Download Chapter 22 Reflection and Refraction of Light Wavelength The

Chapter 22
Reflection and Refraction
of
Light
Wavelength
The distance between any two crests of the
wave is defined as the wavelength
The Nature of Light
• “Particles” of light are called photons
• Each photon has a particular energy
–E=hƒ
– h is Planck’s constant
• h = 6.63 x 10-34 J s
– Encompasses both natures of light
• Interacts like a particle
• Has a given frequency like a wave
Dual Nature of Light
• Experiments can be devised that will
display either the wave nature or the
particle nature of light
• Nature prevents testing both qualities at
the same time
Geometric Optics – Using a Ray
Approximation
• Light travels in a straight-line path in a
homogeneous medium until it
encounters a boundary between two
different media
• The ray approximation is used to
represent beams of light
• A ray of light is an imaginary line drawn
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
Diffuse refection: The objects
has irregularities that spread
out an initially parallel beam of
light in all directions to produce
diffuse reflection
QUICK QUIZ 22.1
Which part of the figure below shows specular reflection of
light from the roadway?
Specular reflection (mirror):
When a parallel beam of light
is directed at a smooth
surface, it is specularly
reflected in only one direction.
The color 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 white 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 wavelengths equally well, a black
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
I
di
do
O’
C
O F
1/do+1/di=1/f=2/r
m=hi/ho=- di/do
Spherical Mirror
Sign Conventions for Spherical Mirrors
Quantities
f
do
di
m
Positive
Negative
Concave mirror
Real object
Real image
Upright image
1/do+1/di=1/f=2/r
Convex mirror
Virtual object
Virtual image
Inverted image
Question: All real images
(a) are upright
(b) are inverted
(c) can appear on a screen
(d) cannot appear on a screen
m=hi/ho=- di/do
Answer: c
Question: When an object is reflected in a plane
mirror, the image is always
(a) real
(b) inverted
(c) enlarged
(d) left-right reversed
Example: A dentist wants a small mirror that, when 2.20 cm
from a tooth, will produce a 4.5 X upright image. What
kind of mirror must be used and what must its radius of
curvature be?
Solution: m=hi/ho=-di/do
4.5=-di/(2.20 cm), which gives di=-9.90 cm
The focal length is given by
1/do+1/di=1/f, i.e., 1/(2.20cm) +1/(-9.90 cm)=1/f, which
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
• Refraction refers to the bending of a light
ray when it goes from one medium to
another
• Refraction occurs because light travels at
different speeds in the two media
Index of Refraction
n=c/v
where c is the speed of the light in vacuum
(3x108 m/s), and v the speed in a given
materials
Snell’s Law
n1sini =n2sinr
Or
sini /sinr =v1/v2
Question: A light ray enters one medium from
another along the normal. The angle of
reflection
(a) is 0°
(b) is 90°
(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
medium from another, a quantity that never
changes is its
(a) direction
(b) speed
(c) frequency
(d) wavelength
Question: The index of refraction of a materials
medium
(a) is always less than 1
(b) is always equal to 1
(c) is always greater than 1
(d) may be less than, equal to, or greater than 1
Answer: c
Answer: c
Example: A beam of parallel light enters a
block of ice an angle of incidence of 30o.
What is the angle refraction in the ice?
(nair=1 and nice=1.31)
Solution:
sinr=n1/n2sini=1/1.31sin30o=0.382, thus,
r=22o
Question: When you look down into a
swimming pool, are you likely to
underestimate or overestimate its depth?
Answer: The pool appears shallower than it
is.
QUICK QUIZ 22.2
If beam 1 is the incoming beam in the figure
below, which of the other four beams are
reflected and which are refracted?
QUICK QUIZ 22.3
A material has an index of refraction that increases
continuously from top to bottom. Of the three paths
shown in the figure below, which path will a light ray
follow as it passes through the material?
QUICK QUIZ 22.4
As light travels from vacuum
(n = 1) to a medium such as glass (n >
1), which of the following properties
remains the same: (a) wavelength, (b)
wave speed, or (c) frequency?
QUICK QUIZ 22.2 ANSWER
Beams 2 and 4 are reflected; beams 3 and 5 are
refracted.
QUICK QUIZ 22.3 ANSWER
(b). When light goes from one material into
one having a higher index of refraction, it
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 ANSWER
(c). Both the wave speed and the
wavelength decrease as the index of
refraction increases. The frequency is
unchanged.