Refraction and Lens

Refraction

Refraction: the change in direction of a wave as it crosses the boundary b/w 2 media in which a wave travels different speeds the bending of waves b/c one part of

each wave is made to travel slower (or faster) than another part.

Ex: Your legs appearing shorter in a swimming pool; a pencil appearing bent when in a glass of water

• When light rays enter a medium in which their speed decreases (more dense medium), as when passing from air to water, the rays bend toward the normal.

• When light rays enter a medium in which their speed increases (less dense medium), as when passing from water to air, the rays bend away from the normal.

Lenses

• A lens is a piece of glass or plastic that bends parallel rays of light so they cross and form an image.

• Lenses function off the principle that light refracts thru glass

Light bends b/c it is changing speed going from one medium into another

• There are 3 types of lenses: 1. Plane (flat)2. converging (aka Convex)3. Diverging (aka Concave)

Converging Lenses• A converging lens is thicker in the middle than the

edges. Convex refers to the shape of the lens

• Rays of light that are initially parallel are made to converge (come together) Converging refers to the behavior of the light

• Rays that move

through the center

move slower

than rays moving

through the edges

Diverging Lens• A diverging lens is thinner in the middle than the

edges. Concave refers to the shape of the lens

• Rays of light that are initially parallel are made to diverge (spread apart) Diverging refers to the behavior of the light

• Rays that move

through the center

move faster

than rays moving

through the edges

Focal Points

• The focal point is the point where light converges.

• In a converging lens, light passes through the focal point.

• In a diverging lens, light appears to go through the focal point. Light looks like it originates from a single point.

Lens Image Formation

Lens Outside the focal point

Inside the focal point

Converging

(convex)

Real

Smaller

Inverted

Virtual

Larger

Upright

Diverging

(concave)

Virtual

Smaller

Upright

Virtual

Smaller

Upright

Check Your Understanding

What type of lens is used in a magnifying glass?

A converging lens. It will create a larger (magnified), upright image. Many instruments used for magnification, such as a microscope or telescope, use multiple converging lenses.

Check Your Understanding

Why are diverging lenses not used on overhead or LCD projectors?

B/c diverging lenses only produce virtual images. Only real images can be projected onto a screen.

The Eye

• The eye is similar to the camera.• The amount of light that enters the eye

is regulated by the iris which surrounds the opening called the pupil. Light enters through the transparent cornea through the pupil. The lens focuses the light on a layer of tissue on the back of the eye called the retina.

• The eye uses a converging lens to focus light on the retina.

• In both a camera and the eye, the image that is formed is up side down. Our brain turns the image right side up for us

• Adjustments in focusing is called accommodations.

• Accommodation is accomplished by changing the thickness and shape of the lens.– Our eye does this by contracting or releasing

eye muscles

Check Your Understanding

Why does your brain have to “flip” the image projected onto the back of our eye?

Because our eye uses a converging lens, which inverts images outside the focal point (most objects). Our brain has to flip the image so as not to confuse our balance.

Some Defects in Vision• The eyes of a farsighted person forms images

behind the retina. The eyeball is too short.

• A person with farsighted vision can see objects that are far but not near.

• A converging lens corrects farsighted vision.

• The eyes of a nearsighted person forms images in front of the retina. The eyeball is too long.

• A person with nearsighted vision can see objects that are near but not far.

• A diverging lens will correct nearsighted vision

Summary of Vision Defects Vision Type CAN See CANNOT

SeeCorrective

Lens

Farsighted Far Near Converging

Nearsighted Near Far Diverging

Astigmatism:Astigmatism: a defect of the eye a defect of the eye caused when the cornea is curved more caused when the cornea is curved more in one direction than in anotherin one direction than in another

Total Internal Reflection• When light travels from a more dense

medium to a less dense medium, there is a certain angle to which no light will be refracted and all light will be reflected. This is called the critical angle.

• Total internal reflection is the 100% reflection of light that strikes the boundary between 2 media at an angle greater than the critical angle. Ex: The critical angle for glass is 43°, the critical

angle for water is 48°, and the critical angle for a diamond is 24.6°.

• The dotted line represents the critical angle. The green line represents total internal reflection.

• Optical fibers are transparent fibers that pipe light from one place to another using total internal reflection.

• Optical fibers are important in communications.

Check Your Understanding

What type of lens does one where to correct farsightedness?

A converging lens because it can make objects appear larger (closer) than they actually are.