Lecture 1

Question 1

What is the speed of light? (C)

Answer 1

3×10^8ms^-1

Question 2

Light travels at different speeds in different materials. How is that relationship quantified?

Answer 2

As a ratio between the speed of light in a vacuum and the speed of light in a material.

Question 3

State the equation for the index of refraction (n)

Answer 3

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.

Question 4

What assumptions and eliminations are made in the study of geometric optics?

Answer 4

• 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

Question 5

State the law of reflection

Answer 5

Angle of incidence = Angle of reflection

Question 6

State the equation for reflectivity (R)

Answer 6

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)

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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

Question 9

Describe Specular reflection.

Answer 9

• Light reflected off a smooth surface
• Parallel rays are all reflected in the same direction
• Angle of reflection = angle of incidence

Question 10

Describe Diffuse reflection.

Answer 10

• Light reflected off a rough surface
• Rays are scattered in all directions
• Angle of reflection is random with respect to the angle of incidence

Question 11

Describe the process of image formation.

Answer 11

• 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

Question 12

What is Ray tracing and what does it involve?

Answer 12

• 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

Question 13

State the differences between real and virtual images.

Answer 13

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

Question 14

Name 3 occurances when light strikes an object.

Answer 14

• 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)

Question 15

State Fermat’s Principle.

Answer 15

“Light travelling between two points follows a path taking the least time”