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Home , Diffuse reflection, Refraction

Chapter 22

Reflection and of

Wavelength Dual Nature of Light

The distance between any two crests of the • Experiments can be devised that will is defined as the display either the wave nature or the particle nature of light • Nature prevents testing both qualities at the same time

Geometric – Using a The Nature of Light Approximation • “Particles” of light are called • Light travels in a straight-line path in a • Each has a particular energy homogeneous medium until it –E = h ƒ encounters a boundary between two – h is Planck’s constant different media • h = 6.63 x 10-34 J s •The ray approximation is used to – Encompasses both natures of light represent beams of light • Interacts like a particle •A ray of light is an imaginary line drawn • Has a given like a wave along the direction of travel of the light beams Ray Approximation Geometric Optics

•A wave front is a surface passing through points of a wave that have the same phase and amplitude • The rays, corresponding to the direction of the wave motion, are perpendicular to the wave fronts

Reflection QUICK QUIZ 22.1

Diffuse refection: The objects has irregularities that spread out an initially parallel beam of Which part of the figure below shows of light in all directions to produce light from the roadway?

Specular reflection (): When a parallel beam of light is directed at a smooth surface, it is specularly reflected in only one direction.

The of an object we see depends on two things: Law of Reflection The angle of incidence = the angle of reflection The kind of light falling on it and nature of its surface

For instance, if light is used to illuminate an object that absorbs all color other than red, the object will appear red. What color will the object show if green light is used instead?

A white object reflects light of all equally well, a object, on the other hand, absorbs light of all wavelengths, and it appears black no matter what color light reaches it. Reversed Image

Spherical Mirror

Focal Length f=r/2 f=r/2 (r=radius of the mirror)

Three rays that determine an image

O’

C O F

O’

C O F

di do O’ I C O F 1/do+1/di=1/f=2/r m=hi/ho=- di/do

Spherical Mirror Sign Conventions for Spherical Question: All real images Quantities Positive Negative (a) are upright f Concave mirror Convex mirror (b) are inverted do Real object Virtual object (c) can appear on a screen di Real image Virtual image m Upright image Inverted image (d) cannot appear on a screen

m=h /h =- d /d 1/do+1/di=1/f=2/r i o i o Answer: c

Question: When an object is reflected in a plane Example: A dentist wants a small mirror that, when 2.20 cm from a tooth, will produce a 4.5 X upright image. What mirror, the image is always kind of mirror must be used and what must its radius of (a) real curvature be? Solution: m=hi/ho=-di/do (b) inverted 4.5=-di/(2.20 cm), which gives di=-9.90 cm (c) enlarged The is given by 1/do+1/di=1/f, i.e., 1/(2.20cm) +1/(-9.90 cm)=1/f, which (d) left-right reversed yields f=2.83 cm Focal length is positive, the mirror is concave with a radius of r=2f=5.7 cm

Answer: d

Refraction Index of Refraction • Refraction refers to the bending of a light ray when it goes from one medium to n=c/v another where c is the speed of the light in • Refraction occurs because light travels at (3x108 m/s), and v the speed in a given different speeds in the two media materials Question: A light ray enters one medium from another along the normal. The angle of Snell’s Law reflection (a) is 0° n1sini =n2sinr Or (b) is 90°

sini /sinr =v1/v2 (c) equals the critical angle (d) depends on the indexes of refraction of the two media.

Answer: a

Question: When a beam of light enters one Question: The index of refraction of a materials medium from another, a quantity that never medium changes is its (a) is always less than 1 (a) direction (b) is always equal to 1 (b) speed (c) is always greater than 1 (c) frequency (d) may be less than, equal to, or greater than 1 (d) wavelength

Answer: c Answer: c

Example: A beam of parallel light enters a Question: When you look down into a block of ice an angle of incidence of 30o. swimming pool, are you likely to What is the angle refraction in the ice? underestimate or overestimate its depth? (n =1 and n =1.31) air ice Answer: The pool appears shallower than it Solution: o is. sinr=n1/n2sini=1/1.31sin30 =0.382, thus, r=22o QUICK QUIZ 22.2 QUICK QUIZ 22.2 ANSWER

If beam 1 is the incoming beam in the figure below, which of the other four beams are reflected and which are refracted? Beams 2 and 4 are reflected; beams 3 and 5 are refracted.

QUICK QUIZ 22.3 QUICK QUIZ 22.3 ANSWER

A material has an index of refraction that increases (b). When light goes from one material into continuously from top to bottom. Of the three paths shown in the figure below, which path will a light ray one having a higher index of refraction, it follow as it passes through the material? refracts toward the normal line of the boundary between the two materials. If, as the light travels through the new material, the index of refraction continues to increase, the light ray will refract more and more toward the normal line.

QUICK QUIZ 22.4 QUICK QUIZ 22.4 ANSWER

(c). Both the wave speed and the As light travels from vacuum wavelength decrease as the index of (n = 1) to a medium such as (n > refraction increases. The frequency is 1), which of the following properties unchanged. remains the same: (a) wavelength, (b) wave speed, or (c) frequency?