Chapter 8: Nature of Light, Reflection at Plane Surfaces

Keywords
eclipse
light
luminous bodies
non-luminous
bodies
penumbra

reflection

shadow

umbra

By the end of this chapter, the learner will be able to:

know illuminated and light source objects in everyday life.

understand how shadows are formed and that eclipses are natural forms of shadows.

understand how the reflection of light from plane surfaces occurs and how we can make use of this.

8.1: Introduction

What can happen if there is no light at all? How do we see objects? Have you ever wondered how shadows are formed? For example, is called photosynthesis (process by which green plants and certain green plants use sunlight to make their own food. This phenomenon other organisms make their own food). Humans and animals need it to
enable them to see.

In this chapter, you will investigate how some objects emit light, while other objects, such as a mirror, simply reflect light. You will also understand the applications of properties of light.

8.2: Light Source

Sources of light are categorised into two sources:
Luminous sources (emiting light, self-luminous or primary). These sources of light produce or make their light.
Non-luminous sources (reflecting light, illuminated sources or secondary source) do not produce their own light but reflect it from luminous sources.
Both luminous and non-luminous sources are further categorised into natural (God – made) and artificial (man – made) sources.
Activity 8.1 Discussing light sources

1. (a) In groups, suggest and discuss the available examples of luminous and non-luminous sources.
   (b) Categorise the examples stated into natural and artificial sources of light.
2. In groups, use a battery, wires, a bulb and a bulb holder to make a lit bulb.

8.3: Transmission of Light

What happens when light falls on a material?
How light behaves when it strikes an object depends on many factors including the material from which the object is made. Materials can be:

Transparent: For example, glass, mica and water. These allow light to pass freely through them. You can see through them. Translucent: For example, ground glass and oiled paper. These allow a portion of the light to pass through them and absorb the rest. Light is scattered and transmitted somehow. You can see through them.
Opaque: Many bodies entirely prevent the light from passing through them, and are said to be opaque. Light is either
reflected or absorbed. Therefore, you cannot see through them.

8.4: Rectilinear Propagation of Light

Activity 8.2 Demonstrating that light travels in a straight line
Key Question: How can we know that light travels in a straight line?
What you need
Three cardboard screens having small holes in their centres and source of light.
What to do

Take three pieces of cardboards.

Set up the cardboard so that the holes are in a straight line, as shown below:

Light a candle in front of point A.
Observe the light candle at point C.

Observe what happens when one cardboard is moved
so that the holes are no longer in a straight line. What conclusion can you draw?
Share your findings with the whole class.

8.5: Light Ray and Light Beam

From this experiment, we can conclude that “light travels along a straight line”. These lines are referred to as rays.
The “ray” is the direction or path taken by light. A beam is a stream of light energy, and may be represented by a number of rays which be diverging, converging or parallel, as shown in Figure 8.1.

8.6: Shadows

What happens when an opaque object is placed in front of a source of light? Since light travels in a straight line, its passage is stopped by the opaque object. Some areas, located behind the opaque object, do not receive light and the dark region having the shape of the opaque object is formed. This dark region is known as a shadow.

Types of shadows

Activity 8.3 Investigating the types of shadows

1. (a) In groups, using the internet and other sources like textbooks, discuss and write some notes on different types of shadows and explain how they are formed.
   (b) Present your work to the rest of the class.

Pinhole camera

Activity 8.4 Constructing a pinhole camera
Key Question: How does a pinhole camera work?
What you need
Dark room, a cuboid made of cardboard, any object.
What to do
a) Take a small piece of cardboard of cubic form and pierce a pinhole in one of its faces.
b) On the opposite face, place a piece of oiled paper.

c) Put a lighted candle in front of the pinhole. What do you observe on the oiled paper?

d) What happens to the image when the object is moved closer to or farther away from the pinhole camera?
e) Now, try to make the pinhole larger. What happens to the image?
f) Share your observations with the rest of the class members.

EXAMPLE 8.1
An object whose height is 2 m is distant from a hole of a pinhole camera by 6 m. The length of the camera is the same as image distance 14 cm. Find the height of the image and the magnification of the camera.

Nature of image formed in a pinhole camera

It is upside down or an inverted image.

Image is real (it can be formed on the screen).

Image is less bright (the clarity of image will depend on the position of the object and on size of the pierced pinhole).

Image has a different size to the lamp object (enlarged image is larger than actual object while reduced image is smaller than object).

According to the size of the pinhole, the image may be slightly blurred. A blurred image results if the pinhole is made larger than the normal.
If an extended source is used, the shadow is seen to be edged with a border of partial shadow called a penumbra and a dark of total shadow or umbra.
Shadows are formed because light travels in straight lines.

Eclipse

An eclipse occurs when the sun, moon and the Earth lie in a straight line.
As the moon and the Earth revolve around each other and the sun, they block some of the sun’s light. When the sun or moon is blocked out by another object, an eclipse occurs. There are two types of eclipse: lunar eclipse and solar eclipse.

