Refraction & Lenses Chapter 18

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

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

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

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

Snell’s Law n 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)

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

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

n Draw a diagram showing the two media. Label them, indicating the two indices of refraction, n i and n r n Draw a normal to the surface n Use Snell’s law to calculate the angle of refraction Problem Solving Strategy

n 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 n Eventually the incidence angle is so great that all the light reflects back into the medium n No refraction takes place Total Internal Reflection

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

water air critical ray Calculate the critical angle cc

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

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

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

n 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 n Same in the evening, the rays bend and reach us after the sun has actually set

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

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

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