Geometric Optics

Question 1

What is the wavelength of the visible light spectrum?

Answer 1

390nm - 760nm

Question 2

What are electromagnetic waves?

Answer 2

Transverse waves of perpendicular electric and magnetic fields propagating in the direction of light

Question 3

What is visible light?

Answer 3

Small part of the electromagnetic spectrum that the photoreceptors of the eye are capable of absorbing

Question 4

What are the units of wavelength of light?

Answer 4

Nanometers (nm) or 10^-9m

Question 5

List the electromagnetic spectrum in order of shortest to longest wavelength

Answer 5

Gamma rays, X-rays, UV rays, Visible light, Infrared, Radio, Microwaves

Question 6

What is UV light used for in optometry?

Answer 6

UV light is used to visualize invisible structures when dyed with fluorescein (e.g., Fluorescein Angiography of retinal blood vessels, corneal epithelium to detect damage or fit of contact lens)

Question 7

What is the speed of light (c) in a vacuum?

Answer 7

3.0 x 10^8 m/s

Question 8

Compare the speed of light in a vacuum vs in other media (non-vacuum space).

Answer 8

Light slows down the denser the media

For air, the difference is minimal so for calculation purposes, use the speed of light in a vacuum for air as well

Question 9

What is frequency (f)?

Answer 9

Number of waves passing a given point in space per unit time

Frequency = cycles/second = f (Hz)

Question 10

What is a period?

Answer 10

How long it takes for one wavelength to pass a given point (seconds/cycle)

Question 11

What is the relationship between velocity, frequency and wavelength?

Answer 11

Velocity (c); frequency (f), wavelength (λ )

Velocity = Frequency x Wavelength

c = f λ

Question 12

How does light change as it passes from a less dense medium into a more dense medium?

Answer 12

Velocity decreases, Wavelength decreases, Frequency stays the same

F = c/λ

Question 13

How does light behave in an isotopic media?

Answer 13

Light travels as a straight line (ray) or as EM waves

Light scatters from a point source in all directions

Question 14

Know basic conversion factors

1 mile = ? Meters = ? Feet

1 inch = ? Cm

Answer 14

1 mile = 1609 meters = 5280 feet

1 inch = 2.54 cm

Question 15

What is the difference between a point source of light and an extended source of light?

Answer 15

Point Source of Light: An infinitely small point of light infinitely far away

Extended Source of Light: a source of light not infinitely small

Question 16

Define an isotropic medium and a homogeneous medium.

Answer 16

Isotropic = (iso = same, -trope = turning) = medium with same optical properties in all directions

Homogeneous = same optical properties throughout the entire volume

Question 17

Define bundle of rays, pencil of light, chief ray.

Answer 17

Bundle of rays = beam of light rays from a point source; a collection of of pencils of light

Pencil of light = narrow cone of light coming from a point source after passing through an aperture

Chief ray = central ray in each pencil of light

Question 18

What is the rectilinear propagation of light?

Answer 18

In an ideal, homogeneous medium (in a vacuum), light travels in straight lines (rays)

We often consider light traveling in air to be equivalent to light traveling in a vacuum but different factors of air (heat gradients) may create a nonuniform environment causing disturbances in the direction of light

Question 19

What is the difference between rays and wavefronts?

Answer 19

Ray = trajectory of light orthogonal (perpendicular) to a wavefront of light

Wavefronts = plane of waves that propagate away from a light source at the same speed in all directions creating spherical waves that are centered on the source

Rays and wavefronts provide interchangeable information

Question 20

How does the curvature of the wavefront change as it propagates away from a point source?

Answer 20

Near the point source, the wavefront has a very steep (high) curvature and small radius. As it moves away from the source, the curvature gets flatter (decreases) and the radius increases.

Eventually, we reach a point with a large enough distance from the point source in which the wavefront has zero curvature.

Radius of curvature (r in meters): 1/r (m^-1)

Question 21

Compare expanding wavefront vs contracting wavefront.

Answer 21

Expanding wavefront (-) = light propagates from a point, radius of curvature increases

Contracting wavefront (+) = light propagates towards a point, radius of curvature decreases

Question 22

How do we get a contracting wavefront?

Answer 22

A lens is required to produce a contracting wavefront from an expanding wavefront.

Question 23

In optometry, what distance is considered at optical infinity?

Answer 23

20 ft or 6 m

At this point, rays are considered to be traveling parallel to each other and orthogonal to the wavefronts

Question 24

Compare contracting wavefront and plane wavefront.

Answer 24

Both require a lens to change the direction of a diverging or expanding bundle of light.

Contracting wavefronts direct rays toward a point source

Plane wavefronts produce parallel rays orthogonal to the wavefront.
(Light rays will also become parallel at optical infinity but lenses can be used to redirect light at varying distances)