During a lunar eclipse, the Earth is between the sun and the moon and its shadow darkens the moon. The moon passes through the Earth’s shadow. A total lunar eclipse occurs when the moon passes

8.7: Reflection of Light on a Plane Surface

We see houses, chairs, our friends, and other objects in the world because light bounces off them and produce the sensation of vision. This is called reflection.

Activity 8.5 Investigating reflection in a plane mirror
Key Question: How does reflection occur in a plane mirror?

What you need
Plane mirror, softboard and pins, ruler and set square, protractor.
What to do

1. Support the mirror vertically on one side of the rectangular block.
2. Draw a line perpendicular to the plane mirror on a plain sheet of paper placed on the softboard.
3. Mark this line NO.
4. By the use of a protractor, draw an angle of 30° from NO. Erect two pins P, and p2 on the line corresponding to 30o. Label this line PP, and it meets the mirror at O.
   2
5. By looking into the mirror, place two pins P, and P, to be in line with the images of P, and P2-
6. Label this line P, P4, to meet the mirror at O.
7. Measure the angle between P,P,and NO. Comment on the results.
8. Compare the two angles that are P, ON and P, ON.
9. Make your observation and conclude.

Laws of reflection

In reflection, the direction of the reflected ray is determined perpendicular to the surface at the point where the ray hits as shown in the angle the incident ray makes with the surface normal, a line known as the glancing angle, g. Since the angle of reflection is equal Figure 8.5 below. The angle made by the incident ray with the mirror is to the angle of incidence, the glancing angle made by the reflected ray
with the mirror is also equal to g.

Figure 8.6: Reflection on a plane surface
The two laws of reflection on plane surfaces include:
The incident ray, the reflected ray and the normal at the point of incidence which all lie in the same plane.
Angle of incidence which is equal to the angle of reflection.

Ray diagram

You can use the simple geometric techniques shown in Figure 8.6 to examine the properties of the images formed by flat mirrors. The line joining any point on the object to its corresponding point on the image cuts the mirror at right angles.

Image formation on a plane mirror

Activity 8.6 Observing images formed by a plane mirror
Key Question: What are the properties of images formed by a plane mirror?
What you need
A large dressing mirror, metre rule or tape measure.

What to do

1. In groups, select one or two members from each group.
2. Let one at a time stand some distance in front of the plane make some mirror while the other members of the group observations.
3. What do the members observe regarding the size of the image formed compared to the size of the object?
4. Let the member move nearer and away from the mirror. What happens to the image?
5. Present your findings as a group to the entire class.

NOTE
ALL rays must have ARROWS indicating the forward direction of the light rays.

Research 8.1
Realising characteristics of images formed by plane mirrors.

Regular and diffuse reflection

If a parallel beam of light falls on a plane mirror, it is reflected as a parallel beam and regular reflection is said to occur. Most surfaces, however, reflect light irregularly and the rays in an incident parallel beam are reflected in many directions. Reflection by a rough surface, such as paper, is irregular and is called irregular or diffuse reflection. The diagram below depicts two beams of light incident upon a rough
and a smooth surface.

Parallel and inclined mirrors

Images formed by parallel mirrors
An infinite number of images are formed of an object placed between two parallel mirrors. They all lie on a straight line for the object perpendicular to the mirrors. The position of the images may be found by the usual construction, remembering that each image seen in one mirror will act as a virtual object and produce an image in the other mirror.

Images formed in two mirrors inclined at 90°

Two flat mirrors are at right angles to each other, as illustrated in Figure 8.10 below, and an object is placed at point O. In this situation, multiple images are formed. Locate the positions of these images. When two mirrors are inclined at right angles, we have not only the images I and I, formed by a single reflection but, in addition, one other images produced by two reflections.

Figure 8.10: Images formed in two mirrors inclined at 90°

Geometrically, the object and all images lie on a circle whose centre is at the intersection of the mirrors. When mirrors are at 60° to each other, five images are seen, and at 30°, eleven images are observed. The number of images is given by:
n =360/α -1
where n is the number of images and a the angle between mirrors.

8.8: Applications of Reflection of Plane Mirrors There are several applications of plane mirrors.

These include:
Viewing ourselves when injured on the face, applying makeup, appreciating beauty and combing hair.
Looking for explosives underneath a vehicle.
Keeping an eye on the customers in shops.
In blind turns on busy roads, seeing the vehicles coming from the other side.
Mirrors can be used in the following pieces of equipment:

Periscope

A simple periscope consists of a tube containing two plane mirrors, fixed parallel to and facing each other. Each makes an angle of 45° with the line joining them. Light from the object is turned through 90°

Kaleidoscope

In the kaleidoscope, multiple images are formed by two mirrors usually at 60° to each other. The mirrors are fixed at one end of the display window or pavilion and items to be advertised are placed in between them. Multiple images of the item can be seen by the window shoppers.

Chapter Summary

In this chapter, you have learnt that: Light is a form of energy that enables us to see objects that reflect it and travels in a straight line. A ray is the direction of the path taken by a narrow stream of light energy. A beam is a collection of many light rays.

You can ask our super AI Physics teacher below questions of Physics s1-s4 and get answers