Chapter 17/18Refraction and Lenses

When a ray of light passes from one medium to another it may be reflected, refracted or both.

Refraction

• Bending of light when it enters a new medium is called refraction.

Refraction• When light rays refract it will change:

1. speed2. direction (bending of a wave) at the boundary

between two media.

Speed of a wave depends on mediumVideo clip: https://www.youtube.com/watch?v=stdi6XJX6gU

Tractor Example

What happens when you pull your toy tractor from smooth (less dense) pavement to a rough patch (more dense) of grass?

Grass

Path of Tractor through Grass (no angle)

The truck will slow down

Tractor Example Continued…

How is it different if the tractor is at an angle so only one wheel hits the grass at a time?

(at an angle)

Changes Direction

How does light bend when it encounters a new medium at an angle?

less dense medium

more dense medium

TOWARD THE NORMAL or AWAY FROM THE NORMAL

How does light bend when it encounters a new medium at an angle?

TOWARD THE NORMAL or AWAY FROM THE NORMAL

more dense medium

less dense medium

Least Time Principle• Light follows the “Least Time

Principle” which can be illustrated by a lifeguard saving a swimmer who needs help. He would travel the farthest distance possible on the sand (where he can run fast) and swim the shortest distance possible in the water (where he swims slow).

• Light would do the same thing! A

ray of light bends towards the normal when crossing a boundary from a medium in which is travels fast into a medium in which it travels slow.

Video…..

Optical Illusions and Refraction

• light travels only through air (no refraction)

• light travels from water (to glass) to air (refraction, or lots of bending, occurs!)

• NOTE: The image of the pencil is located where the refracted rays intersect!

• (simulation: http://www.physicsclassroom.com/mmedia/optics/bp.cfm)

Visual Distortions: objects appear to be wider or broken

Image located where refracted rays intersect

Quick Review:

• When light goes from a less dense to a more dense medium, it bends ___________the normal and when light goes from a more dense to a less dense medium, it bends ____________ the normal.

towards

away

Index of Refraction• Compares how fast light travels through a

material to its maximum velocity (3x108 m/s)

Index of Refraction

Material Index of Refractionvacuum 1.00

air1.0003 (approximately

1.00)ice 1.309

water 1.33ethyl alcohol 1.36

plexiglass 1.51crown glass 1.52flint glass 1.61diamond 2.42

What do you notice about all the index of refraction values?

Note: Higher the index of refraction it has a HIGHER (optical) density, and slower speed.

RefractionThe more the bending, the more the refractive

index or the optical density of the medium.

Here the light ray travels from a more optically densemedium (water) to a lessoptically dense medium(air)[to enter our eyes], -and so the ray bends away from the normal.

Angle of Refraction

The angle the refractedray makes with the normal line

Index of Refraction Example

• If the speed of light in quartz is 1.95 x 108 m/s, what is its index of refraction?

Try it!

• If the index of refraction of water is 1.33, what is the speed of light in water?

Snell’s Law

n1 = Index of refraction of incident medium

n2 = index of refraction “refracted medium”

1 = angle of incidence

2 = angle of refraction

Snell’s Law Example

If a ray of light hits the surface of a pool at an angle of 350, at what angle will it travel through the pool? (n=1 for air, n=1.3 for water)

n1sinθ1=n2sinθ2

If the ray of refraction is bent toward the normal the medium is

more dense. If it is bent away from the normal it is less dense.

Dispersion

• Separating white light into ROYGBIV• Each color of light has its own wavelength and

will have its own refractive index. Because of this when light hits certain surfaces each color will refract at a slightly different angle … rainbow

LT Quiz #1

-What makes diamonds sparkle?– How do fiber optics keep us communicating?– How do those cool holiday decoration light up?

– This property of light is called total internal reflection.

• When light passes from a more dense medium (high n value) to a less dense medium (low n value) at a certain angle, all light is reflected back into the first medium and there is no refracted ray.

Critical Angle • any angle < critical angle refracted• any angle = critical angle runs along the boundary• any angle > critical angle totally reflected

• The angle at which this will occur is called the critical angle.

• TIR results in a refracted angle of 90°.

• This means that Snell’s Law for TIR is:n1sinѲ1 = n2sinѲ2

n1sinѲc = n2sin(90°)n1sinѲc = n2(1)sinѲc = n2 n1

total internal reflection.

Make sure your calculator is in degree mode!

Examples• What is the critical angle

for light traveling from diamond (n = 2.42) to air? sinѲc = n2

= 1.00 Ѳc = 24.4° n1 2.42

• What about diamond to water?

sinѲc = n2 = 1.33 Ѳc = 33.3°

n1 2.42

• What about water to diamond?– Not possible!

Demos

Total internal reflection

• There is no refracted ray

How to Find the Critical Angle for Any Material The critical angle is when light travels along theboundary of two mediums (900 to the normal).To find critcal angle use θ2 = 90

n1sinθ1=n2sinθ2

sin(900) = 1 … Snell’s Law at critical angle becomes …

n1sinθ1=n2

Solving for the critical θ1 … Snell’s Law becomes …

θ1= Sin-1(n2/n1)

LT Quiz #3

Lenses

Convex and Concave Lenses

Convex LensThicker in the middle than at the edges – converges rays

Produces real and virtual images depending on distance away from the lens

Concave LensThinner in the middle than at the edges – diverges rays

Produces virtual images upright and smaller

Lenses can be used to magnify of de-magnify images

How the Eye Works….

• Video clip: https://www.youtube.com/watch?v=POLSLQrelCw

Myopia - Nearsighted -Light rays focus too early-Can see things clearly that are near

Nearsightedness corrected using a diverging lens

What type of lens would be used to correct this problem?

Hyperopia–Light rays focus too far behind retina–Farsighted–Can see things clearly far away

What type of lens would be used to correct this problem?

Farsightedness is corrected using a converging lens

LT Quiz #4

LT Quiz #5