chapter 13 optics

Converging Lens

A converging lens is thickest in the middle, causing incident parallel light rays to converge at a single point after refraction.

Diverging Lens

A diverging lens is thinnest in the middle, causing incident parallel light rays to separate after refraction.

Converging Lenses

In a converging lens, the principal focus (F) is located to the right of the lens, while the secondary principal focus (F’) is to the left of the lens.

Diverging Lenses

In a diverging lens, the principal focus (F) is located to the left of the lens, while the secondary principal focus (F’) is to the right of the lens.

Emergent Ray

Light that leaves a lens after refraction.

Rules for Converging Lens Diagrams

1. A ray parallel to the principal axis is refracted through the principal focus (F).

2. A ray through the secondary principal focus (F’) is refracted parallel to the principal axis.

3. A ray through the optical centre (O) continues straight through without refraction.

Rules for Diverging Lens Diagrams

1. A ray parallel to the principal axis is refracted as though it came through the principal focus (F).

2. A ray that appears to pass through the secondary principal focus (F’) is refracted parallel to the principal axis.

3. A ray through the optical centre (O) continues straight through without refraction.

Converging Lens Locations

• Object placed beyond 2F’ or between 2F’ and F’ produce real and inverted image.

• Object placed at F’ will not produce an image.

• Object placed between F’ and the lens will produce an upright, larger, virtual image.

Diverging Lens Locations

Diverging lens always form a smaller, upright, virtual image on the same side of lens as object.

Thin Lens Equation

• d_{o = always +}

• f _{= + if lens is converging}

• _{f = - if lens is diverging}

• d_{i = + if image is real and located opposite side of object}

• d_{i = - if image is virtual and located same side as object}

Magnification Equation

• ho= + when object is upward

• ho = - when object is inverted

• hi = + when object is upward

• hi = - when image is upward

• m = + when image is upward

• m= - when image is inverted

Camera

• Uses a converging lens

• Produces an: inverted, real image when object is at a distance greater than 2F’ on film or sensor

• Image formed between F and 2F

• In order for camera to focus on the film or sensor at 2F, lens is moved in and out

Movie Projector

• Opposite of a camera

• Takes small objects (film) and projects: large, inverted, real image on screen

• Film is located between 2F' and F'

• Since image is inverted, film is placed upside down in projector so image is seen upright

Magnifying Glass

• Uses a converging lens

• Object located between F’ and lens

• No real image produced at object location

• Brain extends refracted rays backwards producing: large, virtual image located on same side of lens as object

Compound Microscope

• Uses 2 converging lenses

• 2 enlarged, inverted images formed: 1 real, 1 virtual

• Real image not seen since lens is inside body tube of microscope

• Eye piece produces a virtual image, eye piece lens is seen

Refracting Telescope

• Similar to compound microscope

• Object seen is far away

• 2 lenses: 1 real, 1 virtual

• Larger, virtual image is only seen

Terrestrial Telescope

• 3 Converging lenses

• Objective lens, third lens (in between), eye piece lens

• Third lens corrects inverted image produced by eye piece lens

Human Eye

• The eye is an optical device that observes its surroundings by detecting light.

• The lens and cornea refract light, causing it to converge, forming an image on the retina.

• The iris (coloured tissue around pupil) acts as an opening, controlling the amount of light entering the eye.

• After the retina detects light, the optic nerve transmits image information to the brain. The small area where the optic nerve enters, the eye is immune to light, resulting in a blind spot.

Unique Features

Unique feature that other optical devices don’t have is that the focus of the eye is adjusted by changing the focal length of the lens, rather than the distance between the lens and the retina.

Seeing with our brain

When we look at an object, our eyes act like a converging lens and produces a smaller, inverted, real image, on our retina. Our brain takes the inverted image from the retina and flips it so it’s seen upright.

Hyperopia

Far-sightedness

• Person can see far objects clearly but not close ones .

• Distance between retina and lens is too short, or cornea and lens are too weak.

• Focuses behind retina.

• Converging lens fixes problem.

Myopia

Near-sightedness

• Person can see close objects but not far ones

• Distance between lens and retina is too strong, or cornea and lens converge light too strongly.

• Diverging lens fixes problem

Presbyopia

Form of Far-sightedness

• Occurs with old age

• Difficulty reading small print

• Eye lens loses elasticity

• Converging lens fixes problem