Light class 8 questions and answers: Science Class 8 chapter 13 question answer
| Textbook | Ncert |
| Class | Class 8 |
| Subject | Science |
| Chapter | Chapter 13 |
| Chapter Name | Light ncert solutions |
| Category | Ncert Solutions |
| Medium | English |
Are you looking for Class 8 Science chapter 13 question answer? Now you can download ncert solutions for class 8 science chapter 13 pdf from here.
Question 1: Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain.
Answer 1: When we are in a dark room then we cannot see objects in the room. We can see the objects outside the room, because out of the room the light is available and the rays of light can enter our eyes after reflection from the objects.
Question 2: Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Answer 2:
| Aspect | Regular Reflection | Diffused Reflection |
|---|---|---|
| Surface Type | Smooth and polished surface (e.g., mirrors, calm water). | Rough, uneven surface (e.g., walls, unpolished metal). |
| Reflection Pattern | Reflects light in a single, predictable direction (regular). | Light is scattered in many different directions. |
| Angle of Incidence = Angle of Reflection? | Yes, obeys the law of reflection strictly. | Yes, but light scatters in multiple directions after hitting the surface. |
| Reflection Behavior | Creates clear, sharp images or reflections. | Creates a diffuse, blurry or scattered reflection. |
| Example | Mirror reflection. | Reflection on a rough wall or paper. |
No, diffused reflection does not imply the failure of the laws of reflection. The law of reflection (Angle of incidence = Angle of reflection) still holds true at every individual point on the rough surface. However, since the surface is uneven, each small section of the surface reflects light at different angles, which leads to the scattering of light in multiple directions. Thus, the law of reflection is still valid, but the result is a diffuse reflection due to the nature of the surface.
Question 3: 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
Answer 3: (a) A polished wooden table will have regular reflection because its surface is smooth.
(b) A chalk powder will give diffused reflection because the chalk powder particles are not smooth.
(c) Cardboard surface has minute irregularities on it, so it will have a diffused reflection.
(d) Marble stone with water spread over it will have regular reflection if the water is stable. However, if waves are formed, diffused reflection will take place.
(e) Any mirror has a shiny and smooth surface. Therefore, regular reflection will be observed.
(f) A piece of paper has a rough surface. Therefore, diffused reflection will be observed.
Question 4: State the laws of reflection.
Answer 4: The law of reflection states that:
- a) The angle of reflection and the angle of incidence 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.
Question 5: 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.
Answer 5: To perform this experiment take a plane mirror and place it on a plane sheet of paper in a standing position with a block. Then draw an incidence line AB on this paper. Mark the points on the paper by carefully looking at the mirror. The line that goes after it appears on the screen.
Draw a perpendicular on the mirror line and then remove the mirror. Join the points to make the reflected ray on the paper. You will observe that incident ray, reflected ray and normal will be in the same plane, i.e. on the sheet of paper. This verifies the law of reflection.
Question 6: Fill in the blanks in the following.
(a) A person 1 m in front of a plane mirror seems to be _________ m away from his image.
(b) If you touch your _________ ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with _________.
(c) The size of the pupil becomes _________ when you see in dim light.
(d) Night birds have _________ cones than rods in their eyes.
Answer 6: (a) A person 1 m in front of a plane mirror seems to be 2 m away from his image.
(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.
(c) The size of the pupil becomes large when you see in dim light.
(d) Night birds have fewer cones than rods in their eyes.
Question 7: Choose the correct option in Questions 7-8.
Question 7.
The angle of incidence is equal to the angle of reflection
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never
Answer 7: (a) Always
Question 8: Image formed by a plane mirror is:
(a) virtual, behind the mirror and enlarged.
(b) virtual, behind the mirror and of the same size as the object.
(c) real at the surface of the mirror and enlarged.
(d) real, behind the mirror and of the same size as the object.
Answer 8: (b) virtual, behind the mirror and of the same size as the object.
Question 9: Describe the construction of a kaleidoscope.
Answer 9: Kaleidoscope is a device based on the principle of multiple reflections. It consists of three long and narrow strips of plane mirrors inclined at an angle of 60° to one another forming prism. This is fitted in a tube. One end of this tube is closed by a cardboard disc having a hole at its centre. To the other end touching the mirrors plane glass plate is fixed on which broken pieces of coloured bangles are placed. This end of the tube is closed by a ground glass plate.
Question 10: Draw a labelled sketch of the human eye.
Answer 10:
Question 11: Gurmit wanted to perform Activity 13.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advise?
Answer 11: Gurmit’s teacher advised her not to do so because the intensity of the laser light is very high, and 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.
Question 12: Explain how you can take care of your eyes.
Answer 12: The following advice can help someone take better care of their eyes:
- a) Reading in both strong and dim light is not recommended.
- b) We ought to schedule frequent check-ups with an ophthalmologist.
- c) Instead of rubbing our eyes if tiny insects or dust particles get inside, rinse them out right away with cold water.
- d) We ought to refrain from exposing our eyes directly to sunshine.
- e) When reading, keep your gaze at least 25 centimeters away from the book.
Question 13: What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Answer 13: If ∠i = angle of incidence, and ∠r = angle of reflection
Then, ∠i + ∠r = 90° (given)
But ∠i = ∠r (according to the laws of reflection)
∴ ∠i + ∠i = 90°
or, 2∠i = 90°
or, ∠r = ∠i = 45°
Therefore, the angle of incidence of the ray will be 45°.
Question 14: How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Answer 14: Since the mirrors are kept parallel to each other, and Infinite no of images will be formed due to multiple reflections between them.
Question 15: Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 13.19. Draw the reflected ray from the second mirror.
Answer 15:
Question 16: Boojho stands at A just on the side of a plane mirror as shown in Fig. 13.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?
Answer 16: A plane mirror forms a virtual image behind the mirror. The image is as far behind the mirror as the object is in front of it. A cannot see his image because the length of the mirror is too short on his side. However, he can see the objects placed at points P and Q, but cannot see the object placed at point R (as shown in the given figure).
Question 17: (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 13.21).
(b) Can Paheli at B see this image?
(c) Can Boojho at C see this image?
(d) When Paheli moves from B to C, where does the image of A move?
Answer 17: (a) The image of A in the mirror will be as far behind as the object in front of the mirror.
(b) Yes, Paheli can see the image.
(c) Yes, Bhoojho can see this image.
(d) When Paheli moves from B to C, the image of A will not move.
