What is the speed of light? (C)
3×10^8ms^-1
Light travels at different speeds in different materials. How is that relationship quantified?
As a ratio between the speed of light in a vacuum and the speed of light in a material.
State the equation for the index of refraction (n)
n=c/v
Where n is the index of refraction,
C is the speed of light in a vacuum,
V is the speed of light in a material.
What assumptions and eliminations are made in the study of geometric optics?
- Ignores the wave nature of light
- Treats light as rays that travel in straight line paths
- Assume a homogeneous medium of uniform refractive index
- Speed of light never changes in the same material
- Does not consider interference
- Macroscopic - objects in the lights path must be much larger than the wavelength of light
State the law of reflection
Angle of incidence = Angle of reflection
State the equation for reflectivity (R)
R = (n2 - n1)² + (k2 - k1)²
—————————-
(n2 + n1)² + (k2 + k1)²
n = refractive index (n1 is material 1)
k = extinction coefficient (describes absorbtion)
Calculate the reflectivity of light from a HeNe laser passing through air and interacting with Aluminium.
For Al:
n=1.3729
k=7.6177
For air at 633nm:
n=1.00
k= 0.00
R=(1.3729-1)² + (7.6177-0)² / (1.3729+1)² + (7.6177-0)²
= 0.3729² + 7.6177² / 2.3729² + 7.6177²
= 0.13915441 + 58.02935329 / 5.63065441 + 58.02935329
= 58.1684077 / 63.6600077
= 0.914
= 91.4% reflectivity
Calculate the reflectivity of a HeNe laser passing through air and interacting with glass.
Glass:
n=1.5
k=0.0
Air
n =1.0
k=0.0
R= (1.5-1.0)² + (0.0-0.0)² / (1.5+1.0)² + (0.0+0.0)²
= 0.5² / 2.5²
= 0.25 / 6.25
= 0.04
= 4% reflectivity
Describe Specular reflection.
- Light reflected off a smooth surface
- Parallel rays are all reflected in the same direction
- Angle of reflection = angle of incidence
Describe Diffuse reflection.
- Light reflected off a rough surface
- Rays are scattered in all directions
- Angle of reflection is random with respect to the angle of incidence
Describe the process of image formation.
- Light from object enters the eyes
- Light is used by the brain to form an image of an object
- Brain extrapolates the light rays back to their origin
What is Ray tracing and what does it involve?
- The process of following individual rays as they travel from a source/object to another point.
- Involves geometry
- Infinite number of possible rays but only the relevant ones should be drawn
State the differences between real and virtual images.
Real:
- Can be projected onto something e.g. a screen or photographic film
- Formed by the intersection of light rays at a point in space
Virtual:
- Cannot be projected onto something
- Formed because rays appear to come from a source, but in fact come from somewhere else
- The image in a mirror is a virtual image
Name 3 occurances when light strikes an object.
- Some or all can be reflected
- Some or all can be absorbed (Absorbed light is transformed into thermal energy)
- Some or all can be transmitted through (e.g. with materials such as glass or water)
State Fermat’s Principle.
“Light travelling between two points follows a path taking the least time”
