Maharashtra Board Class 8 Science Chapter 16 Reflection of Light PDF Download

16. Reflection Of Light

We get different feelings through senses. The sense of vision is the most important among our five senses. Because of this we can see the hills, rivers, trees, people and objects around us. Only with this we can see the beautiful views of nature like clouds, rainbow, flying birds, Moon, stars etc.

16.1 Reflection Of Light

Switch off the light in your room at night for some time and then turn it on again.

Could you see the objects in the room clearly when the light was switched off? What did you feel when it was turned on again?

From the above activity you can notice that there is some connection between the sense of vision and light. When we switch off the light at night, the objects in the room cannot be seen and they can be seen as before when the light is switched on again. Thus, we can see objects when the light coming from these objects enters our eyes. The light entering our eyes may be emitted by the object or may be reflected by that object. What is meant by light reflected by an object? To understand this, let us learn about the reflection of light.

Reflection of light: When light rays fall on an object their direction changes and they turn back. This is called the reflection of light.

Material: Torch light, mirror, a stand for hanging the mirror, black paper, comb, white paper, drawing board.

Activity

1. Fit a white paper tightly over a table or drawing board.

2. Leaving out some portion in the middle of the comb, cover the rest with black paper so that light can only pass through the open central portion.

3. Hold the comb perpendicular to the white paper and throw torch light on its central portion.

4. Adjust comb and torch so as to get light rays on the white paper. Now keep a mirror in the path of this ray of light as shown in the figure.

5. What do you observe?

In this activity, light rays which fall on the mirror, get reflected and travel in a different direction. The rays falling on any surface are called incident rays. The point at which an incident ray falls is called the point of incidence. The rays going away from the surface after reflection are called reflected rays. The direction of the reflected rays is decided by some rules which are called laws of reflection. Let us learn some definitions before learning about these laws.

Teacher's Note

When you look in a mirror, you see your face clearly because light bounces back from the smooth mirror surface. If you look at a rough wall instead, you cannot see your reflection because the light scatters in many directions.

Exam Trick

Remember: Incident ray hits the mirror and reflected ray bounces back. They always make equal angles with the normal (the perpendicular line). Think of a ball bouncing off a wall at the same angle it came in.

Points to Remember

Reflection happens when light bounces off a surface.
The incident ray is the light coming towards the mirror.
The reflected ray is the light bouncing away from the mirror.
The normal is the perpendicular line at the point where light hits the mirror.
The angle of incidence equals the angle of reflection.

16.2 Reflection Of Light

Some terms related to reflection are as follows.

i. Ray AO is the incident ray

ii. Point O is the point of incidence

iii. Ray OB is the reflected ray

iv. Line ON is the normal

v. The angle \(\angle AON\), between incident ray and the normal is the angle of incidence (i)

vi. The angle \(\angle BON\) between the reflected ray and the normal is the angle of reflection (r)

Laws of reflection

There are three laws of reflection. These are as given below.

1. The angle of reflection is equal to the angle of incidence.

2. The incident ray, the reflected ray and the normal lie in the same plane.

3. The incident ray and the reflected ray are on the opposite sides of the normal.

Teacher's Note

The laws of reflection work the same way everywhere. Whether light reflects from a bathroom mirror or from a lake surface, these rules always apply. Light always follows these three simple rules.

Exam Trick

Remember the three laws with "Equal Angles" - the angle going in equals the angle going out. Like a tennis ball bouncing off a wall at the same angle it came in.

Points to Remember

The angle of reflection always equals the angle of incidence.
The incident ray, normal, and reflected ray all lie in the same flat plane.
The incident and reflected rays are on opposite sides of the normal.
These three laws work for all types of reflecting surfaces.
The normal is always perpendicular to the mirror surface.

16.3 Verification Of The Laws Of Reflection

Equipment: Mirror, drawing board, pins, white paper, protractor, scale, pencil

Activity:

1. Fit a white paper on the drawing board tightly as possible.

2. On the paper draw a line PQ indicating the position of the mirror.

3. Draw a perpendicular ON to PQ at point O.

4. Draw a ray AO making an angle of 30° with ON

5. Fix two pins S and R along AO.

6. Fix the mirror to a stand and place it along PQ perpendicular to the drawing board.

7. Fix pins at T and U along the line joining the bottom of the reflected images of the pins at S and R.

8. Remove the mirror and join the points T and U and extend it up to O.

9. Measure \(\angle TON\).

10. Repeat steps 4 to 9 for angle of incidence equal to 45° and 60° and write down the angles in the following table.

What relation do you find between the angle of incidence and the angle of reflection? If you have done the experiment carefully, you will find that the angle of incidence is equal to the angle of reflection in all three cases. This verifies the laws of reflection.

Teacher's Note

When you do this experiment in class with a mirror and pins, you will see that the angles you measure are always the same. This proves that the law of reflection is true in real life, just like we see in our homes when we look in the mirror.

Exam Trick

In the exam, if they give you the angle of incidence, just remember it equals the angle of reflection. If angle i = 30°, then angle r = 30°. No calculation needed!

Points to Remember

Always measure angles from the normal, not from the mirror surface.
The angle of incidence should equal the angle of reflection in your experiment.
If your measurements are different, check if the mirror is at the right angle.
This experiment proves the first law of reflection in a practical way.
Repeat the experiment several times to get accurate results.

This is a preview of the first 3 pages. To get the complete book, click below.