2.1 - Basic optical concepts

Question 1

What are rays

Answer 1

Straight lines along which light energy propagates

Question 2

What is a point source

Answer 2

Origin

Point where light starts from

Question 3

What are wavefronts

Answer 3

• Surfaces of constant phase or more easily constant optical distance from the source
• Waves spread out - wavefronts - spread out from point - source
• Surfaces at right angles to rays

Question 4

What happens when you drop stone in pond

Answer 4

Series of ripples moving out from point at which stone enters pond i.e. point source
One point of wave - focus on peaks of water outwards - circular waves travelling outwards

Question 5

When do objects and images occur

Answer 5

• These occur at the intersection of at least 2 rays (paraxial optics approximation) - all rays from an object intersect at an image point in the paraxial optics approximation
• Image and objects are points if rays offed to that
• Where rays intersect is the image or object

Question 6

Where do rays emanate from

Answer 6

From 1 particular point - has to be image or object

Question 7

Example of object image system

Answer 7

Object - light bulb
Lens = forms image
Image = relayed onto screen

Question 8

What does a spherical surface do

Answer 8

Form images of objects

Snell’s Law and paraxial approximation – predicts ideal image formation

Question 9

What are lenses

Answer 9

2 spherical surfaces (usually)

Question 10

What do lenses do

Answer 10

Form images

Question 11

What happens in positive lens

Answer 11

Convering light leaves lens = forms real image = solid rays to 1 point on RHS
Image on RHS ( bringing light together )

Question 12

Where is positive lens found

Answer 12

Eye
Magnifying glass
Projector

Question 13

What happens in negative lens

Answer 13

Forms image same side as object = extrapolate rays backwards
Diverging light leaving lens

Question 14

What happens in real object

Answer 14

Produces diverging light as it hits lens

Question 15

What happens in real image

Answer 15

Converging light leaving positive lens

Question 16

What happens in virtual image

Answer 16

Diverging light leaving lens i.e. negative
Rays from O’
Had to extrapolate backwards rays to find out where they intersect at O’

Question 17

What happens in virtual object

Answer 17

Converging light striking lens

Question 18

What happens in diverging lens

Answer 18

Real object

Light diverging as it this lens = light spreads out = diverging from object point = rays of light

Question 19

What happens in converging lens

Answer 19

Rays converge from lens to image point

Light leaving lens comes to image = intersects at O’

Question 20

What is reduced vergence

Answer 20

Linked to the curvature of the wavefront
Relates to “diverging” and “converging” light
Quantifies what the light is doing

Question 21

What is reduced mergence measured in

Answer 21

Dioptres (D)

Question 22

What is formula for reduced vergence

Answer 22

L = n/l

L = reduced vergence 
n = refractive index
l = distance from object ( m )

Question 23

What sign is reduced vergence

Answer 23

If diverging rays = negative

If converging rays = positive

Question 24

What is link between distance of wavefront from object and its curvature AND relationship between reduced vergence and object distance

Answer 24

Longer radius of curvature = shallower curvature
As light diverges from point object values of reduced vergence increases - magnitude decreases but value along scale increases as vergence becomes less negative ( towards 0 which is higher on scale )
e.g. 10cm = -10D
50cm = -2D
As the light proceeds away from the object, value increases along the scale getting closer to 0
= VERGENCE GETTING CLOSER TO 0 IS INCREASING

Question 25

What is axial objects/images

Answer 25

Lie on the optical axis

Show location of objects/images

Question 26

What is off axis objects/images

Answer 26

Lie above or below the optical axis
Show heights of objects/images
Links magnification

Question 27

What does geometrical approximation assume

Answer 27

Light travels as rays in a homogenous medium

Question 28

What happens when you pick up a negative lens and look at a pencil on a desk

Answer 28

The pencil is a real object, the image formed by the negative lens is virtual and the image formed on the back of your eye is real to trigger a response from the photoreceptors