LIGHT I

Light
 
 
POINTS TO REMEMBER
 
 
1.  Light travels in a straight line.
 
2. Mirror:A smooth shining surface, which rebounces the light back in same or in different directions is called a mirror.
 
3. Reflection of light:Scattering back of the light by shining, and smooth surfaces is called Reflection.
 
4. Lateral inversion:Phenomenon of changing left to right and right to left by the mirror while forming images is called lateral inversion.
 
5. Diffused reflection:The light reflected from non-polished surfaces is not in a well-defined direction. It spreads out in all directions. This is called diffused reflection.
 
6. The incident ray:A ray of light from a source striking a given surface is called the incident ray.
 
7. The point of incidence:The point at which the incident ray strikes the surface is called the point of incidence.
 
8. Normal:The perpendicular to the surface at the point of incidence is called normal.
 
9. The angle of incidence:The angle between the normal and the incident ray is called the angle of incidence.
 
10. The angle of reflection:The angle between the normal and the reflected ray is called the angle of reflection.
 
11.The two laws of reflection are as follows:
  1. The incident ray, the normal at the point of incidence and the reflected ray are lie on the same plane
  2. The angle of reflection is equal to the angle of incidence.(     i =        r)
 
12. Plane of incidence:The plane that contains both the incident ray and the normal to the plane is called the plane of incidence.
 
13. Kaleidoscope:This is an interesting device based on the principle of multiple reflections in inclined mirrors. It is called Kaleidoscope.
14. Periscope:It is a device which is used to see objects, which are not in the direct line of sight.
 
15. Real image:It is formed if two or more reflected rays actually meet.
 
16. Virtual image:It is formed if two or more reflected rays appear to meet.
 
17.Real image can be obtained on a screen.
 
18.Virtual image can not be obtained on a screen.
 
19. Lens:Lenses are widely used in spectacles, telescopes and microscopes. Lenses are of two types:
     (a) Convex lens and (b) Concave lens.
 
20. Spherical mirror:Mirrors having curved surfaces are known as spherical mirrors.
 
21. Concave mirror:Its reflecting surface is in curving.
 
22. Convex mirror:Its reflecting surface is bulged out.
 
23. Erect image:If the direction of image and object are same is called erect image.
 
24. Magnifying glass:Any object viewed through a convex lens is seen as magnified.
It is used for observing small or minute object.
 
25. Magnified image:If the size of the image is larger than the object it is called magnified image.
 
26. Prism:It is a transparent object triangular in shape that separates white light into different colors.
 
27. Rainbow:A big arch of a band of seven colors is formed in the sky in the direction opposite to the location of the sun. We identify seven colors in a rainbow as Violet, Indigo, Blue, Green, Yellow, Orange and Red.
 
28. Rear view mirror:Convex mirror are widely used as rear view mirrors in cars and other vehicles. In rear view mirror, a virtual, upright and diminished image is seen.
 
29. Side mirror:Rear view mirror is also used as side mirror in vehicles.
 
 
 
 
Q.1. Fill in the blanks:
  1. An image that cannot be obtained on a screen is called ………….. .
  2. Image formed by a convex ................. is always virtual and smaller in size.
  3. An image formed by a ………… mirror is always of the same size as that of the object.
  4. An image which can be obtained on a screen is called a ………….. image.
  5. An image formed by a concave ………… cannot be obtained on a screen.
Ans. (a) virtual image                (b) mirror                     (c) plane
        (d) real                               (e) lens.
 
Q.2. Mark ‘T’ if the statement is true and ‘F’ if it is false:
  1. We can obtain an enlarged and erect image by a convex mirror. (T/F)
  2. A concave lens always form a virtual image. (T/F)
  3. We can obtain a real, enlarged and inverted image by a concave mirror. (T/F)
  4. A real image cannot be obtained on a screen. (T/F)
  5. A concave mirror always form a real image. (T/F)
Ans.  (a) F                                (b) T                          (c) T
          (d) F                               (e) F.
 
