Light - 2 | General Awareness for SSC CGL PDF Download

Optical Instruments

With the application of generalised rules of optics optical instruments are developed and used.

Simple Microscope

• It is a convex lens of small focal length used for observing magnified images of objects.
• The magnifying power of a simple microscope are given in two cases.
  When final image is formed at least distance of distinct vision (D), then
  m = 1 + D/f
  where, f = focal length of the lens.
• When final image is formed at infinity, then m = D/f

Compound Microscope

• It is a combination of two convex lenses, called objective lens and eyepiece, separated by some distance. Both lenses are of small focal lengths but f_o < f_e.
  Where, f_o and f_e are focal lengths of objective and eyepiece, respectively. 
• It is used for getting larger magnification.

Magnifying Power

The magnifying power of a compound microscope in two cases.

• When final image is formed at least distance of distinct vision (D), then
  where, v_o = distance of image formed by objective lens.
  u_o = distance of object from the objective lens
• When final image is formed at infinity (normal adjustment), then
  

Astronomical Telescope

• It is also a combination of two lenses, called objective lens and eyepiece, separated by a distance. 
• Objective lens is a convex lens of large aperture and large focal length while eyepiece is a convex lens of small aperture and small focal length. 
• It is used for observing distinct images of heavenly bodies like stars, planets, etc.

Magnifying Power

The magnifying power of an astronomical telescope in two cases are 

• When final image is formed at least distance of distinct vision (D ), then
  
  where, f_o and f_e are focal lengths of objective and eyepiece respectively.
  Length of the telescope (L) = f_o + u_e 
  where, u_e = distance of object from the eyepiece
• When final image is formed at infinity, then m = f_o/f_e
  Length of the telescope (L) = f_o + f_e

Kaleidoscope

It is a device consists of three plane rectangular mirrors of equal lengths and breadths inclined at an angle of 60° enclosed in a cylindrical tube. The reflecting surfaces of the mirrors are towards inward. Glass pieces of bright colours are placed in it. When we see through its one end, coloured symmetrical pattern is obtained which changes continuously on rotating the tube.

Periscope

• A periscope is a device composed of two plane mirrors set at a 45° angle to each other, with their reflecting surfaces oriented in opposite directions. 
• Light rays first strike the upper (or forward-facing) mirror, are reflected onto the lower mirror, and after another reflection, they travel to the observer's eyes. Periscopes are commonly used in submarines to view objects above the water's surface.

Optical Fibre

• The principle behind optical fibres is total internal reflection. An optical fibre consists of a central core with a higher refractive index, encased in a cladding with a lower refractive index, and covered by a plastic jacket. Light entering the core is repeatedly reflected within it due to total internal reflection, allowing it to travel through the fibre. 
• Optical fibres are used in decorative table lamps, telecommunications, and networking, as they can transmit multiple signals, each with a different light wavelength. In medical applications, optical fibres enable endoscopy, where fibres are inserted into a patient's stomach to observe internal structures by reflecting light through the fibres and back.

Scattering of Light

• When light travels through a medium containing particles whose sizes are comparable to the wavelength of the light, the light is scattered in various directions upon hitting these particles. This phenomenon is known as the scattering of light. Consequently, light with longer wavelengths, like red, is scattered less, while light with shorter wavelengths, like violet, is scattered more. 
• Red is used for danger signals because it scatters the least and can be seen from greater distances. 
• The sky appears blue because, in sunlight, blue light is scattered the most. This scattered blue light is what we see when we look up at the sky. 
• Clouds look white because they consist of large water droplets and dust particles that scatter all colors of light equally. Since white light is a mixture of all colors, clouds appear white. 
• From space, the sky appears black to astronauts because there is no atmosphere to scatter the light.

Interference and Diffraction of Light (Wave)

• Light is an electromagnetic wave having electric vibrations and magnetic vibrations. These are also known as electric and magnetic vectors. 
• The phenomena of redistribution of energy due to superposition of waves is called interference of light waves. The interference taking place at points of maximum intensity is called constructive interference. The interference taking place at points of minimum intensity is called destructive interference. 
• The bending of light wave around the corners of an obstacle or aperture is called diffraction of light.

Polarisation

• The phenomena of restricting of electric vectors of light into a single direction is called polarisation. Ordinary light has electric vectors in all possible directions in a plane perpendicular to the direction of propagation of light. 
• Polaroids are thin and large sheets of crystalline polarising materials. These are used in the sun glasses. In wind shield of an automobile, window panes of a train and as polaroid spectacles in camera.