Lenses

How does a lens form an image?

• by refracting light

• in a convex lens parallel rays of light are brought to a focus at the principle focus

• distance from the lens to principle focus is called the Focal Length

convex lenses

• makes parallel light ray converge to a focus

• the principal focus lens shows you where it will focus the parallel light rays

• a clear, distinct image of a distant object will be formed from a converging lens

How do distant/nearby objects appear in a convex lens?

• distant object focused using a converging lens appear focused and inverted → telescope

• nearby objects can be magnified using a converging lens will be upright → magnifying glass

concave lens

• makes parallel light rays spread out (diverge)

• the point at which light rats appear to come from is called the principle focus and is in front of the lens

• these are used to correct short sight

magnification eq

image height/object height

real images

• eye

• projectors

• cameras

virtual images

all diverging lens → looking through a converging lens such as a magnifying glass

Concave + convex similarities

• refracts light

• focus point ( always a principal axis)

• parallel light rays used in correcting eyesight

Concave + convex differences

• shape of lens

• light rays diverge/converge

• concave lens image always on same side of object

• convex lens could have image on either side

• convex is real/virtual

• concave is always virtual

image

a visual copy of an object

real

an image that can be formed on a screen

virtual

an image that cannot be formed on a a screen

inverted

upside down

diminished

smaller

enlarged

bigger

converge

rays meet and cross paths

diverge

rays separate from each other and never cross paths

convex ray diagram

1. draw a ray from top of the object straight through optical centre

2. draw a ray from top of the image parallel to the principle axis until it hits the lens plane

3. continue ray through focal point F

4. label image eg real, inverted and diminished

concave lens ray diagram

1. draw ray from top of object straight through optic centre

2. draw a ray from top of image, parallel to principle axis until it hits lens plane

3. construct divergent ray that traces back to point