CBSE Class 8 Science Chapter 16: Light NCERT Solutions

In order to see any object, light must first strike the object and then reflect off it, eventually reaching our eyes. If you are in a completely dark room, there is no external light source present. Therefore, no light can reflect off the objects within the room and travel to your eyes, making it impossible to see them. However, if there is light outside the room, and the room is not completely sealed, some light might enter. If objects are located outside the room where there is light, you can see them because light reflects off them and travels to your eyes.

The key differences between regular and diffused reflection are:

• Surface Type: Regular reflection occurs from a smooth surface (like a mirror), while diffused reflection occurs from a rough or irregular surface (like a wall or paper).
• Ray Behavior: In regular reflection, parallel incident rays reflect as parallel rays, all traveling in a particular direction. In diffused reflection, parallel incident rays scatter in many random directions.
• Examples: Reflection from a plane mirror is an example of regular reflection. Reflection from a road surface or a piece of paper is an example of diffused reflection.

Regarding the laws of reflection: Diffused reflection does NOT mean the failure of the laws of reflection. The laws of reflection state that the angle of incidence equals the angle of reflection, and the incident ray, reflected ray, and normal all lie in the same plane. These laws hold true for every individual ray of light, even on a rough surface. The reason for the scattering in diffused reflection is that the surface is uneven, meaning the normal to the surface is at a different angle for each point of incidence. Thus, while each ray follows the laws, their collective scattering creates the diffused effect.

1. Polished wooden table: Regular reflection will take place. A polished surface is smooth, causing incident parallel rays of light to reflect as parallel rays.
2. Cardboard surface: Diffused reflection will take place. A cardboard surface, though it may appear smooth to the eye, has microscopic irregularities. These irregularities cause the reflected rays to scatter in different directions.
3. Chalk powder: Diffused reflection will take place. Chalk powder consists of many small particles with uneven surfaces, leading to the scattering of light in all directions.
4. Mirror: Regular reflection will take place. A mirror has a very smooth surface, ensuring that parallel incident rays are reflected as parallel rays.
5. Marble floor with water spread over it: Regular reflection will take place. The layer of water spread over the marble floor creates a very smooth and level surface, similar to a mirror, resulting in regular reflection.
6. Piece of paper: Diffused reflection will take place. Even though a piece of paper may seem smooth, it has numerous tiny bumps and creases at a microscopic level. These irregularities cause light to scatter, resulting in diffused reflection.

The two fundamental laws of reflection are:

1. The angle of incidence is always equal to the angle of reflection. Mathematically, $i\; =\; r$, where $i$ is the angle of incidence and $r$ is the angle of reflection.
2. The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.

Here is an activity to demonstrate that the incident ray, reflected ray, and normal lie in the same plane:

1. Take a sheet of white paper and fix it on a drawing board or a flat table using drawing pins.
2. Take a comb and block all its openings except for one in the middle. This will allow a single beam of light to pass through.
3. Hold the comb perpendicular to the paper.
4. Direct a beam of light from a torch through the single opening of the comb, so that it falls on the paper from one side. You should see a ray of light visible on the paper on the other side of the comb. Adjust the torch and comb slightly to get a clear, thin ray of light on the paper.
5. Now, place a strip of a plane mirror vertically in the path of this light ray on the paper. Ensure the mirror is positioned such that the light ray strikes its reflecting surface.
6. Observe the reflected ray. You will see the reflected ray appearing on the paper.
7. To demonstrate the concept of the plane, carefully fold the paper. You can fold the paper such that the part where the incident ray falls and the part where the reflected ray is seen are no longer in the same plane (i.e., they are at an angle to each other).
8. When you fold the paper, you will notice that the reflected ray disappears when viewed from certain angles. This happens because the reflected ray is only visible when it lies in the same plane as the incident ray and the normal. If you fold the paper, you are essentially breaking this plane, and thus the reflected ray is no longer seen in the folded section. This activity visually confirms that the incident ray, reflected ray, and normal all lie within the same plane.