what is optics
branch of physics which describes the properties + behaviour of light + the interaction of light w matter
history of light
wave vs particle
- Hooke & Huygens proposed that light existed as series of waves in hypothetical medium called ‘luminiferous aether’
- Isaac Newton argued that light is comprised of particles which are emitted in all directions from a light source
- light also only travels in straight lines + so can’t be a waveform as they were known to bend around obstacles
- Faraday believed light was a high-frequency electromagnetic vibration, which could propagate even in absence of a medium
- Hertz found that radio waves has same properties of visible light
e.g. could be refracted (bend through medium) + reflected (bounce off medium)
wave vs particle investigations
by 19th century, wave theory generally accepted but could not explain some phenomena regarding light
- Michelson-Morley experiment
- photoelectric effect experiment
Michelson-Morley experiment
hypothesis
- believed that light needed a medium to move in - ‘luminiferous aether’
- if aether exists, light should travel faster when moving along aether than when moving against
- devised experiment that split beam of light in 2
Michelson-Morley experiment
results
photoelectric effect experiment
hypothesis
- if light is a wave, then any energy transfer would be related to intensity
photoelectric effect experiment
results
conclusion
- Albert Einstein rvised Newton’s laws of motion to explain the constant speed of light revealed by M-M experiment
- explained photoelectric effect by resurrecting the particle theory of light
- concluded that light is a wave-particle duality; exhibits properties of both waves + particles
quantum theory
- idea that visible light and other electromagnetic radiation is emitted in discrete packets of energy called quanta
- photon
- exhibit wave-particle duality
quantum theory
the idea that
- the energy of a photon is proportional to its frequency
- and inversely proportional to its wavelength
i.e. shorter wavelengths = higher energy + vice versa
quantum theory
equation
E = hv = (hc)/λ
where:
E = energy of a photon
h = Planck’s constant
v = frequency of the wave
c = speed of light
λ = wavelength of light
frequency
- denoted by v
- no. of complete waves produced in 1 second
- measured in Hertz (Hz)
- difference between 2 peaks/troughs
speed of light
- velocity of light (v) is related to the frequency (v) and wavelength (λ)
- units = ms-1
types of light sources
natural - e.g. sun
artificial - e.g. lamps, lasers
point - emits in all directions (almost never exist)
extended
electromagnetic spectrum
from left to right; short to long λs
- λ measured in nanometres (nm)
- 1nm = 1 x10-9 m
- monochromatic = light comprised of 1 λ
visible spectrum
- anything the human eye can detect
- from ~400 to ~800
electromagnetic spectrum
shorter λ; higher frequency; higher energy
- cosmic rays
- X-rays
- UV
- visible
- infrared
- micro-waves
- radio-waves
longer λ; lower frequency; lower energy
white light
- not actually white
- combination of some of the visible wavelengths of light
- can be split into its constituent colours by a prism; dispersion
- sun emits all λs of light
how does light interact w objects?
- transmission/refraction
- absorption
- reflection
how do we perceive colour
- white light from sun reaches object
- some λs get absorbed by the object
- some λs are reflected by the object
- the reflected λs are what we perceive as colour
i.e. a yellow object is absorbing all the λs except the ones corresponding to the colour yellow
absorption spectra
- determines the absorption of a material
- can be measured by a spectroscope
- black lines represent what the sun absorbs; can be used to detect materials/gases that the sun contains/emits
- called Fraunhofer lines
emission spectra
- if you raise these same gases to ‘incandescence’ they will emit the same wavelengths which can be measured as the emission spectra
-
continuous vs discrete spectra
continuous = λs seamlessly blend together
discrete = individual λs are visible
photopic sensitivity curve
