LENSES.

Presentation on theme: "LENSES."— Presentation transcript:

LENSES

30.1 – Converging & Diverging Lenses
Light is refracted as it passes through glass Converging (Convex) lens – light intersects (converges) on backside, thicker in middle Diverging (Concave) lens – light spreads out (diverges) on backside, thinner in middle

Focal point = the point that parallel rays of light cross after going through lens
Principle axis – a perpendicular line going through center of lens Focal length – the distance from center of lens to focal point

30.2 – Image Formation by a Lens
Converging Lenses Consider an object very far away (at infinity: f  ∞) The spreading is so small, the rays are considered to be parallel After 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 intersect Can only be seen in lens Diverging Lenses Regardless of location of object image is: Virtual Upright smaller

30.5 – Common Optical Instruments
Camera Uses lens (or lenses) to produce  real, inverted image on light sensitive film Exposure of light is regulated by shutter/diaphragm Telescope A 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 Microscope First lens (objective) produces real image (inverted) Second: inside focal length of image producing  virtual, larger, inverted

30.6 – The Eye Light 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 spot
Muscles 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 Vision
Farsightedness Eyeball is too small, the lens focuses light behind the eye Eye cannot focus on objects too close A converging lens is placed in front of the eye to begin the curving process

Nearsightedness Eye is too long or lens is curved too much causing focal point to be before the retina Cannot see objects far away Must wear diverging lenses to “slow the convergence” Astigmatism The cornea is not uniformly curved Causes light to focus well at certain locations on retina, but not all Corrective lenses are specially designed

30.8 – Some Defects of Vision
Distortions of an image – abberation Spherical 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 Abberation Due to dispersion of colors as they pass through lenses Therefore each color has its own focal point