Lenses

Refraction (p 308) Refraction occurs when a wave changes the direction in which it is moving This is caused by a change in speed as the wave passes from one medium to another

Similar to reflection, the angles of incidence and the angles of refraction are measured with respect to the normal (p308) the index of refraction the greater the greater the change in direction: Diamond: n = 2.42; ice: n = 1.31; air n = 1.00

( p 310) Snell’s Law: Question: what happens to the frequency of the waves or light when it passes from one medium to another? Answer: The frequency done not change

(p 310) when light passes from a low index of refraction to a higher index of refraction, light slows down & bends toward the normal (add in margin) bending toward normal: from smaller “n” to a larger “n”

(p 310) when light passes from a high index of refraction to a lower index of refraction, light speeds up & bends away from the normal (add in margin) bending away from normal: from larger “n” to a smaller “n”

(add to margin p 310) Example 1: H 2 O Air Glass air

Example 2:Example 3 : (H 2 O drop) Example 4:Example 5: white light rainbow A ray on the normal does not bend

Where would you have to aim the spear to catch the fish? (p 312) A pool of water is deeper than it appears because the light waves are sharter in water

Total internal reflection (add to p 313) total internal reflection only occurs when light rays are traveling from larger to smaller “n”

The angle of incidence which results in a refracted angle of 90 o is called the critical angle

Critical Angle Calculation (add diagram to p315 #3) What is the critical angle for air/lucite (n =1.51) interface?

(p 312)The highway can appear to be wet. The wet road is a mirage. What is seen is a reflection of the sky on the hot road (or sand) The index of refraction of hot air is less that the index of refraction of cool air. Total reflection can occur-you see a reflection of the sky the twinkling of stars or distant lights is due to refraction. The direction of the light changes as it passes through the air at different temperatures

You see an image of the sun for a few minutes before it rises and after it sets because the earth’s atmosphere refracts the light from the sun Diagram:

Fiber Optics (p 313) Fiber optics make use of total internal reflection Fiber optics are replacing electric ciruits in communication technology Physicians use fiber optics to look inside your body

Refraction from Lenses Lenses are a practical application of refraction (p 326) there are 2 types of lenses: concaveconvex

(p326)A convex lens is a converging lens: it brings light rays together (Diagram p 327)

(p 326) A concave lens is a diverging lens: it spreads out light rays (Diagram p 327)

(p 328) Because light can travel in either direction through a lens, we usually indicate a focal point on both sides of a lens A convex lens has a real focal point A concave lens has a virtual focal point (add to margin p328) concaveconvex mirror f +f - lensf -f +

Ray Diagram for Convex (Converging) Lenses We will use the same rays as with mirrors (p329a) Ray 1: parallel and through “f” (p 330c) Ray 3:through “f” and then parallel

(add p330)(remember rays pass through lenses but bounce off (reflect from) mirrors!) object outside “2f”: imageinverted, smaller real object at “2f’:imageinverted same size real Object betweenimageinverted “2f” and “f”larger real

Object at “f”no image produced Object inside “f”image erect larger virtual

The image is erect, larger and virtual Ray Diagram for Convex Lens: object inside “f” We still follow the same pattern but we have to extend the refracted rays:

Ray Diagram for Diverging Lens (p 332) The image is erect, smaller, virtual

Chromatic Aberration (add to margin p333) Different wavelengths of light refracted by a lens focus at different points –Violet rays are refracted more than red rays –The focal length for red light is greater than the focal length for violet light Chromatic aberration can be minimized by the use of a combination of converging and diverging lenses

Sign Conventions for lenses (p332) the characteristics of images formed by convex lenses are the same as those formed by concave mirrors the characteristics of images formed by concave lenses are the same as those formed by convex mirrors The sign conventions for mirror and lenses are the same: real + virtual – erect + inverted -

Lens Formulae Lens equation: Magnification: (p 333/334)See example problems 1 & 2