NCERT Q&A for Class 8 Science Chapter 16 Light
1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Soln:
If a person is inside the room where there is no light, it is then impossible to visualize the object inside the room but the object out of the room can be seen easily.
When light falls on eyes after reflecting from the object, it becomes visible. If the room is dark, then the object which is in the room reflects no light. Hence, the person is not able to see the objects in the room where there is no light.
2. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water spread over it
(e) Mirror
(f) Piece of paper
Soln:
a) The wooden table that has been a polished-Regular reflection
The surface that has been recently polished can be a good example of a smooth surface. The wooden table that has been polished has a surface that is smooth.
b) White Chalk powder that is used in school- Diffused reflection
Chalk powder spread on a surface is an example of an irregular surface. Hence, it is rough. Therefore, the diffused reflection will appear from chalk powder.
c) Cardboard surface- Diffused reflection
The surface of the cardboard is a kind of irregular surface. Hence, the diffused reflection will take place from a cardboard surface.
d) Marble floor – Regular reflection
Marble floor can be a good example of a surface that is regular. Since water makes the ceramic glossy, thus, the reflections that are regular occur on this surface.
e) Mirror- Regular reflection
A mirror has a very smooth surface hence it gives a regular reflection.
f) Piece of paper- Diffused reflection
Although a piece of paper may look smooth, it has many irregularities on its surface. Due to this reason, it will give a diffused reflection.
3. State the laws of reflection.
Soln:
The law of reflection states that
a) The angle of reflection and the angle of incidence both are always equal to one another.
b) The reflected ray, the incident ray, and the normal to the reflective surface at the point of incidence all come on the same plane.
4. Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Soln:
On a table, place a plane mirror perpendicular to the plane of the table. Make a small hole in a paper and hold it perpendicular to the plane of the table. Try to do this experiment in a dark room. Take one more piece of paper and place it on the table so that it makes contact with the mirror. Draw a line perpendicular to the mirror on the piece of paper which is on the table. Now beam light rays with the help of a torch through the small hole such that the beam of light hits the normal at the bottom of the mirror. The ray of light will be reflected in the light rays from the hole are incident on the mirror. Looking at the piece of paper on the table, we can easily show that the incident ray, the normal line and the reflected ray at the point of incidence lie in the same plane.
5. Describe the construction of a kaleidoscope.
Soln:
The construction of a kaleidoscope:
Take three rectangular mirror strips of dimensions 15cm x 4cm (l x b) and join them together to form a prism. A prism is fixed into a circular cardboard tube. The circular cardboard tube should be slightly longer than the prism. This circular tube is now closed at one end with a cardboard disc. This disc has a hole through which we can see. At the other end of the circular tube, a plane glass plate is fixed. It is important that this glass plate touches the prism mirrors. On this glass plate, several small and broken pieces of coloured glass are placed. This end is now closed by a round glass plate allowing enough space for the coloured glass pieces to move.
6. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advise?
Soln:
Her teacher advised her not to do so because of the intensity of the laser light is very high, it is harmful to the human eyes. It can cause damage to the retina and leads to blindness. Hence, it is advisable not to look at a laser beam directly.
7. Explain how you can take care of your eyes.
Soln:
The following points help a person to take care of his eyes:
a) Reading should not be done in bright light as well as in dim light.
b) He should visit an eye specialist on a regular interval of time.
c) If any small insects or dust particles enters his eyes, do not rub them but clean them immediately with cold water.
d) He should avoid direct exposure of sunlight to the eye.
e) While reading, there should be a distance of at least 25 cm between the eyes and the book.
8. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Soln:
If the reflected ray is at the angle of 90o to the incident ray, then the angle of incidence is 45o. According to the law of reflection, the angle of incidence and the angle of reflection are equal. Therefore, the angle of incidence and the angle of reflection both are 90/2=45o.
9. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Soln:
If a candle is placed between two parallel plane mirror separated by 40 cm, then the multiple and infinite images will be formed due to the multiple reflections between the mirrors. The infinite numbers of images are formed when two mirrors are placed parallel to each other.