W6

• How Colors are Perceived by the Human Eye

  • When light reaches the eye, it falls into a receptor cell in the retina
  • The optic nerve sends signals to the brain, which interprets the image with colors

  
• How Colors are Perceived by the Human Eye

  The color perceived by the eye depends on what makes up the material, what colors it absorbs, and what color it reflects that reaches the human eye

  
• Filter


  A 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
  • Different light sources do not emit the colors with the same proportion

  
• Reflection of Light


  • Incandescent light bulbs emit long-wavelength colors such as red, orange, and yellow
  • Light waves coming from fluorescent light sources are mostly short-wavelength colors, such as blue and violet

  
• The colors of the objects change when shone with different light sources
• Reflective and Refractive Illusions


  • The human brain tends to assume that it can draw a straight line toward all objects being observed
  • When light bends unexpectedly, illusions are created
• Plain Mirrors


  • The reflected light waves in plane mirrors are parallel rays
  • Looking at the mirror is like looking at your replica on the other side of the glass

  
• Concave Mirror


  • A concave mirror converges the light rays at a particular point in front of it, with you seeing an upright or inverted image, depending on whether you are looking at the light before or after it reaches the converging point

  
• Convex Mirror


  • When light rays strike a convex mirror, the rays tend to scatter and appear to be diverging from a point on the other side of the reflecting surface

  
• Mirage


  An optical phenomenon that creates a displaced image of an object due to the refraction of light in the air

  
• Mirage happens because the air just above the ground has a higher temperature than the layer of air above it
• Mirage


  • On a highway mirage or heat haze, you will notice how the distant objects are reflected upside down as if there is a body of water
• Scattering of Light


  • Light passes through the gases in the atmosphere and ends up being scattered
  • The color of the sky that you can see is the scattered light being sent toward you
  • The atmosphere is made of several gases, mainly nitrogen and oxygen, which can scatter the light coming from the sun
  • The different colors are scattered differently when the white light from the sun enters the atmosphere. Those with short-wavelengths are scattered the most, whereas those with long-wavelengths are scattered the least

  
• Blue Sky


  • The blue light passing through the atmosphere is scattered in all directions when the sun is at or near the zenith
  • Zenith is the point in the sky directly above the observer's location
  • People see most of the sky as blue
  • You can see blue light during the day because the atmosphere scatters the blue color the most
• Sunset and Sunrise


  • During sunrise or sunset, the sky appears as shades of red, orange, and yellow
  • Colors with longer wavelengths are scattered the least. Thus, they are the ones reaching your eyes

  
• White Clouds


  • Clouds are masses of water droplets floating in the atmosphere
  • It scatters light differently from that of the surrounding atmosphere
  • Clouds appear white because the water droplets in the clouds scatter all the color frequencies of white light equally
• Rainbow


  • Rainbow is the dispersion of sunlight by the water droplets in the atmosphere. It usually appears after rainfall
  • Primary rainbows are caused by sunlight undergoing one internal reflection in the water droplets as it splits into colors
  • Secondary rainbows, a dimmer rainbow usually found above the brighter primary rainbow, are caused by sunlight undergoing two internal reflections in the water droplets as it splits into colors
  • Supernumerary arcs are bands of faint bows inside the primary rainbow

  
• Halo


  • A light phenomenon that happens when light shines through clouds composed of ice crystals
  • It is also called gloriole, icebow, or nimbus
  • A halo is usually a bright white ring but may also have colors due to the dispersion of light upon striking the ice crystals

  
• Parhelia


  • Commonly called as sundogs
  • These two bright spots seen on either side of the sun or the moon, usually visible when they are close to the horizon
  • Parhelia happen due to the refraction of light upon hitting the small crystals that make up cirrus or cirrostratus clouds
  • The spots are formed on either side of the sun or the moon when light strikes them at a minimum angle of 22°

  
• Your reflection on the concave and convex sides of a spoon looks different
• Light from a red laser passes more easily through red cellophane than green cellophane
• Clothing of certain colors appear different in artificial light and in sunlight
• Haloes, sundogs, primary rainbows, secondary rainbows, and supernumerary bows are light phenomena

  
• Clouds are usually white and rain clouds dark
• The sky is blue and sunsets are reddish

  
• Hertz

  Produced radio pulses
• Special Relativity

  • Relativity of simultaneity
  • Time dilation
  • Length contraction
  • Mass-energy equivalence
  • Cosmic speed limit
• General Relativity

  • Correct predictions of shifts in the orbit of Mercury
  • Gravitational bending of light
  • Black holes
• Maxwell's equations explain the relationship between electricity, magnetism, and light
• Changing electric field induces a magnetic field
• Changing magnetic field induces an electric field
• Changing electric fields and magnetic fields can mutually induce each other, causing them to propagate at a speed of c in the form of electromagnetic waves
• Radio waves

  Electromagnetic waves with the longest wavelength outside the visible light range
• Hertz's device to produce radio waves

  1. Induction coil produces sparks between brass balls

  2. Bent loop of wire with a small gap
• Special Relativity

  Explains how motions can be compared in different inertial frames

  
• Postulates of Special Relativity

  • Physical laws have the same mathematical form for all frames of reference moving at a constant velocity
  • Speed of light in a vacuum is independent of the motion of its source and of the observer

  
• Special Relativity is only applicable to uniform motion, not change-in-curvature motions
• Length contraction

  The length of an object seems to contract when travelling at relativistic speeds
• Time dilation

  A person who is initially stationary tends to observe a slower "clock" than a person travelling at a speed of light