General Science

PHYSICS: WAVES

Overview

• A wave is a disturbance that travels across space and time
• Propagation of waves usually involves transference of energy without transferring mass. This is achieved by oscillations or vibrations around fixed locations
• Mechanical waves require a medium for transmission (e.g. sound)
• Electromagnetic waves do not require a medium and can travel in vacuum (e.g. light)
• Longitudinal waves are those with vibrations parallel to the direction of wave propagation. E.g. sound waves
• Transverse waves are those with vibrations perpendicular to the direction of travel. E.g. electromagnetic waves including light
• Waves on a string are an example of transverse waves

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Properties of waves

1. Reflection: It is the change in direction of a wave at the interface between two media. Examples include reflection of light, sound etc
2. Refraction: It is the change in direction of a wave due to a change in its speed. Examples: refraction of light when it passes through a lens
3. Diffraction: Bending of waves as they interact with obstacles in their path. Example: rainbow pattern when light falls on a CD or DVD
4. Interference: Superposition of two waves that come into contact
5. Dispersion: the splitting up of waves by frequency
6. Polarization: the oscillation of a wave in only one direction. Exhibited only by transverse waves (like light), not exhibited by longitudinal waves (like sound)

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Wave properties in everyday life

• The floor of a lake or the ocean appears closer than it actually is. This is because of refraction of light
• The red ring around the Sun is due to diffraction of light
• We can hear but not see across corners, this is because of diffraction of sound (e.g. we can hear but not see a person in the next room)
• The rainbow and the blue colour of sky are both due to dispersion of light
• Sunglasses use polarization filters to block glare

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SOUND WAVES

About Sound

• Sound is a mechanical wave that is transmitted as longitudinal waves through gases, plasma and liquids. However, in solids it can travel as both longitudinal and transverse waves
• Sound cannot travel in vacuum, it needs a medium for propagation
• The speed of sound in air is 330 m/s

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Perception of sound

• The frequency range 20 Hz to 20 MHz is known as the audible range, where human beings can detect sound waves
• The upper frequency limit decreases with age i.e. as we get older, our ability to detect higher pitches (shrills) decreases
• Other species uses different ranges for hearing. E.g. dogs can perceive frequencies higher than 20 KHz
• Increased levels of sound intensity can cause hearing damage. Hearing can be damaged by sustained exposure to 85 dB or by short term exposure to 120 dB sound. A rocket launch usually involves about 165 dB

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Sonar systems

• Sound Navigation and Ranging is a technology that uses sound propagation for navigation and communication
• Primarily used under water because light attenuates very quickly in water whereas sound travels farther
• First developed by R.W. Boyle and A.B. Wood in 1917 in Britain
• Applications include military, fisheries, wave measurement, ocean-floor mapping etc
• Sonar is used by marine mammals (like dolphins and whales) for communication as well
• Bats communicate by means of SONAR at frequencies over 100 MHz (beyond the human range)

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ELECTROMAGNETIC WAVES

Electromagnetic Spectrum

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Electromagnetic radiation and applications

Radiation	Applications
Radio waves	RADAR, TV, cell phones, microwaves
Microwaves	Wi-Fi
Infrared (IR)	Night vision, thermography, imaging
Visible light	Sight
Ultraviolet (UV)	Sun burn, water disinfection
X-rays	Astronomy, medicine
Gamma rays	PET scans, cancer therapy, astronomy, food sterilization

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Radar systems

• Radio Detection and Ranging is a technology that uses radio waves to identify moving and fixed objects
• Developed by Robert Watson-Watt in 1935 in Britain
• Radar works by measuring the waves that are reflected back from an object. Radar can detect objects at ranges where sound or visible light would be too weak
• Applications include aircraft detection, air traffic control, highway speed detection, weather detection etc

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More about electromagnetic waves

• Radio waves are reflected by the ionosphere and hence can be received anywhere on the earth.
• TV transmission penetrates the ionosphere and hence is not received like radio waves. Thus TV transmission is limited to line-of-sight
• At night, the radio reception improves because the ionosphere is not exposed to sunlight and hence is more settled
• Bats communicate by means of SONAR at frequencies over 100 MHz (beyond the human range). Other animals like dolphins and whales use SONAR as well

