1. Light-
ü The natural agent that stimulates sight and makes things visible.
ü Light is reflected from all surfaces.
ü Regular reflection takes place when light is incident on smooth, polished and regular surfaces.
ü Irregular reflection takes place from rough surfaces.
2. Reflection Of Light-
Figure 01- Laws of reflection
ü Bouncing back of light after striking the surface, in the same medium, is called reflection.
Figure 02- Regular and irregular reflection
ü Regular Reflection- When a narrow beam of light strikes a mirror, the light will not reach your eye unless your eye is positioned at just the right place where the law of reflection is satisfied.
ü Irregular Reflection- When light is incident upon a rough surface, it is reflected in many directions.
ü Two laws of reflection are-
n The angle of incidence is equal to the angle of reflection.
n Incident ray, reflected ray and the normal drawn at the point of incidence to the reflecting surface, lie in the same plane.
ü Splitting of light into its constituent colors is known as dispersion.
Note: Image formed in a plane mirror undergoes lateral inversion. Two mirrors inclined to each other give multiple images. Beautiful patterns are formed in a kaleidoscope because of multiple reflections. Sunlight, called white light, consists of seven colors.
3. Braille System-
ü Visually challenged persons can read and write using Braille system.
ü Visually challenged persons develop their other senses more sharply to improve their interaction with their environment.
4. Parts Of Human Eye-
Figure 03- Human eye
ü Cornea- Transparent bulge on the front surface of the eyeball which protects the eye and helps in refraction of light.
ü Iris- Colored diaphragm behind the cornea which controls the amount of light entering the eye.
ü Pupil- Dark hole in the middle of iris through which light enters the eye.
ü Eye Lens- Transparent, crystalline structure behind pupil and iris.
ü Ciliary Muscles- Hole the eye lens in position and control the focal length of the eye lens.
ü Retina- Surface of the rear part of the eyeball where the light entering the eye is focused.
ü Rods and Cones- Rod cells respond to the brightness of light while cone cells respond to colors.
ü Blind Spot- It is the least sensitive point where no rods and cones are present.
ü The space between the cornea and the eye lens is filled with aqueous humor.
ü The space between the eye lens and the retina is filled with vitreous humor.
NCERT PRACTICE QUESTIONS
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 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. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Ans.-
Regular Reflection | Irregular Reflection |
It occurs when the surface is smooth. | It occurs when the surface is a rough surface. |
Reflected rays move in a particular direction. | Reflected rays scattered in random directions. |
Example: Reflection by the plane mirror | Example: Reflection by the road surface. |
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) Regular reflection b) Diffused reflection c) Diffused reflection d) Regular reflection e) Regular reflection f) Diffused reflection
4. State the laws of reflection.
Ans.- The angle of reflection and the angle of incidence both are always equal to one another. The reflected ray, the incident ray, and the normal to the reflective surface at the point of incidence all come on the same 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.- On the 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.
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.
Ans.- (a) 2m (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.- (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-
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 colored glass are placed. This end is now closed by a round glass plate allowing enough space for the colored glass pieces to move.
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 advise?
Ans.- 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.
12. Explain how you can take care of your eyes.
Ans.- Reading should not be done in bright light as well as in dim light. He should visit an eye specialist on a regular interval of time. If any small insects or dust particles enters his eyes, do not rub them but clean them immediately with cold water. He should avoid direct exposure of sunlight to the eye. While reading, there should be a distance of at least 25 cm between the eyes and the book.
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 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.
14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Ans.- 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.
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.- 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 of 60°.
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?
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.- a) Image of an 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. 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.
Junior Science Topic - Light. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 8, CBSE syllabus, chapter 16)
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