STUDIO
PHY 2054 College Physics II
Drs. Dubey & Bindell
[ THE REFLECTION OF LIGHT:
MIRRORS
PART I]
1|Page
Aim: To understand how we see reflected objects.
1. Describe why you cannot see the object when the lights are off.
2. Why you cannot see the object when something is blocking it even though the lights are on?
3. Consider how you might represent light coming from a light bulb (turned on) and a flashlight
pictured below. Sketch how you would represent the light coming from the bulb and the flashlight
in the diagram below. Describe what aspects of the light your diagram is attempting to represent.
2|Page
4. Could you consider light coming from a light bulb (not turned on) and a tennis ball? If so, sketch
how you would represent the light coming from the bulb and the tennis ball pictured below.
Describe what aspects of the light your diagram is attempting to represent.
To discuss reflection, it is necessary to understand the concept of a wave front and a ray of light.
Both sound and light are kinds of waves. Sound is a pressure wave while light is electromagnetic
wave. The ideas of a wave front and a ray apply to both. A hemispherical view of a sound wave
emitted by a pulsating sphere. The rays are perpendicular to the wave fronts.
These surfaces of constant phase are called
wave fronts. The figure shows hemispherical
view of the wave fronts. If the wave fronts are
drawn through the crests, as they are in the
picture, the distance between adjacent wave
fronts equal the wavelength λ. The radial
lines pointing outward from the source and
perpendicular to the wave fronts are called
rays. The rays point in the direction of the
velocity of the wave.
3|Page
Following figure shows curved wave fronts.
For the following figure,
(a)
draw the wave fronts.
(b)
What is the shape of the wave front (curved or plane)?
The concepts of wave fronts and rays can be used to describe light waves. For light waves, the
ray concept is particularly convenient when showing the path taken by the light. We will make
frequent use of light rays, which can be regarded essentially as narrow beams of light much like
those that lasers produce.
4|Page
Law of Reflection
Most objects reflect a certain portion of the light falling on them. The goal of the following
experiment is to study the law of reflection.
EQUIPMENT NEEDED:
Optics Bench, Light Source, Ray Table and Base, Component Holder, Slit Plate, Slit Mask, Ray
Optics Mirror.
Introduction
The shape and location of the image created by reflection from a mirror of any shape is determined by just
a few simple principles. One of these principles you already know: light propagates in a straight line.
In this experiment, you will observe the reflection of a single ray of light from a plane mirror. The
principles you discover will be applied, in later experiments, to more complicated examples of reflection.
Procedure
Set up the equipment as shown in Figure 2.1. Adjust the components so a single ray of light is aligned
with the bold arrow labeled “Normal” on the Ray Table Degree Scale. Carefully align the flat reflecting
surface of the mirror with the bold line labeled “Component” on the Ray Table. With the mirror properly
aligned, the bold arrow on the Ray Table is normal (at right angles) to the plane of the reflecting surface.
Rotate the Ray Table and observe the light ray. The angles of incidence and reflection are measured with
respect to the normal to the reflecting surface, as shown in Figure 2.2.
5|Page
Figure 2.2 Incident and Reflected Rays
By rotating the Ray Table, set the angle of incidence to each of the settings shown in Table 2.1. For each
angle of incidence, record the angle of reflection (Reflection1).
Table 2.1
Angle of: Angle of:
Incidence Reflection1
θi
θr
0°
10°
20°
30°
40°
50°
60°
70°
80°
90°
6|Page
Repeat your measurements with the incident ray coming from the opposite side of the normal. For each
angle of incidence, record the angle of reflection (Reflection 2) in the table 2.2.
Table 2.2
Angle of: Angle of:
Incidence Reflection2
θi
θr
0°
10°
20°
30°
40°
50°
60°
70°
80°
90°
1. Are the results for the two trials the same? If not, to what do you attribute the differences?
2. Part of the law of reflection states that the incident ray, the normal and the reflected ray all lie in
the same plane. Discuss how this is shown in your experiment.
3.
What relationship holds between the angle of incidence and the angle of reflection?
7|Page
4. The Law of Reflection has two parts. State both parts.
5. The drawing shows top view of an object located to the right of a mirror. A single ray of light is
shown leaving the object. After reflection from the mirror, through which location, A, B, C, or D,
does the ray pass?
6. For the following two surfaces, draw the reflected rays.
(a) In figure (a), are the reflected rays parallel to each other?
8|Page
(b) In figure (b) are the reflected rays parallel to each other?
When parallel light rays strike a smooth, plane surface, the reflected rays are parallel to each other. This
type of reflection is known as specular reflection. An irrefular surface reflects the light rays in various
directions. This type of reflection is known as diffuse reflecton.
Image Formation in a Plane Mirror
EQUIPMENT NEEDED:
Optics Bench
Light Source
Ray Table and Base
Component Holder
Slit Plate -Ray Optics Mirror
Introduction
Looking into a mirror and seeing a nearly exact image of yourself hardly seems like the result of simple
physical principles. But it is. The nature of the image you see in a mirror is understandable in terms of the
principles you have already learned: the Law of Reflection and the straight-line propagation of light.
In this experiment you will investigate how the apparent location of an image reflected from a plane
mirror relates to the location of the object, and how this relationship is a direct result of the basic
principles you have already studied.
Procedure
Set up the equipment as shown in Figure 3.1. Adjust the Slit Plate and Light Source positions for sharp,
easily visible rays.
As shown, place a blank, white sheet of paper on top of the Ray Table, and place the Ray Optics Mirror on
top of the paper. Position the mirror so that all of the light rays are reflected from its flat surface. Draw a
line on the paper to mark the position of the flat surface of the mirror.
Look into the mirror along the line of the reflected rays so that you can see the image of the Slit Plate and,
through the slits, the filament of the Light Source. (Rotate the mirror as needed to do this.)
9|Page
1. Do the rays seem to follow a straight line into the mirror?
With a pencil, mark two points along one edge of each of the incident and reflected rays. Label the points
(r1,r2, etc.), so you know which points belong to which ray.
Remove the paper and reconstruct the rays as shown (Figure 3.2), using a pencil and straightedge.
If you need to, tape on additional pieces of paper. Draw dotted lines to extend the incident and reflected
rays. (If this ray tracing technique is unfamiliar to you, review ray tracing in Experiment 1: Introduction to
Ray Optics.)
On your drawing, label the position of the filament and the apparent position of its reflected image.
2. What is the perpendicular distance from the filament to the plane of the mirror (distance d1, as shown in the
Figure 3.2)?
3. What is the perpendicular distance from the image of the filament to the plane of the mirror (distance d2, as
shown in the Figure)?
10 | P a g e
Change the position of the mirror and the Light Source and repeat the experiment.
1. What is the relationship between object and image location for reflection in a plane mirror?
2. How does the size of the image reflected from a plane mirror relate to the size of the object?
3. Is the image upright with respect to the object?
4. A friend is standing 2 m in front of a plane mirror. You are standing 3 m directly behind your friend.
(a) How far is your image behind the mirror?
(b) What is the distance between you and the image of your friend?
5. Suppose that you are walking perpendicularly with a velocity of +0.90 m/s toward a stationary plane
mirror. What is the velocity of your image relative to you? The direction in which you walk is
positive direction.
11 | P a g e
12 | P a g e