1. LENSES

2. 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

3. 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

4. 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)

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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