Week 10 - TOTAL Internal Reflection & DISPERSION.

Total internal reflection (1) When the light ray (from water) is emerging at the critical angle, the refracted ray will be along the interface. along Refracted Ray is along the surface AIR along water Incident Ray Refracted Ray

Total internal reflection (2) When the incident angle is larger than the critical angle, total internal reflection will occur (at the interface). AIR water Next

Total internal reflection (3) View under water!! AIR water

Total Internal Reflection Can occur when ray goes from higher n to lower n. Above a Critical angle (of incidence) the ray is reflected, not refracted For problems, set the angle of refraction to 90, and solve for critical angle

Problem Find the critical angle for a light ray that is incident from water to air. Answer: 48.8 o

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.

Q4 True or false: If the speed of… True or false: If the speed of light in raindrop is equal to that in air, there would be no rainbows. (T/F)

Reflection Specular Reflection. Diffuse reflection.   Specular Reflection. If the reflection interface is very smooth, specular reflection will occur. Specular reflection is mirror-like (forms images). Diffuse reflection. If the reflection interface is rough (non-metallic), diffuse reflection will occur. Diffuse reflection is where an incident ray is reflected at many different angles as opposed to specular reflection with only one angle of reflection. Laws of Reflection: The normal, incident ray, and reflected ray all lie on the same plane. The incident ray is the same angle from the normal as the reflected ray: The incident ray and reflected ray are on opposite sides on the normal.

Diffraction diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path The amount of diffraction increases with increasing wavelength. Examples: Diffraction of water waves is observed in a harbor as waves bend around small boats and are found to disturb the water behind them. The same waves however are unable to diffract around larger boats since their wavelength is smaller than the boat. http://www.acoustics.salford.ac.uk/feschools/waves/diffract.php

INTERFERENCE Constructive Destructive. Occurs when two waves meet each other. You calculate the height of the new wave or pulse by adding the values of the wave heights along the intersection. When waves or pulses collide, they do not stop, but go on with their original amplitude as if nothing had happened. Constructive Two waves reinforce each other. Destructive. Two waves cancel out each other.

Lenses

Converging Lens

Rules For Converging Lenses 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. 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. 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 Lenses 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). 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. 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

Problem The index of refraction for crown glass for red light is 1.514. What is the speed of red light in crown glass? Answer: 1.98 x 10 8 m/s