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general science#8


Working of the Human Eye
  • Light entering the eye passes through the cornea and the pupil
  • Then, the lens focuses light onto an array of photoreceptor cells in the back of the eye, called the retina
  • There are two types of photoreceptor cells:
    • Rod cells: they are responsible for black and white vision, night vision and peripheral vision. They are more in number
    • Cone cells: they are responsible for colour vision. They are less numerous in number
Keywords: ias, upsc, civil service, study material, general studies

Defects in vision
  • Presbyopia: as people age, the lens becomes less flexible and near point recedes from the eye. As a result objects far away cannot be see. Can be corrected using a converging lens
  • Hyperopia: lens cannot decrease focal length to focus on nearby objects and so nearby objects cannot be see. Corrected using a converging lens
  • Myopia: lens cannotincrease focal length to  focus on far away objects and so farther objects cannot be seen. Corrected using a diverging lens
  • Astigmatism: occurs when the cornea is not spherical but instead elongated. Results in distorted images. Corrected using a cylindrical surface lens
Keywords: ias, upsc, civil service, study material, general studies

Applications of Mirrors
  • Kaleidoscope: A toy in which multiple images are formed by two mirrors placed inside a tube
  • Periscope: Two plane mirrors fixed facing each other  45 degrees. Used in submarines
  • Concave mirror: When used close to face gives magnified image. Used for shaving, personal care etc
  • Convex mirror: Produces smaller image but gives wider range of view. Used in rear view mirrors
  • Parabolic mirror: A concave mirror whose section is the shape of a parabola, helps in focusing. Used as reflectors in search lights, car head lights etc
Keywords: ias, upsc, civil service, study material, general studies

Optical instruments and their applications
Instrument Working principle Applications
Microscope Convex lens (converging lens) system consisting of very short focal length eyepiece and longer focal length objective Magnifying tiny objects: molecular studies
Telescope Convex lens system that provides regular magnification Magnifying distant objects: astronomy
Binocular Pair of telescopes mounted side-by-side General use
Interferometer Superposition of waves To study interference properties of light
Photometer Uses a light sensitive element (like photomultiplier) to measure light intensity Used to measure reflection, scattering, fluorescence etc
Polarimeter Light from a source passing through a polarizer and then measured Measures dispersion or rotation of polarized light
Spectrometer Works by measuring light intensity Used to measure light properties: astronomy
Autocollimator Projects and image onto a target mirror and measures deflection of returned image Component alignment, measure deflection in optomechanical systems
Keywords: ias, upsc, civil service, study material, general studies

Optics in the atmosphere

Observed effect Underlying cause Description
Blue colour of sky Rayleigh scattering Higher frequencies (blue light) get more scattered than lower frequencies
Red colour of sunrise and sunset Mei scattering Scattering due to suspended particles (like dust) when sun’s rays have to travel longer distance
Halos/afterglows Scattering Scattering off ice particles
Sundog Scattering Scattering off ice crystals causing bright spots on the sky
Mirage Refraction
Novaya Zemlya effect Refraction Sun appears to rise earlier than predicted
Fata Morgana Refraction due to temperature inversion Objects beyond the horizon can be seen elevated
Rainbow Total internal reflection
Keywords: ias, upsc, civil service, study material, general studies

Optics for photography

Desired effect Approach
Close up Use macro lens
Long shot Telephoto lens
Panoramic pictures Wide angle lens
Handle low light conditions Increase exposure time (decrease shutter speed)
Fast moving objects Decrease exposure speed (increase shutter speed)
Increase depth of field (foreground and background both in focus) Increase aperture i.e. f-number
Keywords: ias, upsc, civil service, study material, general studies

Optical Fibres

  • Optical fibres are glass or plastic fibre that carries light
  • Advantages include
    • low signal loss
    • immunity from electromagnetic interference
    • higher bandwidth (data rate)
    • low power consumption
  • Optical fibres work on the principle of Total Internal Reflection
  • Applications include long distance communication, endoscopy, light decorations etc
Keywords: ias, upsc, civil service, study material, general studies


About fertilizers

  • Fertilizers are soil amendments applied to promote plant growth
  • Can be applied to soil or directly to leaves
  • Main nutrients in a fertilizer are nitrogen, phosphorous and potassium
Keywords: ias, upsc, civil service, study material, general studies

