light behaves as a:
particle - this can explain refraction and reflection
wave - this can explain diffraction and polarization
Ray Model of Light
- light travels in straight path - direction changes only if there is an obstruction.
A ray is a straight line that represents the path of light.
Sources of light
Luminous source: object that emits ( give off) light. eg sun, light bulbs, lasers, LED
Illuminated source: object becomes visible when light is reflected off it.
eg moon, bicycle reflector
We can see illuminated sources because light reflects off them into your eyes
We can classify material according to the amount of light they reflect, transmit or absorb
Opaque media: material that do not transmit light ( cannot see through them)
eg bricks
Transparent media: material that transmit light eg clear glass, air
Translucent media: material that transmit light but objects cannot be seen clearly.
eg frosted light bulbs, lamp shades
Questions:
Why would you choose a curtain for your window that is opaque?
Quantity of light
Luminous flux (P): Rate at which light energy is emitted ( given off/comes out)
from a luminous source
Units: lumen (lm)
Luminous flux does not depend on the distance it strikes a surface
( total number of light rays emitted does not change)
Illuminance (E) Rate at which light strikes a surface [luminous flux per unit area]
units: lux (lx) = lm/m2
Illuminance depends on the distance between the source of light
and the surface it strikes
For a point source illuminance follows an inverse square relationship
with distance 1/r2
Luminous Intensity : Luminous flux that falls on 1m2 of a 1m radius sphere.
units: candela (cd)
Equation to calculate illuminance:
E = P/4πr2
P = luminous flux (lm)
r = distance from light source (m)
E = illuminance (lx) = lm/m2
Inverse square Relationship
Increasing the distance from the source, decreases the illuminance by the
square of the distance
Luminous flux | Distance from source | Illuminance is proportional to… |
1750lm | 2m | 1/22 or 1/4 |
1750lm | 3m | 1/32 or 1/9 |
Question: refer to the table
A lamp is moved from 2m to 3m away from a book. Compare the illumination
of the book before and after it has been moved.
Illuminance at 2m = 34.8lx [use the equation above]
Illuminance at 3m = 15.5lx
Equation to calculate luminous intensity
Luminous intensity = P/4π units: candela (cd)
How much brighter must the light bulb be than the candle to have the same illuminance on the screen?
Problem solving:
1. What is the illuminance, in lux, for a piece of paper on a table 3.0 m from a point light source that is
producing 1700 lm of luminous flux
Given: r = 3.0m P = 1700 lm E = ??
Equation to use: E = P/4πr2
3. A 32-cd point source of light is 2.0 m above the surface of a desk. What is the illumination of the desk’s
surface in lux?
Given: Luminous intensity = 32 cd r = 2.0m E = ??
equations to use: Luminous intensity = P/4π but E = P/4πr2
therefor E = Luminous intensity/r2 [ I/r2 ]
4. Two lamps illuminate a screen equally – lamp A at 6.0 m, lamp B at 2.0 m. If lamp A is rated 50 cd, what
is lamp B rated?
lamp A: r = 6.0m Luminous intensity = 50cd
Lamp B r = 2.0m Luminous intensity I = ?? E lamp B = E lamp A
since Luminous intensity is given, use the equation: E = Luminous intensity/r2
E lamp A = E lamp B
I/r2 = I/ r2
Complete the above problems and work those given in class
2. What is the luminous intensity, in candelas, of a bulb with 4.5 x 103 lm of luminous flux?
Given : P = 4.5 x 103 lm Luminous Intensity = ??
equation to use: Luminous intensity = P/4π