Chapter 16 Light

We cannot see objects in the dark room due to the absence of the light. But we can see the objects outside the room due to the presence of light outside the room. The light falls on the objects and gets reflected. These reflected rays then reaches to out eyes which enables us to see the objects.

Difference between the regular reflection and the diffused reflection are as follows:

No, diffused reflection does not mean the failure of laws of reflection. The laws of reflection are also followed in diffused reflection.

1. Polished wooden table: Regular reflection will take place because rays of light reflected from the polished wooden table (smooth surface) are parallel.
2. Chalk powder: Diffused reflection will take place because rays of light reflected from the chalk powder are not parallel due to its round surface.
3. Cardboard surface: Diffused reflection will take place because rays of light reflected from the cardboard surface; are not parallel due to its rough surface.
4. Marble floor with water spread over it: Regular reflection will take place because rays of reflect light reflected from the marble surface/ floor spreads on it. Smooth surface are parallel.
5. Mirror: Regular reflection will take place because rays reflected from the mirror are parallel.
6. Piece of paper: Diffused reflection will take place because rays of reflection from the piece of paper will not parallel due to its rough surface.

There are two laws if reflection:

1. The angle of incidence (∠i) is always equal to the angle of reflection (∠r).
2. The incident ray, the normal at the point of incidence and the reflected ray, all lie in the same point.

The laws of reflection of light can be verified by the following activity:

1. Pin a white sheet of plane (mirror) paper on a cardboard.
2. Place a strip of plane mirror M^’ on the plane paper.
3. Pined a 2 pins P₁ and P₂. Obliquely in front of the plane mirror in a straight line so that the line joining the pins can make an angle of 30◦, 45◦ and 60◦ with the normal drawn at the point of incidence.
4. See through the mirror from other side and observe the pins P₁ and P₂ seen on the mirror and pins P₃ and P₄ appears in the straight line.
5. Remove the plane mirror and the pins. Encircle the points where pins were fixed on the paper.
6. Join the P₁ and P₂ extended the line to meet at point O of the mirror. The ray AO so obtained is an incident ray.
7. Similarly join P₃ and P₄ and extend the line to meet at point O of the mirror. The ray O so obtained is a reflected ray.
8. Draw a normal OC.
9. Measure the angle of incidence ∠I and the angle of reflection ∠r. We find that ∠i = ∠r

Conclusion: This activity shows that the mirror incident ray, the reflection ray and the normal at the point of incidence, all lie on the same plane. The angle of the incidence is also equal to the angle of reflection. Thus, this activity verifies the laws of reflection.  

(a) 2m

Plane mirror formed images eject as of the image. The distance between the object and image are same here. So, the images 1 + 1 = 2 m. away from the person.

(b) Left, left hand

This is due to the lateral inversion of images which is formed by the plane mirror.

(c) Large

The amount of light entering in the eyes is very little in dim light. Pupil expands to increase the light.

(d) Less

Night birds cannot see in day time because they have large number of rod cells and a few cone cells on their retina.

(a) Always

According to the first law of reflection, the angle of the incidence is always equal to the angle of reflection (∠i = ∠r).

(b) Virtual, behind the mirror and of the same size as the object

Plane mirror formed the image eject as the size of the object behind the mirror. Images formed by the plane mirror are virtual that is why they cannot be obtained on the screen.

A simple device is used to obtain the colorful designs by multiple reflections is called kaleidoscope.

Principle: It works on the principle of multiple reflection of light.

Construction:

1. Get three rectangular plane mirror strips, each about 15 cm long and 4 cm wide.
2. Join them together to form a prism.
3. Pin them in a cylindrical cardboard tube, and make sure that the tube is slightly longer than the mirror strips.
4. Close the one end of the tube by a cardboard disc having a hole in its center through which you can see.
5. At the other end, touching the mirrors fix circular glass plate.
6. Place several small pieces of the coloured glass on this glass plate.
7. Close (disc) this end of the tube by ground glass plate. Allow enough space for the coloured pieces to move around. This is kaleidoscope.

When seen through the hole, we will be able to see a variety of patterns in the tube.

Gurmit’s teacher advised her not to do so because laser torch can injure the retina of her friends and she may loss her eye sight permanently or temporarily. The intensity of laser light is very high.

We should take care of our eyes properly. If there is any problem in the eyes then we should consult eye specialist.

1. We should not read in the too much light or too little light.
2. Never see sun or other powerful source of light directly.
3. We should not rub the eyes if the dust or some other contaminants get falls into the eyes.
4. Always read and write by keeping books at a normal distance for a vision.

∠i + ∠r = 90◦

But, by the first law of reflection;

∠i = ∠r

∠i + ∠r = 90◦

∠i + ∠i = 90◦

2∠i = 90◦

∠i = 90◦ = 45◦                                                                                                                                                                                                                                              2

Infinite number of images will be formed due to multiple reflections of light. The number of images can be determined as;

The first law of reflection is used to obtain the ray of reflected light.

Here, we can see that the given ray of light will reflect from the second mirror at an angle of 60 degree.

Boojho cannot see his image as he is not standing in front of the mirror. He can see the objects situated at P and Q as the light from these two points get reflected by the mirror and reaches his eyes. He cannot see the object at R.

1. This image of the object place at the A, will be formed behind the mirror and it will be at the same distance from the mirror as the object A is placed or present.
2. Yes, Paheli at B can see the image as the light from B gets reflected by the mirror and reaches A.
3. Yes, Boojho at C point can see the image as the light from C gets reflected by the mirror and reaches A.
4. The image A will be stationary even if Paheli moves from B to C.