The two types of lens
Convex (converging)
Concave (diverging)
What does a convex lens do
Causes rays of light which are parallel to the axis of the lens to converge at the principal focus
Where is the principal focus of a convex lens
Where rays hitting the lens parallel to the axis meet
Rules for convex lenses
- 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 centre of the lens carries on in the same direction
- An incident ray passing through the principal focus before meeting the lens refracts through the lens and travels parallel to the axis
What does Concave lens do
Causes parallel rays of light to diverge
Where is the principal focus of a concave lens
Where the rays hitting the lens parallel to the axis appear to come from
What are virtual rays
Rays that aren’t actually there but show the path that it looks like the light has taken
What is the focal length
The distance between the centre of the lens and the principal focus
3 rules of concave lenses
- An incident ray travelling parallel to the axis refracts through the lens and travels in line with the near side principal focus (so it appears to have come from the principal focus)
- An incident ray passing through the centre of the lens carries on in the same direction
- An incident ray passing through the lens towards the far side principal focus refracts through the lens and travels parallel to the axis
What kind of images are formed from lenses
Virtual and real
Real images
Formed when the light rays from a point on an object come together to form an image -> they pass through the same point
Where does the screen have to be placed
Exactly where the rays meet otherwise it’ll be out of focus
Virtual images
Formed when the light rays from a point in an object are diverging after they have left the lens -> so the light from the point on the object appears to be coming from a different place
- the light rays don’t actually pass through that point they jsut appear to
What to mention when describing an image
- how big it is compared to the object
- whether it’s upright or inverted
- whether it’s real or virtual
Images formed by concave lenses
- virtual
- upright
- smaller than the object
Convex at different points
>2F: inverted, real, smaller
At 2F: inverted, real, same size
Between 2F and F: inverted, real, bigger
Between F and lens: upright, virtual, bigger + on the same side
What lenses do magnifying glasses use
Convex lenses
How do magnifying glasses work
By creating a virtual, upright image that is bigger than the object and on the same side as the object
What has to happen for an object to be magnified
The object being magnified must be closer to the lens that the focal point
