2.1 - Basic optical concepts Flashcards
What are rays
Straight lines along which light energy propagates
What is a point source
Origin
Point where light starts from
What are wavefronts
- 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
What happens when you drop stone in pond
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
When do objects and images occur
- 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
Where do rays emanate from
From 1 particular point - has to be image or object
Example of object image system
Object - light bulb
Lens = forms image
Image = relayed onto screen
What does a spherical surface do
Form images of objects
Snell’s Law and paraxial approximation – predicts ideal image formation
What are lenses
2 spherical surfaces (usually)
What do lenses do
Form images
What happens in positive lens
Convering light leaves lens = forms real image = solid rays to 1 point on RHS
Image on RHS ( bringing light together )
Where is positive lens found
Eye
Magnifying glass
Projector
What happens in negative lens
Forms image same side as object = extrapolate rays backwards
Diverging light leaving lens
What happens in real object
Produces diverging light as it hits lens
What happens in real image
Converging light leaving positive lens
What happens in virtual image
Diverging light leaving lens i.e. negative
Rays from O’
Had to extrapolate backwards rays to find out where they intersect at O’
What happens in virtual object
Converging light striking lens
What happens in diverging lens
Real object
Light diverging as it this lens = light spreads out = diverging from object point = rays of light
What happens in converging lens
Rays converge from lens to image point
Light leaving lens comes to image = intersects at O’
What is reduced vergence
Linked to the curvature of the wavefront
Relates to “diverging” and “converging” light
Quantifies what the light is doing
What is reduced mergence measured in
Dioptres (D)
What is formula for reduced vergence
L = n/l
L = reduced vergence n = refractive index l = distance from object ( m )
What sign is reduced vergence
If diverging rays = negative
If converging rays = positive
What is link between distance of wavefront from object and its curvature AND relationship between reduced vergence and object distance
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
What is axial objects/images
Lie on the optical axis
Show location of objects/images
What is off axis objects/images
Lie above or below the optical axis
Show heights of objects/images
Links magnification
What does geometrical approximation assume
Light travels as rays in a homogenous medium
What happens when you pick up a negative lens and look at a pencil on a desk
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
