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Concave and Convex
A lens uses refraction to change the direction of rays of light.
Red light is refracted the LEAST and purple light is refracted the MOST.
Lenses are used throughout the world for many different things such as glasses to correct
Two types of lenses:
CONVEX: This lens is thickest at the centre and
looks as below. It focuses the rays and
CONVERGES them to a point that is why it is also
known as a CONVERGING lens. The rays converge
to the point called the focal point or focus, F. The
image formed is real, and can be seen on a screen.
CONCAVE: This lens is thinnest at the centre and looks as below. It focuses the rays and
DIVERGES them to a point that is why it is also known as a DIVERGING lens. The
rays are refracted OUTWARDS. A virtual focal point is also formed behind the lens but it
doesn't actually exist the rays could bounce back to create it but they refract outwards.
Real and virtual: In a real image, the light rays forming it do actually meet, but in a virtual
images, they do not meet but just appear to meet for example the virtual image in a plane
mirror is as if it is behind the mirror, but it isn't. So in a virtual image, say in a mirror, the
`image' is also inverted flipped so the right side is on the real left side. An example of a
real image is a photo.
There are two focal points on a lens one on either side. They both are the same distance
from the lens and are on the principal axis. One is virtual and one is real.
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The blue circle is called the VIRTUAL FOCUS. It is a focal point that is formed, but is not
actually there. It isn't real, unlike the other focal points. It is behind the lens, rather like a
A thinner lens is weaker and has a longer focal length.
A lens can also focus rays that are NOT PARALLEL, in this case the rays will NOT meet
at the focal point, but a little ahead of it.…read more