Lecture 23 - Means of Light Flashcards
(42 cards)
What is light?
Transverse electromagnetic waves that extend from gamma rays (high energy) to radio waves (low energy).
-If undisturbed, these propagate in straight lines called “rays” (geometrical optics)
What is the the wavelike representation of light ?
accounts for a number of diffraction and interference phenomena
What is the particle or quantum nature of light is particularly useful to?
describe the exchange of energy when light is absorbed and scattered.
When does light exist ?
only when we can detect it
What is one wavelength indicated by in a graph?
a sinusoidal variation in electric field associated with a photon
What is the total distance traveled every second of one wavelength (SPEED OF LIGHT) ?
c= lamda (WAVELENGTH) x v (frequency of radiation)
What is c in a vacuum?
- 998 x 10 ^8 ms-1
- high due to v being high
What does each photon /and each particle have?
- has small amount of energy associated by it and it is only determined by its frequency nu
What is SE equation?
SE = hv
What does the higher frequency mean?
the higher the energy
What is h?
planks constant
6.626 x 10_34 Js
What are the 3 different ways of describing light?
- wavelike nature
- rays
- particle aspect of light which allows us to understand the energy in each photon.
What are the 3 typical components of a photometric system ?
- Source- of electromagnetic radiation which may be the sun (most important), a light emitting diode, a candle or an electric lamp- it then travels the light until it hits a detector which is the time when light is detected.- detecting light means absorbing energy- which means destroying photons.
- A detector - which might be the human eye, or a photocell, or a tomato plant
- A filter or modulator -which is anything in between these two, such as the atmosphere, the macular pigment in the eye, or a slit aperture or filter in an optical system- (it absorbs mostly short wavelength of lights) - after absorption of these filters including lens of human eye- light is converted into electrical signals by absorption in photoreceptor pigments.
What is a very important element?
detector of radiation
What are the decetors of the human eye ?
photoreceptor pigments
How is the most important detector of the eye formed?
by combining. signals from long wavelengths ( L cones ) and medium wavelengths (M )
-They are usually in ratio of 2L : 1M = Photopic
What does the average human retina contain ?
twice as many long wavelength cones (L CONES) than M cones
What is at low light levels?
rod receptors indicated by small diameter photoreceptors
What is the density of photoreceptor in centre fovea ?
- lots of cones a
- Note the size of cone receptors in the fovea and the absence of rod receptors.
What do the cone receptors do?
increase rapidly in diameter and end up having a larger diameter than surrounding rod photoreceptors
What is the density of photoreceptors 1.2 mm in the periphery (RHS section)?
more rods
-Note the large number of rods in the near periphery and the fewer, but much larger cones.
What is the ideal detector of radiation ?
-Spectral responsivity, V(lamda), linear response, large dynamic range and high sensitivity.
What are the good properties of the ideal detector of radiation ?
- spectral repsonsiovity- which is the spectral range over which you get a measurable response
- the response linearity of the detector - its dynamic range which is arranged by levels over which the detector produces a linear response
- The sensitivity of detector- which is the smallest amount of radiant flux for which the detector produces a measurable response
How can we measure the spectrum responsively of the detector?
- a graph
- use silicion light detector
- we allow light of a particular wavelength to fall on its surface and measure the signal generated
- usually when you produce a spectrum the amount fo radiant flux over a narrow range of wavelength varies as you move the slit over the visual spectrum
- so you may get more light in the long wavelength region os spectrum opposed to short
- so need to divide the signal the detector produces in response to this wavelength of light by the amount of radiant flux at that wavelength which falls onto the surface of the detector