by the objective lens is within the principal focus of the eyepiece, creating a final magnified image at the least distance of distinct vision.
The Astronomical Telescope
An astronomical telescope is used to view distant objects like stars and planets. It consists of two convex lenses:
- Objective lens: Has a long focal length and forms a real image of a distant object at its focal plane.
- Eyepiece: Positioned so that the final image is at infinity.
Credit: PhysicsMax
The telescope produces an inverted image, which is acceptable for astronomical observations but less practical for terrestrial use.
The Human Eye
The human eye’s optical system includes the cornea, aqueous humor, vitreous humor, and lens, which form a real and inverted image of an object on the retina. The retina transmits this image to the brain through the optic nerve, and the brain interprets it. The iris regulates the amount of light entering the eye through the pupil.
Long Sight (Hypermetropia)
People with long sight (hypermetropia) can see distant objects clearly but struggle with close objects. Their near point is beyond the normal 25 cm due to a short eyeball, causing light rays from nearby objects to focus behind the retina. This condition is corrected using a converging lens, which helps focus light properly on the retina.
Short Sight (Myopia)
Short-sighted individuals (myopia) cannot see distant objects clearly because light rays from far objects focus in front of the retina. Their far point is less than infinity. This condition is corrected with a diverging lens, which adjusts the focus so distant objects appear clear.
Credit: PhysicsMax
The Camera
Credit: PhysicsMax
A camera captures photographs by focusing light onto a film or sensor. It consists of:
- Light-proof box: Houses the components, with a converging lens at the front and a light-sensitive film at the back.
- Adjustable lens: Varies the distance between the lens and the film for proper focus.
- Shutter: Opens and closes at variable speeds to control light exposure.
- Aperture: Regulates the amount of light reaching the film.
- Diaphragm: Adjusts the size of the aperture.
To take a photograph, the camera lens is pointed at an object, and the focus is adjusted using the focusing ring. Pressing the shutter button briefly exposes the film to light, capturing the image.
Comparison of the Human Eye and a Camera
Similarities
- Both have an enclosed dark chamber (the eye has a black-pigmented interior, and the camera has a light-tight black box).
- Both contain a light-sensitive surface (the retina in the eye and film in the camera).
- Both use a converging lens system to focus light from external objects.
- The iris in the eye functions similarly to the diaphragm in a camera, regulating light entry.
- The pupil in the eye serves the same purpose as the camera aperture.
Differences
| Human Eye | Camera |
|---|---|
| Lens has a variable focal length. | Lens has a fixed focal length. |
| Image distance is fixed. | Image distance can be adjusted. |
| A biological organ. | A mechanical device. |
| Can suffer from vision defects. | Does not experience defects like the human eye. |
| Fixed position within the eye socket. | Can be moved and positioned freely. |
Slide Projector
A slide projector uses a convex lens to project a real, magnified, and inverted image of a slide or film onto a screen. The slide (object) is positioned between one focal length (f) and twice the focal length (2f) from the projection lens.
Credit: PhysicsMax
Essential Parts of a Slide Projector
- Light Source and Converging Mirror: A small, powerful light source with a converging mirror behind it directs light rays toward the slide.
- Condenser: A combination of two plano-convex lenses that collects light from the source and converges it onto the slide, ensuring uniform illumination. It also helps protect the slide from heat.
- Slide Carrier: Holds the slide in an upside-down position so that the projected image appears upright on the screen.
- Focusing Lens: Placed near the main projection lens to adjust the clarity of the image.
- Projection Screen: A white screen that receives the final image.
Optical Fibre
Optical Fibre Cable as a Wave Guide
Optical fibre cables are used as waveguides for transmitting electromagnetic waves at optical frequencies. These frequencies are on the order of \( 10^{15} \) Hz, which is much higher than the radio wave frequency of \( 10^{10} \) Hz. The amount of information transmitted is directly proportional to the frequency of the signals. As a result, over 15,000 simultaneous telephone conversations can be transmitted through a single optical fibre cable. In contrast, a single pair of metallic cables can only handle 48 simultaneous telephone conversations transmitted as electric currents.
An optical fibre cable consists of a cylindrical core made of silica glass with a radius ranging from 0.0002 cm to 0.001 cm. Around this core, a cladding layer is formed by adding small amounts of boron, germanium, or phosphorus, creating a rarer medium. The cladding is essential for retaining light waves within the core and for providing mechanical strength. A protective buffer jacket made of plastic surrounds the cladding to prevent moisture damage and scratches. Additionally, a tough Kevlar layer provides tensile strength, and the entire cable is enclosed in a black polyurethane outer jacket to ensure flexibility.
Mode of Operation
When an optical signal enters one end of the fibre, it strikes the interface between the core and the cladding. If the angle of incidence is greater than the critical angle, total internal reflection occurs continuously within the core, as illustrated in the diagram. Eventually, the signal emerges from the other end of the fibre, effectively allowing the optical fibre to function as a waveguide.
Credit: PhysicsMax
Advantages of Optical Fibre Cables (O.F.C)
- The fibre material is made of silica glass, which is abundantly available compared to copper or aluminum.
- Silica is significantly cheaper than copper.
- Since no electric current flows through the fibre, there is no risk of electrical hazards.
- Optical fibres are lightweight, small, and flexible, making them easier to handle.
- The optical signal transmitted through the fibre is not affected by radio frequency interference.
- It is impossible to tap into optical fibre systems, ensuring secure communication.
- Optical fibre cables can transmit a much larger number of signals simultaneously compared to metallic cables.
- Energy loss in optical fibre cables is significantly lower than in metallic cables. The loss in optical fibres is approximately \( 12.5 \) dB per km.