1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Ans:
If a person is in a completely dark room, it is impossible to see objects inside the room. However, objects outside the room can be seen if they are illuminated by light.
This is because we see objects when light reflects off them and reaches our eyes. In a dark room, there is no light to reflect off the objects, making them invisible. On the other hand, objects outside the room reflect light, allowing them to be visible.
2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Table
No, the laws of reflection still apply in both cases; the angle of incidence equals the angle of reflection for each individual ray. The difference is in how the rays are scattered in diffused reflection.
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
Ans:
(a) Polished wooden table – Regular reflection
A polished surface is smooth, so the light will reflect in a specific direction.
(b) Chalk powder – Diffused reflection
Chalk powder has a rough surface, causing light to scatter in various directions.
(c) Cardboard surface – Diffused reflection
Cardboard has an irregular, rough surface, leading to scattered light rays.
(d) Marble floor with water spread over it – Regular reflection
The marble surface is smooth, and the water adds a glossy finish, resulting in regular reflection.
(e) Mirror – Regular reflection
Mirrors have a smooth, polished surface that causes light to reflect in a specific direction.
(f) Piece of paper – Diffused reflection
Though smooth, the paper’s surface has microscopic irregularities, causing light to scatter in various directions.
4. State the laws of reflection.
Ans:
The laws of reflection are:
a) The angle of reflection is equal to the angle of incidence.
This means that the angle at which the light strikes the surface (angle of incidence) is equal to the angle at which it is reflected (angle of reflection).
b) The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
This means that all three rays (incident, reflected, and normal) are coplanar, i.e., they lie in the same two-dimensional plane.
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.
Ans:
Activity:
Place a plane mirror perpendicular to the table’s surface.
Take a piece of paper and make a small hole in it. Hold this piece of paper perpendicular to the table.
In a dark room, position another piece of paper on the table, making sure it touches the mirror.
Draw a normal line (a line perpendicular to the mirror) on the paper.
Shine a torch through the small hole, directing the light beam at the normal of the mirror.
Observe the light beam on the paper. The light will strike the mirror and reflect back.
By examining the setup, you will see that the incident ray, the normal, and the reflected ray all lie on the same plane. This confirms that the rays and the normal at the point of incidence are coplanar.
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.
Ans:
(a) 2 m
(b) left, left hand.
(c) large .
(d) fewer
7. The angle of incidence is ________ equal to the angle of reflection.
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never
Ans:
The answer is (a) Always.
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.
Ans: (b) virtual, behind the mirror and of the same size as the object.
9. Describe the construction of a kaleidoscope.
Ans:
The construction of a kaleidoscope involves the following steps:
Mirrors: Take three rectangular mirror strips, each measuring 15 cm x 4 cm. Join them together to form a triangular prism. The reflective surfaces of the mirrors should face inward to form the shape of the prism.
Tube: Attach the prism to a circular cardboard tube, ensuring that the tube is slightly longer than the prism itself. The tube holds the mirrors in place and allows for smooth rotation.
One End: Close one end of the tube with a cardboard disc. This disc should have a small hole through which the viewer can look.
Other End: At the other end of the tube, attach a plane glass plate. This plate should touch the mirrors of the prism, creating a stable base for the colored pieces inside.
Colored Pieces: Place several small, broken pieces of colored glass on the glass plate. These pieces will move freely when the kaleidoscope is rotated.
Closing the Tube: Finally, close the open end of the tube with a round glass plate. This allows the colored pieces to move around inside, creating changing patterns as the kaleidoscope is rotated.
When you look through the hole in the disc, the multiple reflections of the colored pieces create beautiful and symmetrical patterns due to the mirrors.
10. Draw a labelled sketch of the human eye.
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?
Ans:
Gurmit’s teacher advised her not to use a laser torch because laser light has very high intensity, and directly looking at it can be harmful to the eyes. The concentrated light can cause damage to the retina and even lead to permanent blindness. Therefore, it is crucial to avoid direct eye exposure to laser beams.
12. Explain how you can take care of your eyes.
Ans:
To take care of your eyes, you should follow these practices:
a) Avoid reading in both bright and dim lighting conditions.
b) Visit an eye specialist regularly for eye check-ups.
c) If dust or small particles enter the eyes, wash them immediately with clean, cold water instead of rubbing the eyes.
d) Protect your eyes from direct exposure to sunlight.
e) Maintain a distance of at least 25 cm between your eyes and the book when reading.
13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Ans:
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 always equal to the angle of reflection. Since the total angle between the incident and reflected ray is 90°, the angle of incidence and the angle of reflection will both be 45°.
14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Ans:
When a candle is placed between two parallel plane mirrors separated by 40 cm, an infinite number of images will be formed. This happens due to the continuous reflections between the two mirrors, creating multiple images. The number of images depends on the distance between the mirrors and the position of the candle, but in the case of parallel mirrors, there is an infinite number of images formed.
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.
Ans:
To find the reflected ray from the second mirror, we apply the laws of reflection. The steps are as follows:
Incident Ray and First Reflection:
The incident ray OA strikes the first mirror at an angle of 30°.
According to the law of reflection, the angle of incidence is equal to the angle of reflection. Therefore, the ray will reflect off the first mirror at an angle of 30°.
Reflection from the Second Mirror:
The reflected ray from the first mirror, now called OB, strikes the second mirror at point O’.
The angle between the reflected ray OB and the second mirror is 60° (since the total angle between the two mirrors is 90°, and the angle of incidence at the first mirror is 30°, leaving 60°).
According to the law of reflection, the angle of incidence at the second mirror is 60°, so the angle of reflection will also be 60°.
Drawing the Reflected Ray:
After applying the law of reflection at the second mirror, the reflected ray will be at an angle of 60° from the normal of the second mirror. This gives the path of the reflected ray from the second mirror.
Thus, the reflected ray from the second mirror is drawn at an angle of 60° from the normal of the second mirror.
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?
Ans:
Boojho cannot see his image in the mirror because the reflected rays from his body will not reach his eyes, as he is not positioned directly in front of the mirror.
Objects at P and Q: Boojho can see the images of objects situated at P and Q because the rays coming from these objects are reflected by the mirror and reach his eyes. This allows him to see the images of P and Q in the mirror.
Object at R: Boojho cannot see the image of the object situated at R because the rays from object R do not get reflected toward him. As a result, he cannot view the image of the object at R in the mirror.
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?
Ans:
The image of the object placed at A will be formed behind the plane mirror at the same distance as the object A is in front of the mirror. The image is virtual, erect, and laterally inverted.
(b) Can Paheli at B see this image?
Yes, Paheli at B can see this image because the reflected rays from the object (and the image formed behind the mirror) will reach Paheli’s eyes, allowing her to see the image.
(c) Can Boojho at C see this image?
Yes, Boojho at C can also see the image, as the reflected rays from the object will reach his eyes as well, allowing him to view the image.
(d) When Paheli moves from B to C, where does the image of A move?
The image of the object at A will remain in the same position behind the mirror. It does not move when Paheli moves from B to C because the position of the image in a plane mirror is fixed relative to the object and mirror.