Lenses - Physics

Lenses

A transparent object that can refract light.
They can:
- enlarge objects
- improve vision
- etc.

types of lenses

• converging lenses

• diverging lenses

converging lenses

• side curved outwards

• sometimes called a convex lens

• light rays converge at a focal point

diverging lenses

• side curved inwards

• sometimes called concave lenses

• light rays diverge as they pass through the lens

optical centre

• the centre point of the lens

• lies at the intersection of the principal and vertical axis

• labelled as O on a ray diagram

principal axis

• goes through the principal focus and the optical centre

• similar in function to the principal axis in curved mirrors

Vertical axis

• perpendicular to the principal axis, intersecting at the optical centre

• drawn as a dashed line

• light rays refract at the vertical axis

Principal focus

• a point where real or virtual light rays converge

• marked as F on a ray diagram

Secondary principal focus

• an equidistant point on the principal axis from O on the other side of the lens relative to F

• marked as F’ on a ray diagram

focal length

• the distance between the optical centre and the principal focus

• marked as f on a ray diagram

rules for converging lens

1. light rays travelling parallel to the principal axis will refract toward the principal focus (F)

2. light rays travelling through the optical centre (O) will continue without bending

3. light rays travelling through the secondary principal focus (F’) will refract in a ray parallel to the principal axis

SALT for an object behind 2F

• Size - smaller

• Attitude - inverted

• Location - between F and 2F

• Type - real

SALT for an object at 2F

• Size - same size

• Attitude - inverted

• Location - at 2F

• Type - real

SALT for an object between 2F and F’

• Size - larger

• Attitude - inverted

• Location - beyond 2F

Type - real

SALT for an object at F’

No image is formed

SALT for an object between F’ and O

• Size - larger

• Attitude - upright

• Location - on the same side of the lens as the object

Type - virtual

diverging lenses

differences with converging lenses:

• principal focus (F) is now where the virtual rays intersect (on the same side as incident rays)

• Secondary principal focus (F’) is now on the opposite side (same side as emergent rays)

rules for diverging rays

1. light rays travelling parallel to the principal axis will refract as if the ray had come from the principal focus (F)

2. light rays travelling through the optical centre (O) will continue travelling without bending

3. light rays travelling toward to the secondary principal focus (F’) will refract parallel to the principal axis

SALT for an object behind 2F (diverging)

• Size - smaller

• Attitude - upright

• Location - on the same side of the lens as object

Type - virtual