Synthetic fertilizers

  • Synthetic fertilizers are manufactured using the Haber-Bosch process to produce ammonia, which is then used to manufacture other nitrogen fertilizers
  • Urea is the most commonly used fertilizer. It has the highest nitrogen content
  • Synthetic fertilizers do not replace trace minerals in the soil (eg Zinc, copper, magnesium etc)
  • Production of synthetic fertilizers is highly energy intensive. The production of synthetic ammonia currently accounts for 5% of global natural gas consumption
  • Excess and unregulated use of synthetic fertilizers can cause Fertilizer Burn, in which plant tissues die due to excess nitrogenous salts
Keywords: ias, upsc, civil service, study material, general studies


  • Include naturally occurring minerals such as manure, worm castings, compost, etc.
  • Primary sources of biofertilizers are
    • Bacteria: Rhibozium, Azopirillum
    • Fungi: Mycorrhiza
    • Fern: Azolla
  • Cover crops can also be used to enrich soil between plantings of the main crop. Cover crops work through the principle of nitrogen fixation: i.e. convert atmospheric nitrogen into a plant-accessible form
  • Minerals such as limestone, rock phosphate and sulphate of potash can also be used
  • Biofertilizers release their nutrients much slowly compared to synthetic fertilizers and thereby prevent Fertilizer Burn
  • In addition to improving crop yields, biofertilizers also improve the health and long-term productivity of soil
Keywords: ias, upsc, civil service, study material, general studies

Environmental effects of fertilizer use

  • Oxygen depletion: Nitrogen compounds in fertilizer run-off are primarily responsible for serious oxygen depletion in oceans and lakes. This lack of dissolved oxygen causes serious damage to aquatic life in lakes and along coastal areas. Also leads to discolouration of water (green, yellow, red, brown)
  • Soil acidification: Nitrogen containing synthetic fertilizers cause soil acidification
  • Heavy metal accumulation: Synthetic fertilizers, especially those based on phosphates, can contain significant amounts of cadmium, uranium, zinc, lead and radioactive polonium, all of which can be stored in plant tissues, and later enter the food chain in the form of produce
  • Greenhouse gases: Due to the large scale use of fertilizers, nitrous oxide has now become the third most important greenhouse gas (after carbon dioxide and methane).
Keywords: ias, upsc, civil service, study material, general studies


  • A pesticide is a substance that is used to kill pests
  • Pests can include insects, molluscs, birds, weeds etc
  • In addition to preventing crop losses due to pests, pesticides can kill disease-spreading mosquitoes, allergy inducing bees or wasps, and also to control algae levels in lakes
  • Due to its negative effects on birds, DDT has been banned as a pesticide for agricultural use under the 2001 Stockholm Convention. However, it is still used in developing countries for malaria prevention and other vector control
Keywords: ias, upsc, civil service, study material, general studies

Commonly used pesticides

Used to control
Copper sulphate, barley straw
Strychnine, DRC1339, parathion (in diesel oil)
Chlorine, iodine, oxygen, alchohol, phenol
Fungi, oomycete (water molds)
Sulphur, neem oil, tea tree oil, rosemary oil, milk
Dichlorophenoxyacetic acid (2,4D), atrazine, glyphosate
Organochlorine, organophosphates, carbamates, pyrethroids
Methoprene, permethrin, dicofol
Molluscs (slugs and snails)
Metal salts (iron phosphate, aluminium sulphate), metaldehyde
Nematodes (worms)
Nematophagus fungi, neem cake
Anticoagulants, metal phosphides, hypercalcemia
Keywords: ias, upsc, civil service, study material, general studies

