Refraction & Lenses

Refraction & Lenses Chapter 18

Refraction of Light • Look at the surface of a swimming pool • Objects look distorted • Light bends as it goes from one medium into another\ • Why?

When a medium causes a wave to slow down it is more optically dense

Entering more optically dense • Waves moving into a more optically dense medium will cause the wave speed to slow down and then bend toward the normal • Therefore, the angle of refraction is smaller than the angle of incidence

Entering less optically dense • Waves moving into a less optically dense medium will cause the wave speed to increase and then bend away from the normal • Therefore, the angle of refraction is larger than the angle of incidence

Snell’s Law • Dutch Scientist Willebrord Snell discovered that a ray of light bends in such a way that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, index of refraction (n)

ni sin i nr sinr • ni index of refraction for the incident medium •  i angle of incidence • nr index of refraction for the refracting medium •  r angle of refraction

Index of refraction • ratio of speed of light in a vacuum, c, to another medium, v • n=c/v • index of refraction in a vacuum is 1.00 • table on page 486

Total Internal Reflection • If ray travels from a more optically dense into a less optically dense medium the angle of refraction is larger than the angle of incidence • Eventually the incidence angle is so great that all the light reflects back into the medium • No refraction takes place

Critical Angle • The incident angle that causes the refracted ray to lie right along the boundary of the substance • Unique to each substance

Critical Angle air critical ray water qc Calculate the critical angle

Total Internal Reflection • When light enters a thin glass rod light is internally reflected (fiber optics) • Telephone, computer and video signals • Explore the human body • Plants use internal reflection

Effects of Refraction • Mirages • Summertime - the sun hits to road and causes the air above the road to heat up • The index of refraction for warm air is 1.00026 • The index of refraction for cool air is 1.00028

This small change in index of refraction causes the rays to bend • This bending makes the road look like there is a puddle on it

Because light travels slightly slower in Earth’s atmosphere than in outer space the sun rays bend causing the sun to reach us before the sun is actually above the horizon • Same in the evening, the rays bend and reach us after the sun has actually set

Dispersion of Light • Light of all wavelengths travels at the same speed in a vacuum • Other media causes the speed to slow down • The wavelength also determines the speed & index of refraction

In most materials red light travels fastest • It also has the smallest index of refraction • Violet is the slowest and has the largest index of refraction • Red is bent the least, violet the most

Dispersion • The separation of light into a spectrum by refraction • Diamond • Rainbow (water)

18.2 Convex & Concave Lenses • 1303 French physician wrote of using lenses to correct eyesight • 1610 Galileo used two lenses to make a telescope • He then discovered the moons of Jupiter • Now; microscopes, cameras, limitless uses

Type of Lenses • Lens-piece of transparent material used to focus light and form an image • Refractive index needs to be larger than air

Convex Lens • Thicker at the center than at the edges • Sometimes called a converging lens • Why? • What type of image will convex lenses produce? • Similar to concave mirrors

Concave Lens • Thinner in the middle than at the edges • Rays pass through the lens and spread out • Sometimes called a diverging lens • What type of images are formed? • Why? • Similar to convex mirrors

Lens Equations • Same as the mirror equations • Real images on the opposite side of the lens and the image • Real images di is positive • Virtual images are always on the same side of the lens and di is negative • Negative magnification means image is inverted and real

Real Images Formed by Convex Lens • No center of curvature, just F and 2F • F is the focal point and f is the focal length • Object needs to be placed beyond the focal point

Convex Lenses and Virtual Images • Object needs to be between the lens and the focal point • All rays diverge • Virtual image on the same side of the lens and larger in size

Concave Lens • Rays will always diverge • Always produce a virtual image • Same side of lens • Image will always be smaller

Ray Diagrams • Ray 1: Passes parallel to the principal axis to center of lens then bends to pass through the focal point • Ray 2: Passes through the focal point to the center of the lens and bends parallel to principal axis • Ray 3: Straight through the center of the lens at the principal axis

18.3 Applications of LensesLens in the Eye • Light travels through the cornea then the lens and then the image focuses on the retina • Retina works like the screen • The cells in the retina absorb this light and sends information about the image along the optic nerve to the brain

Focusing Images • Light is focused mostly by the air cornea boundary because this is the greatest difference in index of refractions • Lens does the fine focusing by accommodation (muscles around the lens contract, close up, or relax, far away) • changing the shape changes the focal length of the eye

Nearsightedness • Myopia, can’t focus on far away objects • Focal length too short so image forms in front of the retina • Use a diverging lens, concave lens, to spread the rays out so that the image distance increases

Farsightedness • Hyperopia, can’t focus on close up objects • The focal length of the eye is too long and the image forms behind the retina • Use a converging lens, convex lens, to produce a virtual images farther from the eye the image then becomes the object for the eye

Also happens as a person gets older and the lens becomes more rigid, the muscles can’t change the shape of the lens

Refraction & Lenses