Reflection and Refraction

Reflection

The Law of Reflection Law of Reflection – the angle of incidence and the angle of reflection are equal

The Law of Reflection Reflection - a wave bounces back into the first medium when hitting a boundary of a second medium Our brain thinks light travels in straight-lines. Angle of Incidence – angle made by the incident ray and the normal Angle of Reflection – angle made by the reflected ray and the normal

Reflection of Sound An echo is reflected sound Sound reflects from all surfaces of a room Acoustics is the study of the way sound reflects off of objects in a room Reverberations – Multiple reflections of sound within a room The walls of concert halls are designed to make the reflection of sound diffuse

Ex: Sitting in her parlor one night, Gerty sees the reflection of her cat, Whiskers, in the living room window. If the image of Whiskers makes an angle of 40° with the normal, at what angle does Gerty see him reflected?

Reminder: Speed of Light What speed does light travel at in a vacuum? In air? 3 x 108 m/s Does light speed up, slow down, or stay the same speed when it travels through different media? Light travels fastest in a vacuum and slows down in water, glass, and other media

Refraction Refraction: Bending of waves as they change speeds. This change in direction is caused by the fact that light travels at different speeds in different media. The imaginary line (perpendicular to surface) to which we measure the angles is called the normal line. Pool appears shallower, pencil appears bent

Refraction

Snell’s Law Relationship between the angle of incidence (θ1) and angle of refraction (θ2) can be explained by Snell’s Law. Speed of light is greatest in a vacuum (c = 3 x 108 m/s), so it is convenient to compare to this value. This ratio is called the index of refraction (n).

Snell’s Law and Refraction Another way to write Snell’s Law: Crossing a boundary, the frequency of a wave does not change (to conserve energy). θ, v, and λ do change. Our brain thinks light travels in straight lines. Ex: We see the fish shallow and far from the boat.

Refraction

Ex: Hickory, a watchmaker, is interested in an old timepiece that’s been brought in for a cleaning. If light travels at 1.90 x 108 m/s in the crystal, what is the crystal’s index of refraction?

Ex: While fishing out on the lake one summer afternoon, Amy spots a large trout just below the surface of the water at an angle of 60.0° to the vertical, and she tries to scoop it out of the water with her net. a) Draw the fish where Amy sees it. b) At what angle should Amy aim for the fish? (nwater = 1.33).

Ex: Water has an index of refraction (n) of 1.33. Find the speed of light in water. Light from the tuna’s snout heads to the surface at 380 and refracts to Carl’s eye. Find θ. Extend Carl’s line of sight and sketch an imaginary tuna at the location that he envisions it.

Mirrors and Lenses

Lenses Lens – a piece of glass which bends parallel rays so that they cross and form an image Lenses work because light bends as it goes from the air into the glass. (THINK: Refraction) Eye glasses and telescopes use lenses to help us see better.

Eyes Your lenses in your eyes change size all the time. When you look at objects real close up, the lens gets thicker. If you look at objects far away, it gets thinner. It does this to help you focus the correct image on the retina. After light passes through the lens it shines through the vitreous humor to the back of the eye where it hits the retina. The retina takes the light and changes it into nerve impules so the brain can understand what the eye sees. In both the camera and the eye, the image is upside down, our brain flips the image right-side up for us.

Mirrors Your eye cannot tell the difference between an object and its virtual image Virtual Image – the point located behind a mirror where an object appears to originate The image is as far behind a mirror as the object is in front of the mirror

Mirror Image