LENSES

30.1 – Converging & Diverging LensesLight is refracted as it passes through glassConverging (Convex) lens – light intersects

(converges) on backside, thicker in middleDiverging (Concave) lens – light spreads out

(diverges) on backside, thinner in middle

Focal point = the point that parallel rays of

light cross after going through lensPrinciple axis – a perpendicular line going

through center of lensFocal length – the distance from center of lens

to focal point

30.2 – Image Formation by a LensConverging LensesConsider an object very far away (at infinity: f

∞)The spreading is so small, the rays are

considered to be parallelAfter passing through lens, they intersect at focal

point on other side and produce an upside down image

Object at F no image (forms at infinity)

Between F and ∞ real, inverted

Object closer than F = virtual image – cannot be projected onto

screen, rays of light do not intersectCan only be seen in lens

Diverging LensesRegardless of location of

object image is:VirtualUprightsmaller

30.5 – Common Optical InstrumentsCameraUses lens (or lenses) to produce real, inverted

image on light sensitive filmExposure of light is regulated by

shutter/diaphragmTelescopeA second lens is used (eyepiece) as a magnifying

lens to enlarge real image from first lens

A third lens or prisms can be used to make image upright

C0mpound MicroscopeFirst lens (objective) produces real image (inverted)Second: inside focal length of image producing

virtual, larger, inverted

30.6 – The EyeLight enters through

transparent cover – cornea

The amount of light allowed in through the pupil is regulated by the iris – colored part of eye

Light is focused on tissue on back of eye - retina

The nerves that carry light information away (optic

nerve) produce a blind spotMuscles surrounding the lens of the eye constantly

change its shape to adjust for objects close and far away

Recall: the image produced is upside down

30.7 – Some Defects in VisionFarsightednessEyeball is too small, the lens focuses light

behind the eyeEye cannot focus on objects too closeA converging lens is placed in front of the eye

to begin the curving process

NearsightednessEye is too long or lens is curved too much

causing focal point to be before the retinaCannot see objects far awayMust wear diverging lenses to “slow the

convergence”AstigmatismThe cornea is not uniformly curvedCauses light to focus well at certain locations

on retina, but not allCorrective lenses are specially designed

30.8 – Some Defects of VisionDistortions of an image –

abberationSpherical abberation

Light does not focus at same location

Due to varying refraction angles at different distances from center

Corrected by: thin covering at edges or combination of lenses

Chromatic AbberationDue to dispersion of colors as they

pass through lensesTherefore each color has its own

focal point