Understanding Positive and Negative Lenses and Formation of Images, Slides of Physics

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Focus

Distant Source

^

An object moved far to theleft approaches a parallel raysource.^ •^

s^ = o^

∞.

^

The image approaches thefocal point

f^ of the lens.

^  

2 1

R

R

n f

s^ o

R^2

R^1

f

object

image

Negative Lens

^

A biconcave lens is anegative lens.•^

Both terms are negative

-^

R^1

< 0 and

R^2

0

-^

Diverging lens

-^

Focal length is negative

^   

2 1

R

R

n

f

s^ o

R^1

R^2

f

object

image

Focal Plane

^

Parallel rays may enter thelens at a small angle.•^

Focal points on a line

^

This defines the focal plane.

s^ o

R^2

R^1

f

object

image

Real Image



Rays through a converging lens can meet at a focalplane and form a real image.•

Object between

f^ and

2 f

from the lens forms past

2 f

on far side

-^

Screen elsewhere has blurred image

Thin lensequation

f s s^

i o

Virtual Image



Rays through a diverging lens create no real image.•

Virtual image from apparent origin• Focal length and image point are negative

next

f s s^

i o