Lenses

Convex

• Bulges outwards

• Changes a parallel beam into a converging beam

Concave

• Bulges inwards

• Changes a parallel beam into a diverging beam

Centre of a lens is called the optic centre.

Rules for Converging Lenses

Rule 1
Ray that passes the optic centre passes straight through the lens.

Rule 2
Ray that travels parallel to the axis passes through the focus.

Rule 3
Ray that passes through the focus emerges parallel.

Formation of a Real Image in a Converging Lens

Formation of a Virtual Image in a Converging Lens

Converging Lens Formulae

Formula for real images in a concave lens

Formula for virtual images in a concave lens

Formula for magnification

Rules for Diverging Lenses

Rule 1
Ray that passes through the optic centre passes straight through the lens.

Rule 2
Ray that heads towards the focus emerges parallel to the axis.

Rule 3
Ray that travells parallel to the axis emerges as if it came from the focus.

Image in a Diverging Lens

Virtual, upright, and diminished, on the same side of the lens as the object.

Diverging Lens Formulae

$\frac{1}{u}-\frac{1}{v}=-\frac{1}{f}$

Power of a Lens

Power of a Lens $=\frac{1}{focal‎‎‎⠀leng\operatorname{th}\operatorname{}}$

• Converging lens has a positive power.

• Diverging lens has a negative power.

SI Unit: metre^-1 or m^-1

$P=P_1+P_2$

Eye

• The iris controls the amount of light entering the eye. It does this by making the size of the hole in its centre - the pupil - smaller in bright light and larger in dim light.

• Retina allows sight by sending electrical signals to the brain when light strikes it.

• When a real image is brought into focus on the retina that object is seen clearly.

Near-Sight

Can see nearby objects clearly put cannot see distant objects clearly.

Correction of Near-Sight

Use a diverging (concave) lens.

Far-Sight

Can see distant objects clearly but cannot see nearby objects clearly.

Correction of Far-Sight

Use a converging (convex) lens.