Refraction, Lenses, & Sight

Refraction• The change in direction of a wave as is

crosses the boundary between two media in which the wave travels at different speeds.

Index of Refraction

materialinlightofspeed

vacuuminlightofspeedn

Snell’s Law

2211 sinsin nn

Index of RefractionMaterial Index of RefractionVacuum 1.0000

Air 1.0003Ice 1.3100

Water 1.3330Ethyl Alcohol 1.3600

Plexiglas 1.5100Crown Glass 1.5200

Light Flint Glass 1.5800Dense Flint Glass 1.6600

Zircon 1.9230Diamond 2.4170

Rutile 2.9070Gallium phosphide 3.5000

Dispersion

• The separation of light into colors arranged according to their frequency, by interaction with a prism or diffraction grating.

Rainbows• White light separates into different colors

(wavelengths) on entering the raindrop because red light is refracted by a lesser angle than blue light. On leaving the raindrop, the red rays have turned through a smaller angle than the blue rays, producing a rainbow.

Lenses

Converging Lens

Rules For Converging Lenses

1) Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens.

2) Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

3) An incident ray which passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

Image Formation by Converging Lens

Diverging Lens

Rules For Diverging Lenses1) Any incident ray traveling parallel to the

principal axis of a diverging lens will refract through the lens and travel in line with the focal point (i.e., in a direction such that its extension will pass through the focal point).

2) Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

3) An incident ray which passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

Diverging Lens Image Formation

Always Virtual, Smaller, and Right-Side Up

The Human Eye

The Human Eye

Normal Eye Focus

Near-Sighted (Myopia)

Far-Sighted (Hypermetropia)

Astigmatism• Cornea is oval shaped instead of spherical.

This causes light to focus on two or more spots.

LASIK

Laser-assisted In Situ Keratomileusis