Q.3. Match the item given in Column I with one or more item of Column II.
Column IColumn II
  a. A plane mirror
  b. A convex mirror
 c. A convex lens
 d. A concave mirror
 e. A concave lens
  1. Used as a magnifying glass.
  2. Can form image of object spread over a large area.
  3. Used by dentists to see enlarged image of teeth.
  4. The image is always inverted and magnified.
  5. The image is erect and of the same size as the object.
  6. The image is erect and smaller in size than the object.
Ans.
Column IColumn
a. A plane mirror

 b. 

A convex mirror


c. A convex lens
d. A concave mirror


e. A concave lens
5. The image is erect and of the same size as the object.
2. Can form image of object spread over a large area.
1. Used as a magnifying glass.
3. Used by dentists to see enlarged image of teeth.
The image is erect and smaller in size than the object.
 
 
Q.4. State the characteristics of the image formed by a plane mirror.
Ans.Image formed by a mirror (flat) has following features:
  1. Image in virtual and of same size.
  2. Reflected image retains the color of the object.
  3. Image is erected but laterally inverted.
  4. Lateral inversion.Right side of the object appears as left side in the image formed by a plane mirror. For example if we show our right hand, image in the mirror will show it's left hand.
 
          
 
 
                        
 
 
 
Fig.15.1.Lateral inversion by a plane mirror
 
Q.5. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.
Ans. A, H, I, M, O, T, U, V, W, X, Y.
 
Q.6. What is a virtual image? Give one situation where a virtual image is formed.
Ans.A virtual image is formed when two reflected (or refracted) rays do not meet actually. A virtual image cannot be obtained on a screen. Image formed by a plane mirror is always a virtual image.
 
 
 
 
 
 
 Fig.15.2.The image formed by a plane mirror is always virtual
 
Q.7. State two differences between a convex and a concave lens.

Ans.

Convex lensConcave lens
  1. Feel thicker in the middle than at the edges.
  2. Converges the light falling on it.
  3. Can form real and inverted image also form virtual and erect image.
  1. Feel thinner in the middle than at the edges.
  2. Diverges the light falling on it.
  3. Always forms erect, virtual and smaller image.
 
Q.8. Give one use each of a concave and a convex mirror.
Ans.Use of concave mirror:
         Doctors use concave mirrors for examining eyes, ears, nose, throat and teeth.
 
 
 
 
 
 

Fig.15.3.A dentist examining a patient
 
      Use of convex mirror:
      Convex mirror is used as a side view mirror in motor vehicles.
 
 
 
 
 
 
 
 
 

Fig.15.4.Convex mirror as side view mirror
Q.9. Which type of mirror can form a real image?
Ans.  A concave mirror can form a real image.
 
 
 
Fig.15.5. A concave mirror forms a real image of the sun
 
Q.10. Which type of lens forms always a virtual image?
Ans.A concave lens
 
Choose the correct option in questions 11-13:
Q.11. A virtual image larger than the object can be produced bya
  1. concave lens                                              ii. concave mirror
  2. convex mirror                                           iv. plane mirror
Ans.  ii. Concave mirror
 
Q.12. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves I m towards the mirror, then the distance between David and his image will be:
  1. 3 m                                                              ii. 5 m
  2. 6 m                                                             iv. 8 m
Ans.iii. 6 m
 
Q.13. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be:
  1. 1 m/s                                                           ii. 2 m/s
  2.  4 m/s                                                         iv. 8 m/s
Ans.iii. 4 m/s
 
ADDITIONAL IMPORTANT QUESTIONS
 
Q.1. What is rectilinear propagation of light?
Ans.Light travels in a straight line, it is called rectilinear propagation of light.
 
Q.2. What is regular reflection?
Ans.When light falls on polished surfaces they change the direction of light in a well define manner. This is called regular reflection.
 
Q.3. Define ‘Mirror’.
Ans.A smooth shining surface, which rebounces the light back in same or in different direction is called a mirror.
 
Q.4. How many reflected ray can there be for a given single incident ray falling on a plane mirror?
Ans.For one incident ray, there is only one reflected ray.
 
Q.5. Define lateral inversion.
Ans.Phenomenon of changing left to right and right to left by the mirror while forming images is called lateral inversion.
 
Q.6. State two uses of concave mirror.
Ans.  i. Concave mirrors are used in head lights of the cars, buses, etc.
         ii. Used by dentists and doctors.
 
Q.7. What type of image can be obtained by a convex mirror?
Ans.A convex mirror always produces the virtual, erect and smaller images.
 