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CHEMISTRY: RADIOACTIVITY

About radioactivity

• It is the process by which an unstable atomic nucleus spontaneously decays (loses energy) by emitting ionizing particles and radiation
• This decay results in the atom of one type (parent nuclide) transforming into an atom of a different type (daughter nuclide)
• Eg: Carbon-14 emits radiation and transforms into nitrogen-14
• The SI unit of radioactivity is Becquerel (Bq). Another commonly used unit is the Curie (Ci)
• Radioactivity of a material is quantified by its half life. This is the time taken for a given amount of a radioactive material to decay to half its initial value
• Radiation can be measured using scintillation counters and Geiger counters

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History of radioactivity research

• Radioactivity was first discovered by French scientist Henri Becquerel in 1896
• Research in radioactivity of uranium led Marie Curie to isolate a new element Polonium and to separate Radium from Barium
• The dangers of radioactivity was discovered by Nikola Tesla in 1896, when he intentionally subjected his fingers to X-rays
• Henri Joseph Muller was awarded the Nobel Prize in Physiology or Medicine in 1946 for his discovery (in 1927) of the harmful genetic effects of radiation

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Transmutation of elements

• Isotopes: they are atoms of an element with the same atomic number but different mass number (eg uranium-238 and uranium-235)
• Isobars: elements with same mass number but different atomic number. Usually occurs when a radioactive nucleus loses a beta particle (eg. Thorium-234 and palladium-234)
• Isotones: radioactive nuclei that contain the same number of neutrons (eg. Radium-226 and Actium-227)
• Isomers: are different excitation states of nuclei. The higher-energy (unstable) element undergoes isomeric transition to form the less energetic variant without change in atomic or mass number

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Types of radioactive decay

Alpha rays can be stopped by a sheet of paper, beta rays by aluminium shielding, while gamma rays can only be reduced by a thick layer of lead

• Radioactive radiation can be split into three types of beams
• Alpha rays: they are helium particles that carry a positive charge. They have low energy and can be stopped by a sheet of paper
• Beta rays: they are streams of electrons and carry negative charge. They have higher energy than alpha rays
• Gamma rays: they are high energy rays (like X-rays) that carry no electrical charge

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Radioactivity and the Big Bang theory

• According to the Big Bang theory stable isotopes of the lightest elements (H, He, Li, Be, B) were formed immediately after the Big Bang
• Radioactive (unstable) isotopes of these light elements have long since decayed, and isotopes of elements heavier than boron were not produce at all in the Big Bang
• Thus, the radioactive materials currently in the universe were formed later and are relatively young compared to the age of the universe
• These radioactive nuclei were formed in nucleosynthesis in stars and during interactions between stable isotopes and energetic particles
• For instance, carbon-14 is constantly produced in the earth’s upper atmosphere due to interactions between cosmic rays and nitrogen

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Applications of radioactivity

• Radioisotopic labeling: used to track the passage of a chemical through the human body. Some common radio isotopes used for labeling are
  • Tritium: used to label proteins, nucleic acids
  • Sodium-22 and Sodium-36: ion transporters
  • Sulphur-35: proteins and nucleic acids
  • Phosporous-32 and Phosphorous-33: nucleotides (like DNA)
  • Iodine-125: thyroxine
  • Carbon-14 is not used for radioactive labeling due to its long half life (5730 years)
• Random number generators: based on the premise that radioactive decay is truly random
• Radiometric dating: used to date materials based on a comparison between observed abundance of radioactive isotopes and its decay products, using known decay rates. The most common methods of radiometric dating are
  • Carbon dating: when organic matter grows, it traps carbon-14. The age of the organic matter can be estimated by measuring the amount carbon-14 remaining in the body. Used for dating material up to 60,000 years old
  • Potassium-argon dating: used in geochronology and archeology, especially for dating volcanic material. Used for samples older than a few thousand years
  • Uranium-lead: one of the oldest and most refined radiometric dating techniques. Used in geochronology to estimate material from 1 million to 4.5 billion years old. A variant, the lead-lead dating scheme was used by American scientist Clair Cameron Patterson to estimate the age of the earth (4.55 billion years) in 1953

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Radioactive therapy

• Used for palliative and therapeutic treatment
• Common applications include treatment of thyroid eye disease, heterotopic ossification, trigeminal neuralgia
• In low doses, it is used for cancer treatment. However, in large doses, it can cause cancer
• Total body irradiation is used to prepare the body to receive a bone marrow transplant

