LENSES

Lyzinski Physics

Light Speeds

• When traveling through a vacuum, light travels at 3 x 108 m/s. This is the fastest light ever travels. We shall refer to this as the SPEED OF LIGHT.

• When traveling through a different “medium” (substance), light slows down.

• The INDEX OF REFRACTION compares this slower speed to the speed of light in a vacuum.

vcn

Index of refraction

Speed of light

Speed of light in new medium

The Index of Refraction

• What does “refract” mean. It means “to bend”. When light travels from one medium to another it actually bends (thus causing it to either slow down or speed up)

• The index of refraction is always GREATER THAN 1 (because the speed in a medium is always less than the speed of light).

• Optically “DENSE” mediums have a higher index. We will call these mediums “HEAVY”

• Optically “LESS DENSE” mediums will be called “LITE” (mis-spelled )

Some common mediums

Medium Index of Refraction

vacuum 1.00

air 1.003

water 1.33

ethanol 1.36

fluorite 1.43

crown glass 1.52

quartz 1.54

Zircon 1.92

diamond 2.42

Refraction (the bending of light)

i

r

i

r

n =1.00

n =1.33

air

water

n =1.52Crown glass

n =1.33water

If light goes from LITE to HEAVY, it bends TOWARD the normal.

If light travels from HEAVY into LITE, it bends AWAY FROM the normal

Normal to the surface

Normal to the surface

“La-Hite” (LHT) “Ha-La” (HLA)

Total Internal Reflection

orci nn 90sinsin

heavy

lite

Normal

“Ha-La” has a “Phenomena” associated with it.

ci

i

rc n

n1sin

Rrii nn sinsin

ci

SNELL’S LAW Relates the angles to the indexes

i

r

n =1.00

n =1.33

air

water

Normal to the surface

Rrii nn sinsin

How much does the light bend???

Prisms cause light to bend

glassairair

Light bends towards normal in the “heavy” glass

Light bends away from the normal in the “lite” air

IMPORTANT QUESTION: Does all light bend the same amount?

LHT HLA

All light does NOT bend the same amount in a prism.

Increasing index (bend)

Decreasing wavelength

Why does the index increase going from R to I (in ROY-G-BIV)?

The Wave Equation

The frequency of light never changes once it is created. Velocity can change. Wavelength will then also change. But frequency never changes once the wave is generated.

fv wavelength

frequencyWave speed

If v goes , then must go .

If v goes , n goes .

Refraction (the bending of light)

H

L

If light goes from LITE to HEAVY, it slows down AND the wavelength decreases.

If light travels from HEAVY into LITE, it speeds up AND the wavelength increases

Normal to the surface

L

H

Normal to the surface

Concave Lenses

Primary Focus

Principal axis

Concave lenses are similar to stacking two prisms.

Convex Lenses

Convex lenses are also similar to stacking two prisms.

Principal axisPrimary Focus

How do you locate the image formed in a LENSE Situation.

1. Any ray parallel to the principal axis is reflected through the focus.

2. Any ray through the focus is reflected parallel to the principal axis.

3. Any ray through the center of the lens passes directly through the lense following its same path.

Image Formation in CONVERGING Lenses

NO IMAGE!!!VIRTUAL!!!

Outside 2F’

2F’ F’

F 2F

On 2F’Between 2F’ & F’On F’Inside F’

Smaller, Inverted, RealLARGER, Inverted, Real

Same Size, Inverted, Real

Image Formation in Diverging Lenses

F

F’

All images are VIRTUAL and smaller than the object

Cameras ……

….. and the human eye

BOTH use converging

lenses with the object far

beyond the secondary

focus.

Film

Magnifying Glasses

2F’ F’ F 2F

Object that you are looking at (under the lens)

Larger, virtual image that you see

Film ProjectorsW

hite screen

Microscopes

Lens #1

(Objective)

Lens #2

(Eyepiece)

Virtual Image that your eye

sees

Human

eye

The Human EyeCornea: Protective “Window” of

eye

Iris: Colored part that acts like a

camera shutter.

Pupil: Hole in the middle of the

iris.

Lens: has adjustable focal length.

Retina: Where image is formed.

Optic nerve: Sends image to

brain where it is flipped

upside down.

Muscles that “tense” the lens

Near-Sightedness

Furthest Point that the eye can see

clearly

Far Point

Object beyond the far point can’t be focused clearly on the retina

“Fuzzy” vision

Diverging Lens creates a virtual image of the “far

away” object that is inside (or at) the far point. The eye

sees this virtual image clearly.

Far-Sightedness

Closest Point that the eye can see clearly

Near Point

Object inside the near point can’t be focused clearly on the retina

“Fuzzy” vision

Converging Lens creates a virtual image of the “too

close” object that is outside (or at) the near point. The eye sees this virtual image

clearly.

Focus of lens