Converging and Diverging lenses

Lenses form images by refracting light. There are two main types of lenses converging and diverging and they have opposite effects on light rays.

• A converging lenses is convex- causing parallel light rays to converge at the principal focus.
• A diverging lenses is concave- causing parallel light rays to diverge.
• The axis of a lens is a line passing through the middle of the lens.
• The principal focus of a converging lens is where light rays parallel to the axis meet.
• The principal focus of a diverging lens is the point in which the light rays parallel to the axis appear to all come from- where you can trace them back to untill the appear to meet up behind the lens.
• There is a principal focus on each side of a lens. the distance from the center to the principal focus is called the focal length.

Refraction in a converging lense.

• An incident ray that is 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 carries on in the same direction when passing through the center.

Refraction in a diverging lens.

• An incident ray refracts through the lens and travels in line with the principal focus when it is parallel to the axis.
• An incident ray going through the lens towards the principal focus refracts through the lens parallel to the axis.
• An incident ray carries on in the same dircetion when passing through the center.

Real and Virtual images

• When the light from an object comes together to form and image on an 'screen' eg. the filmon a camera or the retina in an eye this is a real image.
• When the light from the object seems to be coming from a completely different place the light rays are diverging so it is a vitual image.
• A refection in a mirror is a virtual image as the objects seems to be behind the mirror.
• When you look through a maginfing lens you get a virtual image as the object appears to be bigger than it is.