Refraction of Light

Light changes direction (bends) as it crosses a boundary between 2 media in which the light moves at different speeds.

Amount of refraction of light depends on properties of media (material type, temperature or density) and angle at which it hits the boundary.

Examples of Light Refraction

Pond or pool looks shallower than it actually is

Straw or spoon in a glass appears bent

White light comes out of prism as rainbow

Air above hot stove seems to shimmer

Stars twinkle

More on Refraction of Light

Light waves travel faster in air than in water and slower in glass than water.

More dense = slower light

When light enters a different medium, speed changes and it bends.

Bending of light due to change in speed = REFRACTION

Index of Refraction

The index of refraction (n) of a medium is equal to the speed of light in a vacuum divided by the speed of light in the medium.

n = c v

In a vacuum, n is equal to 1.

The larger n is, the slower light will travel through a substance.

Common Indices of Refraction

Medium n

Vacuum 1.00

Air 1.0003

Water 1.33

Diamond 2.42

Snell’s Law of Refraction

Though you will not be required to work problems using this formula, it is important to know that you can calculate the incident or refracted angle of light mathematically using Snell’s Law of Refraction.

n1 sin1 n2 sin2

1. WHAT IS THE SPEED OF LIGHT INA DIAMOND THAT HAS AN INDEX OF REFRACTION OF 2 .42?

2. THE PICTURE BELOW REPRESENTS LIGHT GOING FROM A (MORE OR LESS) DENSE MEDIUM TO A (MORE OR LESS) DENSE MEDIUM.

Journal #37 5/7/12

Critical Angle and Total Internal Reflection

When light passes from a substance with a higher index of refraction to a lower index of refraction (such as from water to air), an interesting phenomenon can occur.

If the angle of incidence is increased too much, the incident light will bend so greatly that it cannot escape the substance. This phenomenon is called total internal reflection. The angle at which this happens is referred to as the critical angle.

Critical Angle and Total Internal Reflection

Figure a: Ray A is partially refracted and partially reflected.

Figure b: Ray B is refracted along the boundary of the medium and forms the critical angle.

Critical Angle and Total Internal Reflection

Figure C: An angle of incidence greater than the critical angle results in the total internal reflection of Ray C, which follows the law of reflection.

Fiber Optics

Fiber optics use total internal reflection.

Light is totally internally reflected over and over many times.

Advantages of Fiber Optic Technology

Used to get light to inaccessible places such as car engines, inside a patient’s body, and in communications transmitting telephone messages – replacing electrical circuits and microwave links in communication technology

Can carry more info in high frequencies of visible light than in lower frequency electrical current

Thin glass fibers replace bulky expensive copper cables – more practical in weight, size, cost

USE THE CONCAVE AND CONVEX LENSES ON YOUR TABLE. START WITH THE LENS VERY CLOSE TO YOUR EYE AND SLOWLY BACK IT AWAY. DESCRIBE THE CHANGES TO YOUR

IMAGE UNTIL THE MIRROR IS A FULL ARM’S LENGTH AWAY .

Journal #38 5/8/12

Lenses

A lens is made of transparent material, such as glass or plastic, with an index of refraction larger than that of air, causing light to bend (refract) as it passes through it.

A lens has a curved surface on one or both sides.

Plano-convex

Double-convex

Plano-concave

Double-concave

Types of Lenses

Convex vs. Concave lenses

A convex lens causes parallel light rays to eventually converge and a concave lens causes parallel light rays to eventually diverge.

Convex Lens: Beyond 2F

Image is real, inverted, and reduced.

Check Line

Convex Lens: @ 2F

Image is real, inverted, and same size.

Check Line

Convex Lens: Between FP and 2F

Image is real, inverted, and enlarged.

Check Line

Convex Lens: @FP

No image is formed.

Check Line

Convex Lens: Between FP and Lens

The refracted light rays diverge. The image forms on the same side of the lens

at the object. The image is virtual, upright, and enlarged.

Check Line

The Concave Lens

Rays diverge after they hit the lens. Image will always be virtual, upright, and reduced.

The Concave Lens

Check Line

Applications that Use Lenses

Hand lenses/Magnifying glassesProjectorsRefracting telescopesBinocularsCamerasMicroscopesCorrective Eyeglasses and Contact lenses

Nearsightedness

Nearsightedness (Myopia) occurs when the eyeball is too long, so focal length is too short.

Image forms in front of the retina; causes distant objects to be blurry.

Corrected by a concave lens that forces light to diverge to a farther point on back of retina.

Farsightedness

Farsightedness (Hyperopia) occurs when the eyeball is too short so focal length is too long, also happens with aging as muscles holding the shape of lens relax and allow it to flatten.

Image forms behind wall of the retina; causes objects located close to the eye to become blurry.

Corrected by convex lens that forces light to converge at a closer point on the back of retina.

Lens ChartLens Type/

Placement of Object

Size of image compared to

ObjectReal or Virtual Upright or

Inverted

Double Convex beyond 2F

Double Convex at 2F

Double Convex between 2F &

F

Double Convex at F

Double Convex in front of F

Double Concave (any

placement)

Lens Chart Answers -

Lens Type/ Placement of

Object

Size of image compared to

ObjectReal or Virtual Erect or

Inverted

Double Convex beyond 2F

Smaller Real Inverted

Double Convex @2F

Same Size Real Inverted

Double Convex between 2F &

FEnlarged Real Inverted

Double Convex @ F

No Image No Image No Image

Double Convex in front of F

Enlarged Virtual Upright

Double Concave (any

placement)Reduced Virtual Upright