Q.8. Why should we not look at the sun through convex lens?
Ans.As the light after passing through a convex lens becomes concentrated at a point and can damage our eyes permanently.
 
Q.9. Why do we need a shiny surface for reflection?
Ans.The extent of reflection depends upon the shine and smoothness of the surface. More is the shine and smoothness of the surface, more will be the reflection. That is why, mirrors reflect most of the light falling on it. Hence for reflection, shiny surfaces are required.
 
Q.10. What is reflection?
Ans.When a ray of light falls on a smooth and shiny surface, the whole of light is sent back in the same direction. It is calledreflection (Fig. 15.6). Mirrors do not allow even a small amount of light to pass through them. Mirrors show regular and complete reflection.

 
 
 
 
 
 
 
 
 

Fig.15.6.Reflection of light
 
Q.11. Can you guess how light would be reflected from a mirror if the angle of incidence is 90 degree.
Ans.Incident ray is striking at an angle of 90 degree (perpendicular to the mirror). According to the laws of reflection, the angle of reflection will be 90 degree. The ray of light will be reflected back along the same line.
 
Q.12. While standing before a plane mirror, if you move your right hand, which hand does your image move?
Ans.If we move our right hand, our image will move left hand. It is because in a plane mirror our "left appears right" and "right appears left". This is called lateral inversion. Hence we can say that the plane mirror forms laterally inverted images.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Fig.15.7.Left hand appears on the right side in the image
 
Q.13. Explain the incident ray, point of incidence and normal in brief.
Ans.A ray of light from a source when strikes to a given surface is called the incident ray.
         The point at which the incident ray strikes the surface is called the point of incidence. Whereas, the perpendicular to the surface at the point of incidence is called normal.
 
Q.14. Differentiate between real image and the virtual image.
Ans.Difference between Real image and Virtual image:
Real ImageVirtual Image
  1. It can be obtained on a screen.
  2. It is inverted.
  3. It is formed at the point where the reflected rays actually meet.
  4. It is on the same side of the mirror where the object is present.
  5. Size of the image depends upon the distance of the object from the mirror.
  1. It cannot be obtained on a screen.
  2. It is always erect.
  3. It is formed where the reflected rays appear to meet virtually.
  4. It is always behind the mirror.
 
  1. Size of the image varies according to the mirror used, i.e. image is of same size in plane mirror, larger in concave mirror and smaller in convex mirror
 
Q.15. What are spherical mirrors? Give their types.
Ans.Mirrors having curved surfaces are known as spherical mirrors. Their name is so, because they are considered as a part of a hollow sphere. Mirrors are mainly of two types:
         concave mirrors and convex mirrors:
              (i) Concave mirror. Its reflecting surface is in curving.
             (ii) Convex mirror. Its reflecting surface is bulged out.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Fig.15.8.Convex and Concave Mirrors
 
Q.16. Here are given capital letters of English Alphabet encircle the letters which wi ll not show lateral inversion on facing a plane mirror.
                       A                    B                 C                D               E                    F
                       G                    H                  I                 J                K                   L
                       M                    N                O                P                Q                   R
                       S                     T                 U                V               W                  X
                       Y                    Z
Ans.                                      
 
 
 
 
 
 
 
 
 
Q.17. Why a convex mirror is used as a rear view mirror in cars and other vehicles?
Ans.Convex mirror has a wider field of view than a plane or a concave mirror. So it can well be used to see what is behind us rather well. Hence drivers use convex mirror to see the traffic following him.
Q.18. Draw a ray diagram to explain the formation of a virtual image of a point source of light in a plane mirror.
Ans.See Fig. 15.2 on page 210.
 
Q.19. In Fig. 15.9, complete the image of the coin after reflection at the surfaces of both the mirrors.
Ans.
 
 

\
 
 
 
 
 
 
 
Fig. 15.9
 

Q.20. Explain why the word 'AMBULANCE' is written as in                                       in figure?
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Fig.15.10.An ambulance
Ans. When the driver of a vehicle ahead of an ambulance looks in his/her rear view mirror, she/he can read `AMBULANCE' written on it and give way to it. It is the duty of every one of us to allow ambulance to pass without blocking its way.
           
