NCERT Solutions for Class 8th Science Chapter 16: Light
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:
If we are in a dark room, then it is not possible for us to see objects in the room.
However, objects outside the room are visible to us.
An object becomes visible when light reaches our eye after being reflected from the object. If there is no light in the room, then the objects inside the room cannot reflect any light. Hence, we cannot see in a dark room. If there is light present outside the room, then we can see the objects outside the room.
Question 2:
Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Answer:
Regular reflection takes place from a smooth or a regular surface. In regular reflection, all reflected rays are parallel to each other for parallel incident rays.
Irregular or diffused reflection takes place from an irregular surface. In diffused reflection, the reflected rays are not parallel to each other for parallel incident rays.
This happens because of the presence of irregular microscopic surfaces. Hence, parallel incident rays reflect in different directions. However, each ray obeys the laws of reflection. Therefore, laws of reflections are not violated in diffused or irregular reflections.
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:
(a) Polished wooden table → Regular reflection
A polished surface is an example of a smooth surface. A polished wooden table has a smooth surface. Hence, reflections from the polished table will be regular.
(b) Chalk powder → Diffused reflection
Chalk power spread on a surface is an example of an irregular surface. It is not smooth. Therefore, diffused reflection will take place from chalk powder.
(c) Cardboard surface → Diffused reflection
Cardboard surface is also an example of an irregular surface. Hence, diffused reflection will take place from a cardboard surface.
(d) Marble floor with water spread over it → Regular reflection
Marble floor with water spread over it is an example of a regular surface. This is because water makes the marble surface smooth. Hence, regular reflection will take place from this surface.
(e) Mirror → Regular reflection
Mirror has a smooth surface. Therefore, it will give a regular reflection.
(f) Piece of paper → Diffused reflection
Although a piece of paper may look smooth, but it has many irregularities on its surface. Due to this reason, it will give a diffused reflection.
Question 4:
State the laws of reflection.
Answer:
Laws of reflection:
(i) The angle of reflection is always equal to the angle of iincidence.
(ii) The incident ray, the reflected ray and the normal to the reflective surface at the point of incidence all lie in 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:
Place a plane mirror on the table. Take a paper sheet and make a small hole in its centre. Make sure that the light in the room is not bright. Hold the sheet normal to the table. Take another sheet and place it on the table in contact with the vertical mirror. Draw a normal line on the second sheet from the mirror. Now, light a torch on the mirror through the small hole such that the ray of light falls on the normal at the bottom of the mirror. When the ray from this hole is incident on the mirror, it gets reflected in a certain direction. You can easily observe the incident ray, reflected ray and the normal to the mirror at the point of incidence on the sheet placed on the table. This shows that the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
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 your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with your _________.
(c) The size of the pupil becomes ________ when you see in dim light.
(d) Night birds have _________ cones than rods in their eyes.
Answer:
(a) A person 1 m in front of a plane mirror seems to be __2 __ m away from his image.
(Object distance and image distance are the same from a plane mirror. The image of a person 1 m in front of a mirror is 1 m back to the mirror. Hence, the image is 1 + 1 = 2 m away from the person.)
(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_. (This is because of lateral inversion of images formed in a plane mirror.)
(c) The size of the pupil becomes __large__ when you see in dim light.
(In dim light, the amount of light entering the eye is very little. To increase the amount of light, the pupil expands. )
(d) Night birds have __less__ cones than rods in their eyes.
(Night birds can see in the night, but not in the day. They have on their retina a large number of rod cells and only a few cones.)
Question 7:
Angle of incidence is equal to the angle of reflection.
(a) Always (b) Sometimes
(c) Under special conditions (d) Never
Answer:
(a) The angle of incidence is always equal to the angle of reflection. This is the first law of reflection.
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:
(b) Image formed by a plane mirror is virtual, behind the mirror and of the same size as the object.
The image formed by a plane mirror is of the same size as the object. The image is formed behind the mirror. The image cannot be obtained on a screen. Hence, it is a virtual image.
Question 9:
Describe the construction of a kaleidoscope.
Answer:
Construction of a kaleidoscope:
Three rectangular mirror strips of dimensions 15cm × 4cm (l × b) are joined together to form a prism (fig a). This prism is fixed into a circular cardboard tube.
The circular cardboard tube should be slightly longer that the prism (fig b). This circular tube is now closed at one end with a cardboard disc. This disc has a hole in it through which we can see (fig c) .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 (fig d).
Question 10:
Draw a labelled sketch of the human eye.
Answer:
Question 11:
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 advice?
Answer:
Laser light is harmful for the human eyes, because its intensity is very high. It can cause damage to the retina and lead 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:
To protect our eyes, the given points should be taken into account:
(i) Visit an eye specialist regularly.
(ii) Avoid reading in dim light and very bright light.
(iii) Avoid direct exposure of sunlight to the eye.
(iv) Clean your eyes with cold water quickly if dust particles or small insects enter your eye. Do not rub your eyes.
(v) Maintain a distance of at least 25 cm between the book and your eyes while reading.
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:
If the reflected ray is at an angle of 90° to the incident ray, then the angle of incidence is 45 °. According to the law of reflection, the angle of incidence is equal to the angle of reflection. Therefore, the angle of incidence and the angle of reflection both are = (90/2) = 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:
Infinite or multiple images of the candle will be formed because of multiple reflections between the mirrors. When two mirrors are placed parallel to each other, then infinite numbers of images are formed.
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. 16.19. Draw the reflected ray from the second mirror.
Answer:
The first law of reflection is used to obtain the path of reflected light.
It can be observed that the given ray of light will reflect from the second mirror at an angle 60°.
Question 16:
Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?
Answer:
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. 16.21).
(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:
(a) Image of the object placed at A is formed behind the mirror. The distance of the image from the mirror is equal to the distance of A from the mirror. Image of A is shown in the given figure.
(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. It will remain at the same position when Paheli moves from B to C.
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