UNIT 13-PHY 131-Chapter 23-Reflection and Refraction

23.
1
Unit 13 23.2
Chapter 23 23.3
Reflection and 23.6
23.9
Refraction of light
23.1 WAVEFRONTS AND RAYS
Sources of Light
Most objects we see are not sources of light; we see them by the
light they reflect or transmit.
Some fraction of the light incident on an object is absorbed, some
fraction is transmitted through the object, and the rest is reflected.
The nature of the material and its surface determine the relative
amounts of absorption, transmission, and reflection at a given
wavelength.
Wavefronts and Rays
A hemispherical view of a sound wave emitted by a pulsating
sphere. A ray points in the direction of propagation of a wave and
is perpendicular to the wavefronts. The distance between two
successive wavefronts is called the wavelength ()
At large distances from the source, the wave fronts become less and less curved.
Slide 2
23.2 THE REFLECTION OF LIGHT
The angle of incidence ( θi ) is the angle between an incident ray and
the normal; the angle of reflection ( θr ) is the angle between the
reflected ray and the normal.
Definition: The Laws of Reflection

The incident ray, the reflected ray, and the normal to the surface all lie
in the same plane, and the angle of incidence equals the angle of
reflection.
Slide 3
23.2 THE REFLECTION OF LIGHT
Specular and Diffuse Reflection
Reflection from a smooth surface is called specular
reflection; rays incident at a given angle all reflect at the
same angle.
Reflection from a rough, irregular surface is called diffuse

reflection. For diffuse reflection from rough surfaces, the angles
of reflection for the incoming rays are still equal to their
respective angles of incidence.
However, the normals to the rough surface are at random angles
with respect to each other, so the reflected rays travel in many
directions
Reflection and Transmission
When light reaches a boundary between two transparent media, such as from air to glass, some
of the light is reflected and some is transmitted into the new medium. The reflected light still
follows the same laws of reflection (as long as the surface is smooth so that the reflection is
specular).
For normal incidence on an air-glass surface, only 4% of the incident intensity is reflected; 96%
is transmitted.
Slide 4
23.3 THE REFRACTION OF LIGHT:
SNELL’S LAW
The speed of a wave (e.g. light) in any medium is given by:
𝑣 = 𝜆𝑓
where   the wavelength of a light (wave) in a medium.
f  the frequency of the source of light (wave)
Note: The wavelength of light from a source changes as it travels from one medium to the
next, while the frequency (f) is independent of the medium through which the wave travels but
only dependent of the source.
Light travels through a vacuum at a speed c  3.00 10 m s

8
Light travels through materials at a speed less than its speed in a vacuum.
DEFINITION OF THE INDEX OF REFRACTION
The index of refraction of a material is the ratio of the speed of light in a vacuum to the speed
of light in the material:
Speed of light in vacuum c
n 
Speed of light in the material v
Slide 5
When light travels from a material with
one index of refraction to a material
with a different index of refraction, the
angle of incidence is related to the
angle of refraction by
n1 sin 1  n2 sin  2
Slide 6
Example
A light ray strikes an air/water

surface at an angle of 46 degrees
with respect to the normal.
Calculate the angle of refraction
when the direction of the ray is
(a) from air to water and (b) from
water to air.
Slide 7
Example
n
(a) sin  2  1
sin 1

1.00  sin 46 
 0.54
n2 1.33
 2  33
n1 sin 1 1.33sin 46

(b) sin  2    0.96
n2 1.00
 2  74
Slide 8
Example 23.2
A beam of light strikes one face of a window pane with an angle of incidence of 30.0°. The
index of refraction of the glass is 1.52. The beam travels through the glass and emerges from a
parallel face on the opposite side. Ignore reflections.
(a) Find the angle of refraction for the ray inside the glass.
(b) Show that the rays in air on either side of the glass (i.e. the incident and emerging rays)
are parallel to each other.
Solution
(a)
Slide 9
Example 23.2
Solution
(b)
Slide 10
23.6 THE FORMATION OF IMAGES THROUGH
REFLECTION OR REFRACTION
Real and Virtual Images
There are two kinds of images. For the plane mirror, the light rays seem to
come from a point behind the mirror, but we know there aren’t actually any
light rays back there.
In a virtual image, we trace light rays back to a point from which they
appear to diverge, even though the rays do not actually come from that point.
In a real image, light rays actually do pass through the image point.
The simplest version of describing a real and virtual image is as follows:

Mirrors
• Real image appear on the side of the mirror with the object.
• Virtual image appear on the other side of the mirror with the object.
Thin Lenses
• Real image appear on the other side of the lens with the object.
• Virtual image appear on the same side of the lens with the object.
Slide 11
23.6 THE FORMATION OF IMAGES
THROUGH REFLECTION OR
REFRACTION
Slide 12
Converging lens With a converging lens, paraxial rays that are parallel to
the principal axis converge to the focal point.
Diverging lens: With a diverging lens, paraxial rays that are parallel to the
principal axis appear to originate from the focal point.
Slide 13
RAY DIAGRAMS
Slide 14
IMAGE FORMATION BY A CONVERGING LENS
Properties of the image: In this example, when the object is placed further than
twice the focal length from the lens, the real image, inverted and smaller than the
object.
Slide 15
Properties of image: When the object is placed between F and 2F, the real image,
inverted and larger than the object.
Slide 16
Properties of the image: When the object is placed between F and the lens, the
virtual image, upright and larger than the object.
Slide 17
IMAGE FORMATION BY A DIVERGING LENS
Properties of image: A diverging lens always forms an upright, virtual,

diminished image (i.e. smaller than the object).
Slide 18
The Thin-Lens Equation and the Magnification Equation
1 1 1 hi d
  m  i
do di f ho do
Note the notation used for the thin lens equation and magnification in this slide
in relation to the notation used in the book:
do = p and di = q
h0 = h and hi = h
Slide 19
Summary of Sign Conventions for Lenses
f is  for a converging lens.

f is  for a diverging lens.
do is  if the object is to the left of the lens.
do is  if the object is to the right of the lens.
di is  for an image formed to the right of the lens (real image).

di is  for an image formed to the left of the lens (virtual image).
m is  for an upright image.

m is  for an inverted image.
Slide 20
Example 23.9
A wild daisy 1.2 cm in diameter is 90.0 cm from a camera’s zoom lens. The
focal length of the lens has magnitude 150.0 mm.
(a) Find the distance between the lens and the image sensor.
(b) How large is the image of the daisy?
Strategy
The problem can be solved using the lens and magnification equations.
The lens must be converging to form a real image on the sensor, so f = +150.0
mm. The image must be formed on the sensor, so the distance from the lens to
the sensor is q.
After finishing the algebraic solution, we sketch a ray diagram as a check.
Given: h = 1.2 cm; p = 90.0 cm; f = +15.00 cm
Find: q, h′
Slide 21
Example 23.9
Solution
(a)
(b)
Properties of the image: Real, inverted and smaller than the object
Slide 22
Example 23.9
Solution
Slide 23
Example
Slide 24
Slide 25
End

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23.

Definition: The Laws of Reflection

Reflection from a rough, irregular surface is called diffuse

Light travels through a vacuum at a speed c  3.00 10 m s

A light ray strikes an air/water

n1 sin 1 1.33sin 46

The simplest version of describing a real and virtual image is as follows:

Properties of image: A diverging lens always forms an upright, virtual,

f is  for a converging lens.

di is  for an image formed to the right of the lens (real image).

m is  for an upright image.

(b) How large is the image of the daisy?

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