Light

No, we cannot see objects in a dark room because there is no light to illuminate the objects in the room. When light falls on the eyes after reflecting from the object, it becomes visible. If the light is available outside the room, then the objects present would reflect it and hence we would be able to see objects.

Diffused reflection is not due to the failure of the laws of reflection. It is caused by the irregularities in the reflecting surface, like that of a cardboard.

(a) Polished wooden table — Regular reflection
Reason — The polished wooden table has a smooth surface, hence, the reflections are regular.

(b) Chalk powder — Diffused reflection
Reason — Chalk powder spread on a surface will form an irregular surface, hence, the reflections are diffused.

(c) Cardboard surface — Diffused reflection
Reason — The surface of the cardboard is irregular, hence, the reflections are diffused.

(d) Marble floor with water spread over it — Regular reflection
Reason — Marble floor with water spread over it is a smooth surface, hence, the reflections are regular.

(e) Mirror — Regular reflection
Reason — A mirror has a very smooth surface, hence, the reflections are regular.

(f) Piece of paper — Diffused reflection
Reason — Piece of paper has many irregularities, hence, the reflections are diffused.

Two laws of reflection are :

(i) The angle of incidence is equal to the angle of reflection.

(ii) Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane.

Take a sheet of stiff paper or a chart paper. Let the sheet project a little beyond the edge of the table. Cut the projecting portion of the sheet in the middle. Look at the reflected ray. Make sure that the reflected ray extends to the projected portion of the paper. Bend that part of the projected portion on which the reflected ray falls.

When the whole sheet of paper is spread on the table, it represents one plane. The incident ray, the normal at the point of incidence and the reflected ray are all in this plane.

When we bend the paper we create a plane different from the plane in which the incident ray and the normal lie. Then we do not see the reflected ray.

Hence, the above activity indicates that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.

(a) A person 1 m in front of a plane mirror seems to be 2 m away from his image.

Reason — When a person stands in front of a plane mirror, their image appears to be as far behind the mirror as the person is in front of it.
So, when the person is 1 m away from the mirror. His image will be formed at 1 m from the mirror on the other side, hence, his image will be 1 + 1 = 2 m from the person himself.

(b) If you touch your left ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with your left hand.

Reason — The image formed by a mirror is laterally inverted hence, when we touch our left ear with our right hand in front of a plane mirror, it will be seen in the mirror that our right ear is touched with our left hand.

(c) The size of the pupil becomes large when you see in dim light.

Reason — In dim light conditions, there's less available light for vision. To allow more light to enter the eye and improve visibility in low-light environments, the iris dilates the pupil. When the pupil dilates, it enlarges in size, allowing more light to pass through the eye and reach the retina at the back of the eye.

(d) Night birds have fewer cones than rods in their eyes.

Reason — Having fewer cones and more rods allows night birds to have better night vision. Rods are more sensitive to low levels of light, making them ideal for detecting prey and navigating in the dark. Additionally, having fewer cones means that night birds likely have reduced color vision or even monochromatic vision, which is less important for hunting in low-light conditions.

Always

Reason — According to the laws of reflection :

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

virtual, behind the mirror and of the same size as the object.

Reason — Image formed by a plane mirror is virtual, behind the mirror and of the same size as the object.

To make a kaleidoscope, we will take three rectangular mirror strips each about 15 cm long and 4 cm wide. We will join them together to form a prism as shown in the figure below:

After that, we will fix the arrangement of mirrors in a circular cardboard tube or tube of a thick chart paper. It should be slightly longer than the mirror strips. Close one end of the tube by a cardboard disc having a hole in the centre, through which we can see.

In order to make the disc durable, we will paste a piece of transparent plastic sheet under the cardboard disc. At the other end, touching the mirrors, we will fix a circular plane glass plate.

After that, we will place on this glass plate several small pieces of coloured glass (broken pieces of coloured bangles). Close this end of the tube by a ground glass plate. Allow enough space for the colour pieces to move around.

Our kaleidoscope is ready.

Labelled sketch of the human eye is shown below:

Intensity of laser light is very high. It can cause damage to the retina and lead to blindness. Hence, Gurmit's teacher advised her not to use a laser light for the activity as she was concerned about the risk of accidental exposure to the laser beam, which could potentially cause damage to Gurmit's eyes or the eyes of others nearby.

In order to take care of our eyes we should do the following:

1. We should always read at the normal distance for vision. We should not bring the book too close to our eyes or keep it too far.
2. We should not look at the Sun or a powerful light directly.
3. Too little or too much light is bad for eyes. Insufficient light causes eyestrain and headaches. Too much light, like that of the Sun, a powerful lamp or a laser torch can injure the retina.
4. We should never rub our eyes. If particles of dust go into our eyes, we should wash our eyes with clean water or visit the doctor if need be.
5. If advised, we should use suitable spectacles.

Given,

∠i + ∠r = 90° where

∠i = angle of incident.

∠r = angle of reflection

According to the law of reflection,

∠i = ∠r

Substituting the above law in the given condition we get,

2∠i = 90°

∠i = $\dfrac{90}{2}$ = 45°

Therefore, angle of incidence = 45°.

When a candle is placed between two parallel plane mirrors, infinite images are formed due to repeated reflections between the mirrors.

Let MM' and M'M'' be the two plane mirrors at right angles.

The reflected ray from the second mirror is shown in the figure below:

Given that the ray is incident on mirror MM' at 30°,

∴ ∠AOX = 30°

According to the law of reflection,

∠i = ∠r

∴ ∠XOO' = ∠AOX = 30°

As OX ⊥ MM' and O'X ⊥ M'M''

∴ OX ⊥ O'X

∴ ∠OXO' = 90°

In ΔOXO', by angle sum property.

∠XOO' + ∠OXO' + ∠XO'O = 180°

⇒ 30° + 90° + ∠XO'O = 180°

⇒ ∠XO'O = 180° - 90° - 30°

⇒ ∠XO'O = 60°

∴ Angle of incidence of the ray on M'M'' = 60°

∴ Angle of reflection of the ray from M'M'' = 60° [∵ ∠i = ∠r]

Boojho cannot see himself in the mirror because light rays originating from him do not reach his eyes after reflection.

He can see the image of objects situated at P and Q because the light rays coming from P and Q get reflected by the mirror and reach his eyes.

Boojho can't see the image of object situated at R because the light rays from object R do not reach his eyes after reflection.

(a) Image of the object placed at A is formed behind the mirror and distance of the image from the mirror is equal to the distance of A from the mirror.

(b) Yes, Paheli at B can see this image.

(c) Yes, Boojho at C can see this image.

(d) Image of the object at A will not move, hence, it will remain in the same position when Paheli moves from B to C.