Environmental effects of pesticides

  • Over 98% of insecticides and 95% of herbicides reach a destination other than their target species
  • Pesticides contaminate land and water when they run-off from fields, when discarded, sprayed etc
  • Air pollution: pesticide drift occurs when pesticides suspended in the air get carried away to other areas. Pesticides also react with other chemicals to produce ozone, accounting for about 6% of total ozone production
  • Water pollution: run-off and eroding soil lead to pesticide pollution of water. This affects water solubility, and also the pesticides enter the food chain through water. Some pesticides are toxic to fish, kill off zooplankton (the main food source for fish). Harmful to amphibians such as tadpoles and frogs
  • Soil contamination: nitrogen fixation is affected by pesticides in soil. Pesticides also kill bees and are responsible for pollinator decline, leading to decreased crop yields. Widespread use of pesticides eliminates animals’ food sources causing them to change food habits or starve. Pesticide poisoning also travels up the food chain
Keywords: ias, upsc, civil service, study material, general studies

Pest resistance and rebound

  • Pests may evolve to become resistant to pesticides
  • Managed through pesticide rotation
  • Mixture of pesticides may also be used
  • Certain pests sometimes themselves act as pesticides in the sense that they control other pests. In this case pesticides that target one pest species may lead to a secondary pest outbreak due to the other species
  • Also, sometimes use of pesticides may affect natural enemies of the pest more than the pest itself. In this case, the pesticide may lead to temporary decrease in pest populations, but in the long-term the pest population may increase due to the absence of its natural enemies (especially for mosquitoes). This is called pest rebound.


History of genetics research
  • The father of genetics is Gregor Mendel (Austria-Hungary). In 1866 he published the principle known as Mendelian Inheritance which described the concept of inheritance between parent organisms and offspring
  • In 1869 Friedrich Miescher (Switzerland) discovered DNA
  • 1880: Walther Flemming (Germany) describes division of chromosomes
  • 1933: Jean Brachet (Belgium) establishes DNA is found in chromosomes and RNA in cell cytoplasm
  • 1944: Oswald Theodore Avery, Colin McLeod, Maclyn McCarty (US) identify DNA as genetic material
  • 1953: James Watson and Francis Crick (US) establish double helix structure of DNA. They win the Nobel Prize in Physiology or Medicine in 1962 for this discovery
  • 1968: Hargobind Khorana, Robert Holley and Marshall Nirenberg (US) demonstrate the role of RNA in protein synthesis. Nobel in Medicine 1968
  • 1977: Frederick Sanger (UK) sequences DNA for the first time. He produce the entire genome of bacteriophage X174. Nobel in Chemistry in 1980
  • 1983: Kary Mullis (US) discovers polymerace chain reaction enabling easy amplification of DNA. Nobel in Chemistry 1993
  • 1995: The genome of Haemophilus influenzae is the first genome of a living organism to be sequenced
  • 2001: First draft sequence of the human genome
  • 2003: Human Genome Project successfully completed
  • DNA (deoxyribonucleic acid) is the basis for genetic inheritance. However, certain viruses use RNA for genetic information
  • DNA is composed of a chain of nucleotides. There are four types of nucleotides: adenine (A), cytosine (C), guanine (G), thymine (T)
  • DNA usually exists in a double-helix structure molecule
  • Each nucleotide in one strand of the double-helix pairs with a particular partner nucleotide in the other strand. A pairs with T and C pairs with G
  • Genes are regions within DNA. Genes are arranged in long chains of DNA molecules. These chains are called chromosomes
  • Eukaryotic organisms have DNA arranged in multiple such chromosomes. Bacteria have only one chromosome
  • The combined DNA sequence of all chromosomes is called the genome. This contains all hereditary information of that organism
  • Haploid organisms have only copy of each organism. Eg: male bees, wasps, ants
  • Diploid organisms have two copies of each chromosome. Eg: most plants and animals (including humans). However, in male humans the sex-linked X and Y chromosomes exist only as a single copy.
  • Male: XY, Female: XX
Human Genome Project
  • The Human Genome Project was an international scientific effort to determine the complete genetic code of human beings
  • Launched in 1990, complete results published in 2003
  • Performed by scientists from US, UK, Canada and New Zealand, lead by University of California Santa Cruz
  • Key findings of the project include
    • There are approximately 25000 genes in human beings
    • All human races are 99.99% alike genetically
    • Most genetic mutation occurs in male. Thus males are responsible for genetic evolution and for genetic disorders
  • Human Genome Project mapped nucleotides in haploid sequences. Efforts are currently underway to map diploid sequences as well. Eg: International HapMap Project, Personal Genome Project


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