Physci reviewer

• Spacecraft Photos


  • Satellite photos show that the earth is plainly spherical
  • Astronauts in satellites orbiting in low Earth orbit personally observed curvature of the planet
• Lunar Eclipse

  • During a lunar eclipse, the shadow of the earth formed on one side of the moon shows a dark circle
  • The shape that casts in the moon shows a spherical shape
• Time Zones


  • The time difference in different places in the world is due to the presence of a time zone
  • It can only be explained if the world is round and continuously rotating on its axis
• Riding a Plane


  • Airplanes are capable of traveling in a straight line without falling off any edges even for a period of time and it can circle the earth without stopping
  • The curvature of Earth on the horizon is noticeable if you travel in a transatlantic flight
• Astronomical Observations Prior to the invention of telescopes

  • Observation of the sun and the moon
  • Observation of the rising and setting of the sun in the east and the west
  • Observation of lunar eclipse
  • Observation of solar eclipse
  • Observation of the daily and annual motion of the stars
  • Observation of constellations and planets (Mercury, Venus, Mars, Jupiter, and Saturn)
• Nebra sky disk, one of the earliest records of astronomical observation, was documented


  Approximately 1600 BC
• Supernova that resulted in the Crab Nebula was observed in a petroglyph


  1006 AD
• Hellenistic Observations


  • Eratosthenes and Aristarchus (250 BC) studied the distances between the nearby planetary objects
  • Eratosthenes introduced the spherical Earth model and tried to calculate the circumference of the earth using geometry
• Hellenistic Observations


  • Plato first introduced that the planets follow perfect circular orbits around the earth
  • Heraclides (330 B.C.) presented the first Solar System model where the earth is the center
  • Aristarchus (270 B.C.) presented an alternative model of the Solar System placing the sun at the center, which was termed as the heliocentric theory
• Kepler's Works


  • The planet's path around the sun is elliptical in nature where the sun is the center and located at one focus
  • There is an imaginary line from the center of the sun to the planet and this sweep out equal areas within equal time intervals
  • The ratio of squares of the periods between any two planets will always be equal to the ratio of the cubes of its distances from the sun
• Law of Ellipses


  • Kepler's first law describes that planets are moving around the sun by following an elliptical path
  • An ellipse is a special curve where the sum of the distances from any point within the curve to two other points is always constant
  • The other two points are termed the foci of the ellipse
• Law of Equal Areas


  • Kepler's second law describes the speed of the planet while orbiting around the sun
  • It explains that the speed of the planet within the space is always changing
• Law of Harmonies


  • Kepler's third law compares the orbital period and radius of the orbit of one planet to other planets
  • The T²/R³ ratio between the two planets is the same, validating the law of harmonies that all planets have the same value for the T²/R³ ratio
• Aristotle's Views on Motion


  • Motion is an object's change in position with respect to time
  • Natural motion occurs when an object moves and returns to its natural state depending on the object's composition
  • Violent motion occurs when an object moves after an external force, such as pushing or pulling, is applied
  • Vertical motion of an object depends on the kind of element that makes up the object
  • Heavier objects would fall faster to the ground than lighter objects
  • Horizontal motion is caused by force, which Aristotle referred to as the impetus
  • Aristotle believed that projectiles moved parallel to the ground until it was the object's time to fall toward the ground
• Galileo's View of Motion


  • Vertical motion of objects was downward due to gravity, and objects fall at the same rate even if they vary in weight
  • If the Earth's surface is very flat and extends infinitely, objects that are pushed will not be impeded and will continue to move
  • Projectiles follow a curved path called a trajectory
• Acceleration


  The change in speed per unit time
• Inertia


  The tendency of matter to resist a change in its state of motion, dependent on the mass of an object
• Law of Interaction


  When an object exerts a force on another object, the second object exerts another force that is equal and opposite in direction, called action-reaction pair
• Properties of Light


  • Reflection - The bouncing of light when it reaches a reflecting surface or a boundary between two media
  • Refraction - The bending of light due to the change in its speed when it obliquely passes two different media
  • Diffraction - The spreading of light when it encounters an obstacle or an opening
  • Interference - The combination of two or more waves into one wave whenever they pass through the same point
• Particle Nature of Light

  Light is made up of discrete packets of energy called Photons
• Destructive Interference


  Occurs when the opposite parts of two waves meet, resulting in the cancellation of the two waves
• Constructive Interference


  Occurs when identical parts of two waves meet, resulting in a new wave with the same wavelength but twice the amplitude
• Davisson and Germer's experiment on the diffraction of electrons showed one of the properties of a wave
• Electron diffraction


  • Results in a new wave with the same wavelength but twice the amplitude
  • Davisson and Germer found nothing significant until their setup was broken, then air entered and oxidized the nickel
  • Recrystallizing the nickel made small holes that served as a diffraction grating
• Electron diffraction experiment


  1. Electron beam fired by gun

  2. Beam passes through carbon disc (diffraction grating)

  3. Electrons diffract and produce ring patterns on phosphor screen
• Color perception


  • Light reaches eye, falls into receptor cell in retina
  • Optic nerve sends signals to brain, which interprets image with colors
  • Color depends on what makes up material, what colors it absorbs, and what color it reflects
• Filters


  Colored glass or cellophane that absorbs certain wavelengths of visible light
• Reflection of light


  • Objects reflect certain colors based on the source that illuminates them
  • Incandescent bulbs emit long-wavelength colors
  • Fluorescent light sources emit short-wavelength colors
• Plane mirrors


  • Reflected light waves are parallel rays
  • Looking at mirror is like looking at replica on other side of glass
• Concave mirror


  • Converges light rays at a particular point, can see upright or inverted image
• Convex mirror


  • Scatters light rays, appears to be diverging from a point on other side
• Mirage


  • Optical phenomenon that creates a displaced image of an object due to refraction of light in air
  • Happens because air just above ground has higher temperature than layer above it
• Rainbow


  • Dispersion of sunlight by water droplets in atmosphere
  • Primary rainbows from one internal reflection, secondary rainbows from two internal reflections
  • Supernumerary arcs are faint bows inside primary rainbow
• Heinrich Hertz


  • Used Maxwell's equations to create device that produced radio wave impulses
  • Radio waves are longest wavelength electromagnetic waves outside visible light
• Special theory of relativity


  Physical laws have same mathematical form for all frames of reference moving at constant velocity
• Time dilation


  • Person stationary observes slower "clock" than person travelling at speed of light
  • Significant only at relativistic speeds
  • Equation: At = At' / sqrt(1 - v^2/c^2)
• Length contraction


  • Length of object seems to contract when traveling at relativistic speeds
  • Equation: L = L' * sqrt(1 - v^2/c^2)
• General theory of relativity


  • Gravity is consequence of curvature of space-time, caused by uneven distribution of mass/energy
  • Predicts light is bent by gravity, confirmed by Eddington
  • Predicts existence of black holes
• Cosmology


  • Studies origin, evolution and fate of universe
  • Copernicus and Newton viewed universe as static, Einstein's theory changed this
• Doppler shift


  • When object gets closer, light waves compressed (blueshifted)
  • When object moves away, light waves stretched (redshifted)