Causes parallel rays of
light to converge (move
together) at a focus
Principal focus is where rays
hitting the lens parallel to the
axis meet
Diverging Lens
Concave - caves inwards
Causes parallel rays of light to diverge (spread out)
Principal focus is the point
where rays hitting the lens
parallel to the axis appear to
all come from
The distance from the centre of the lens to the
principal focus is called the focal length
3 Rules for refraction in a converging lens
An incident ray parallel to the axis
refracts through the lens and passes
through the principal focus on the other
side
An incident ray passing through the
principal focus refracts through the
lens and travels parallel to the axis
An incident ray passing through the
centre of the lens carries on in the
same direction
3 Rules for refraction in a diverging lens
An incident ray parallel to the axis refracts through
the lens, and travels in line with the principal focus
(so it appears to have come from the principal
focus)
An incident ray passing through the lens towards
the principal focus refracts through the lens and
travels parallel to the axis
An incident ray passing through the
centre of the lens carries on in the same
direction
Lenses can produce real and virtual images
A real image is where the light from
an object comes together to form an
image on a 'screen' - like the image
formed on the eye's retina
A virtual image is when the rays are
diverging, so the light from the object
appears to be coming from a completely
different place