Q.21. Explain the two types of lenses.
Ans.Lenses are of two types:
        1. Convex lens.Those lenses which feel thicker in the middle than at the edges are convex lenses.
        2. Concave lens.Those lenses which feel thinner in the middle than at the edges are concave lenses.
 
 
 
 
 
 
 
 
 
 
 
 

                                           
                                                        (a)                                            (b)
Fig.15.11.(a) A convex lens and (b) a concave lens
Q. 22. What are converging and diverging lens?
Ans.A convex lens converges (bends inwards) the light falling on it. Therefore, it is called a converging lens. On the other hand, a concave lens diverges (bends outward) the light and is called a diverging lens.
 
Q.23. How many colors are found in a rainbow?
Ans.There are seven colors in a rainbow, though it may not be easy to distinguish all of them. They are as VIBGYOR, and the colors are red, orange, yellow, green, blue, indigo and violet.
 
Q.24. Give an activity to show that seven colors can be mixed to get white light.
Ans.Take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven segments. Paint each segment with the seven rainbow colors, as shown in the following figure. Make a small hole at the centre of the disc. Fix the disc loosely on the tip of a refill
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

                                                (a)                                                          (b)
Fig.15.12.(a) A disc with seven colors (b) It appears white on rotating
 
of a ball pen. Rotate the disc in the day light when the disc will rotate fast, the colors get mixed together and the disc appears to be whitish. Such a disc is popularily known as Newton's disc.
 
Q.25. State the laws of reflection.
Ans.Laws of reflection are:
         (i) The incident ray, the reflected ray and the normal all lie in the same plane at the point of incidence. (Fig. 15.13).
 
 
 
 
 
 
 
 
 
 
 
 

Fig.15.13.Reflection of light
           (ii) The angle of incidence is always equal to the angle of reflection.
                                                                         i =    r
Q.26. Observe the figure (15.14) and fill in the blanks:
  1. Size of your friend is …………. to the size of image.
  2. Distance between mirror and image is ………….  to the distance between your friend and mirror.
  3. Image of your friend is …………….  .
  4. Image of your friend is ……………. inverted.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Fig. 15.14
Ans.  (a) equal                                                             (b) equal
          (c) erect                                                              (d) laterally.
 
Q.27. Write the properties of the images formed by a plane, concave and convex mirror and also concave and convex lenses.
Ans.  (i) Plane Mirror. The image formed by a plane mirror is always erect. It is virtual and is of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it.
In an image formed by a mirror, the left side of the object is seen on the right side in the image, and right side of the object appears to be on the left side in the image.
         (ii) Concave Mirror. A concave mirror can form a real and inverted image. When the object is placed very close to the mirror, the image formed is virtual, erect and magnified.
         (iii) Convex Mirror. Image formed by a convex mirror is erect, virtual and smaller in size than the object.
         (iv) Convex Lens. A convex lens can form real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified. When used to see objects magnified, the convex lens is called a magnifying glass.
         (v) Concave Lens. A concave lens always forms erect, virtual and smaller image than the object.
 
Q.28. Explain the uses of concave and convex mirror and also concave and convex lenses.
Ans.
(a) Concave Mirror
      (i) Doctors use concave mirror for examining eyes, ears, nose and throat.
     (ii) Concave mirrors are also used by dentists to see an enlarged image of the teeth.
    (iii) The reflectors of torches, headlights of cars and scooters are concave in shape.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Fig.15.15.Reflector of a torch
 
 
(b) Convex Mirror
      (i) Convex mirror is used as a side view mirror. These help the drivers to see the traffic behind them.
 
(c) Convex Lens
     (i) It is used as a magnifying glass, microscopes, telescopes etc.
    (ii) Used in spectacles to correct far sightedness.
 
(d) Concave Lens
     (i) Can be used to correct short sightedness in spectacles.
 
 
OBJECTIVE TYPE QUESTIONS
 
Q.1. Match the items given in Column I with those given in Column II:
Column IColumn II
  1. Light travels in
  2. Angle between normal and the incident ray
  3. Device which is used to see objects
  4. Rear view mirror
  5. Reflecting surface in curving
  6. Transparent object triangular as shape
  1. Angle of incidence
  2. Periscope
 
  1. side mirror
  2. Prism
  3. Concave mirror
  4. straight line
 
Q.2. Fill in the blank space in the following statements.
  1. Uneven surfaces show …………… reflection.
  2. Incident ray, the reflected ray and  …………… lie on the same ……………..  .
  3. Laws of ……………. is applicable in playing back shots in the carroms.
  4. Changing of right side to left is called ……………..  .
 