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Radiation poisoning

• It is a form of damage to organ tissue due to excessive exposure to ionizing radiation
• Caused by exposure to large doses of radiation in short periods of time, or by exposure to small doses over long periods
• Increases the probability of contracting other diseases like cancers, tumours and genetic damage
• Common symptoms are nausea and vomiting
• Common occurrances of radiation poisoning include nuclear warfare, nuclear reactor accidents, spaceflight (exposure to cosmic rays), ingestion and inhalation of radioactive compounds (such as strontium in cow’s milk)
  • In Nov 2006, Russian dissident died due to suspected deliberate ingestion of Polonium-210

BIOLOGY: BIOMOLECULES

1.  Lipids
   • They are a broad group of molecules that include fats, fatty acids, sterol, waxes, glycerides and phospholipids
   • Fats are a subgroup of lipids called triglycerides
   • Cholesterol is an example of the type of lipids called sterol
   • The main functions of lipids include energy storage, cell signaling and cell structure
2. Carbohydrates
   • They are organic compounds that contain only carbon, hydrogen and oxygen
   • They belong to 3 types: monosaccharides, disaccharides and polysaccharides
   • Monosaccharides
     • Monosaccharides are the simplest form of carbohydrates, and cannot be broken down any further.
     • Eg: glucose and fructose
     • Monosaccharides dissolve in water, taste sweet and are called “sugars”
     • Used as energy source and in biosynthesis
   • Disaccharides
     • Disaccharides are compounds made by two monosaccharides bound together.
     • Eg: sucrose and lactose
     • Like monosaccharides, disaccharides dissolve in water, taste sweet and are called “sugars”
     • Used for carbohydrate transport
   • Polysaccharides
     • Polysaccharides are compounds made by complex chains of monosaccharides.
     • Eg: cellulose, glycogen
     • Used for energy storage (glycogen) and for cell walls (cellulose)
     • Cellulose is the most abundant organic molecule on Earth
3. Amino acids
   • They are molecules that contain an amine group and a carboxyl group
   • Eg: glycine, monosodium glutamate
   • They are the building blocks of proteins
   • Applications include metabolism, drug therapy, flavour enhancement, manufacture of biodegradble plastics
4. Proteins
   • They are compounds made from amino acids
   • The first protein to be sequenced was insulin, by Frederick Sanger who won the Nobel Prize in Chemistry for this in 1958
   • The first protein structures to be solved were hemoglobin and myoglobin by Max Perutz and Sir John Cowdrey Kendrew in 1958. They won the Nobel Prize in Chemistry for this achievement in 1962
   • Proteins are used as enzymes, in muscle formation, as cell cytoskeleton, cell signaling and immune responses
   • The process of digestion breaks down protein into free amino acids that are then used in metabolism
5. Nucleic acids
   • They are macromolecules formed by chains of nucleotides
   • Common examples include DNA and RNA
   • DNA (Deoxyribonucleic acid)
     • Contains two strands of nucleotides arranged in a double helix structure
     • In cells, DNA is organized into long structures called chromosomes
     • Used primarliy for long term storage of genetic information
     • DNA was first isolated by Swiss physician Friedrich Miescher in 1869
     • The double helix structure was suggested by James Watson and Francis Crick in 1953. They, alongwith Maurice Wilkins won the Nobel Prize in Physiology or Medicine for this discovery in 1962
   • RNA (ribonucleic acid)
     • Contains one strand of nucleic acids
     • Less stable than DNA
     • Used primarily for protein synthesis
     • Messenger RNA carries information from DNA to the ribosome. Translation RNA translates the information in the mRNA
     • RNA synthesis was discovered by Severo Ochoa of Spain, for which he won the Nobel Prize in Physiology or Medicine in 1959

Matching cell functions to biomolecules

Function	Biomolecule
Cell structure	Lipid
Impact protection	Lipids and proteins
Enzymes	Proteins
Energy storage	Carbohydrates, proteins, lipids
Cell movement and support	Proteins (actin and myosin)
Protein synthesis	Nucleic acids (RNA)
Hormones	Proteins
Immediate cellular energy	Carbohydrates (glucose)
Electrical and thermal insulation	Lipids
Storage of amino acids	Proteins
Genetic information	Nucleic acids (DNA)