TEXTBOOK – Chapter 8
Almost half of the brain is dedicated to visual info processing.
Ether: all-encompassing, massless fabric of space. Humans used to believe that light was emitted by objects
and that the light travelled through the ether in straight lines.
Diffraction: bending of waves around small obstacles.
Dual theory of light: theory that light is composed of particles known as photons that have wave-like motion
when they travel. Remains the cornerstone of our current understanding of light.
Electromagnetic radiation: Maxwell came up with this. Energy is radiated in the form of a wave, comes from
motion of oscillating charged particle (photon).
The movement of photons through space produces an oscillating electric and magnetic field (they move
together in the same direction as photon).
SO, electromagnetic radiation is made up of 2 moving wave forms, electric and magnetic field (linked
together). The 2 fields are perpendicular to each other and they fluctuate in intensity together. This basically
represents the waves that move through space.
Wavelength is one cycle (which later repeats itself). Also known as lambda.
Gamma rays have shortest wavelength. ELF waves have longest.
Only a small segment of the electromagnetic spectrum is actually used for sensory function.
Blue: short wavelengths. Green+yellow: middle wavelengths, red: long wavelengths.
Why can’t we see a broader range of wavelengths?
- The sun is the major source of electromagnetic radiation, and its emission spectrum is restricted to a
narrow range of wavelengths
- Visible light spectrum falls comfortable within emission and transmission spectra, so being able to see
that is most important.
Point source: small light source from which light radiates outwards in all directions.
Wave fronts: representation of light propagation in space at a particular point in time through a series of
expanding concentric circles.
Optical infinity: A distance of 6 metres or more at which wave fronts flatten out (lines become parallel)
Light ray: the straight line path of a photon as it travels outwards from the light source.
Inverse square law: the amount of light passing through a specific area is inversely proportional to the square
of the distance. 1/d squared.
One of 2 things can happen when light hits surface of opaque object: absorption or reflection. - Absorption only when its atoms can convert energy to vibrational motion (heat).
- If it can’t create vibration (cuz of nature of the object, way it interacts with light), light will just be
reflected by the object.
- If transparent, light simply passes through (transmission)
The colour of an object can be attributed to the wavelengths of light reflected back and captured by our eyes
(maybe some wavelengths are absorbed, some reflected)
- Objects that reflect all wavelengths appear white.
- Objects that absorb all wavelengths appear black.
Light scattering: when light interacts with gaseous particles. Light is first absorbed by particle, then re-emitted
in a random direction.
- Rayleigh scattering: when very small particles in upper atmosphere (like nitrogen and oxygen) absorb
light, re-emit in random direction. Scatters short wavelengths.
- Sunlight interacts with gas molecules throughout upper atmosphere, resulting in a far greater amount
of diffuse short-wavelength radiation throughout sky. This is why the sky is blue. At sunset/sunrise,
light must travel greater distance within atmosphere, so greater scattering. More of the short
wavelengths are scattered away, leaving greater proportion of long wavelengths to penetrate
atmosphere and reach our eyes (red).
- Mie scattering: Due to larger particles like dust, smoke, water vapour. Not in upper atmosphere but
lower. Doesn’t discriminate between wavelengths. Reason for white glare around sun.
- Non-selective scattering: when particles are much larger than wavelengths. Equally affects all
wavelengths too. Clouds appear white because water droplets and ice crystals in them produce
scattering of light.
Plane-polarized light: A type of light in which the electric field is restricted to only one plane of oscillation.
Unpolarized light: array of electrical fields in all possible orientations. Direct sunlight, artificial light, etc.
How to polarize light? 1) light that is scattered by a particle can be polarized along a certain plane. 2) by
transmission through certain crystals that absorb more light in one plane than another. 3) under certain
conditions reflection polarizes light too.
Sunglasses: the polarizing material absorbs light in all planes except vertical. Also there’s a reduction in the
overall intensity of sunlight passing through, since only a small fraction of the light is contained in the vertical
plane. They also reduce the glare of reflected light!
Refraction: the bending of light that occurs when it travels from one medium to another.
- Due to change in speed of light as it moves through more dense(slower)/less dense(faster) mediums.
Refractive index: the greater the index, the slower the speed.
Medium must be transparent though otherwise light won’t be transmitted through it.
The greater the difference between 2 media’s indexes, the greater the amount of refraction that will occur. Light must also strike the surface at an angle away from the “normal”, which is the line perpendicular to