Q.3. Choose the true and false statements from the following:
  1. Concave mirror is converging in nature.
  2. Convex mirror is diverging in nature.
  3. Plane mirror forms virtual image.
  4. Concave mirror has a virtual focus.
  5. Spherical mirrors do not obey laws of reflection.
  6. During lateral inversion, the image becomes inverted.
  7. Angle between incident ray and reflected ray is double that of the angle of incidence.
           
Q. 4. Choose the correct option in the following:
  1. The path of the light is
(a) always a straight line                            (b) a curved line
(c) a zig-zag line                                        (d) depends on the medium
  1. Which one shows lateral inversion?
(a) Plane mirror                                           (b) Convex mirror
(c) Concave mirror                                      (d) All of these
  1. Image formed by a plane mirror is always
(a) Virtual and erect                                     (b) Real and erect
(c) Virtual and inverted                               (d) Real and inverted
  1. An image which can be obtained on a screen is called
            (a) Erect                                                       (b) Inverted
            (c) Real                                                        (d) Virtual
  1. Image formed by a convex mirror is
            (a) Erect, virtual and smaller                       (b) Inverted, virtual and smaller
            (c) Erect, real and smaller                           (d) Erect, virtual and magnified
  1. Which is used as side view mirror?
(a) Plane mirror                                            (b) Concave mirror
(c) Convex mirror                                        (d) None of these
  1. A concave lens always forms
(a) Erect, virtual and smaller image
(b) Erect, virtual and magnified image
(c) Erect, real and smaller image
(b) Inverted, virtual, and smaller image
  1. White light is composed of
(a) Seven colors                                           (b) Three colors
(c) Five colors                                              (d) Eight colors
  1. The mirror has a wide field of view must be
(a) Convex                                                   (b) Concave
(c) Plane                                                       (d) Both (a) and (b)
  1. The ratio of the size of the image to the size of the object is called
(a) magnification                                          (b) power
(c) transformation ratio                                (d) focal length
  1. The laws of reflection of light are valid for
(a) Plane mirrors only                                  (b) Concave mirrors only
(c) Convex mirrors only                              (d)  All reflecting surfaces
  1. The power of a lens is measured in
(a) cm                                                           (b) cm-1
(c) dioptre                                                    (d) m
  1. The splitting of white light into its seven constituent colors is called
(a) refraction                                                 (b) dispersion
(c) deviation                                                 (d) reflection
  1. A convex lens
(a) has always two convex surfaces
(b) has always two spherical surfaces
(c) is thinner in the middle than at the edges
(d) is thicker in the middle than at the edges.
  1. During the reflection of light angle of reflection is
(a) equal to incidence angle                         (b) smaller than incidence angle
(c) bigger than incidence angle                    (d) equal to incident ray
 
ANSWERS TO OBJECTIVE TYPE QUESTION
 
 
Q.1.  Match the items given in Column I with those given in Column II:
Column IColumn II
  1. Light travels in
  2. Angle between normal and the incident ray
  3. Device which is used to see objects
  4. Rear view mirror
  5. Reflecting surface in curving
  6. Transparent object triangular as shape
  1. Straight line
  2. Angle of incidence
 
  1. Periscope
  2. side mirror
  3. Concave mirror
  4. Prism
 
Q.2. Fill in the blanks:
  1. irregular                                                          ii. Normal, plane
  2.  Reflection                                                   iv. Lateral inversion.  
 
Q.3. True/False
  1. True                  ii. True               iii. True                  iv. False           v. False
  2. False               vii. True.             
 
Q.4. Choose the correct option:
      i.  (a)                   ii. (a)               iii. (a)               iv. (c)             v. (a)
    vi.  (c)                 vii. (a)             viii. (a)               ix. (b)             x. (a)
    xi.  (d)                 xii. (c)             xiii. (b)             xiv. (d)           xv. (a).