ASHAQ HUSSAIN BHAT
GL TEACHER SCHOOL EDUCATION
DEPARTMENT JAMMU AND KASHMIR
LIGHT LESSON NO. 15
Points to Remember
- Light travels along a straight path.
- The change of direction of light by a mirror when it falls on it is called reflection of light.
- Objects are visible only when light reflected by them reaches our eyes.
- Plane mirror shows the images of the object placed in front of them.
- The image formed in a plane mirror is always behind the mirror, erect, of same size and at the same distance from the mirror as object in front of mirror.
- The curved shining surfaces are called spherical mirrors. If the reflecting surface is concave then the mirror is concave or if reflecting surface is convex then the mirror is called convex mirror.
- An image formed on a screen is called Real image.
- Virtual image cannot be obtained on screen.
- Concave mirrors form smaller or larger images which may be real or virtual.
- Concave mirrors have many uses.
- Convex mirror form images erect and smaller in sizes.
- Magnifying glass is a convex lens.
- A lens with thick edges and thin middle portion is called concave lens.
- A lens which is thick in middle and thin at the edges is called convex lens.
- Images formed by a convex lens are visible from the side opposite to that of the object.
- Light of rays bend to a point after passing through a convex lens. So, it is called converging lens.
- A concave lens, diverges the light rays and is called diverging lens.
- White light consists of seven colours.
- A convex lens can form real and inverted images.
- A concave lens always forms erect, virtual and smaller images than the object.
SOME IMPORTANT DEFINITIONS
Reflection of light. The change in direction of light by a shiny surface is called reflection of light.
Screen. The white sheet or surface on which the image is obtained.
Source of light. An object which emits light.
Real image. The image formed when incident rays after reflection actually meet at a point.
Virtual Image. The image formed when incident rays after reflection do not actually meet but appear to meet at a point.
Concave mirror. The spherical mirror whose outer surface is polished and in which reflection takes place from the inner surface .
Convex Mirror. The spherical mirror whose inner surface is polished and in which reflection takes place from the outer surface.
Lens. A lens is made up of transparent medium (glass) bounded by two spherical surfaces.
Lenses are of two types-(i) Convex lens and (ii) Concave lens.
Convex Lens. A convex lens is thick at the centre and thin at the edges.
Concave Lens. A concave lens is thin at the centre and thick at the edge.
TEXT BOOK EXERCISES
Q. 1. Fill in the blanks :
(a) An image that cannot be obtained on a screen is called...........
(b) Image formed by a convex..........is always virtual and smaller in size.
(c) An image formed by a ............ mirror is always of same size as that of an object.
(d) An image which can be obtained on a screen is called ...............image.
(e) An image formed by a concave............... Can not 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
(a) We can obtain an enlarged and erect image by a convex (T/F)
(b) A concave lens always forms a virtual image:(T/F)
(c) We can obtain a real, enlarged and inverted image by a concave mirror.(T/F)
(d) A real image cannot be obtained on a screen. (T/F)
(e) A concave mirror always forms a real image.(T/F)
Ans.
(a)We can obtain an enlarged and erect image by a convex(F)
(b) A concave lens always forms a virtual image.(T)
(c) We can obtain a real, enlarged and inverted image by a(T)
(d) A real image cannot be obtained on a screen. (F)
(e) A concave mirror always forms a real image(F)
Q. 3. Match the items given in Column I with one or more items of Column II.
Column I
(a) A plane mirror.
(b) A convex mirror
(c) A convex lens
(d) A concave mirror
(e) A concave lens
Column II
(i) Used as a magnifying glass.
(ii) Can form image of objects spread over a large area.
(iii) Used by dentists to see enlarged image of teeth.
(iv) The image is always erect and diminished. te
(v) The image is erect and of same size as the object.
(vi) The image is erect and smaller in size than the object.
Ans.
Column I. Column II
(a) ...........................v
(b) ...........................vi
(c) .............................i
(d).............................. ii
(e) ...............................iv
Q. 4. State the characteristics of image formed by a plane mirror.
Ans. Characteristics of image formed in a plane mirror.
(i) The image is as far behind the plane mirror as the object is in front of it.
(ii) The image is laterally (sideways) inverted.
(iii) The image is of the same size as that of the object.
(iv) The image formed in a plane mirror is virtual, erect and of the same size as the object.
(v) The image formed in a plane mirror cannot be obtained on the screen.
Q. 5. Find out the letters of English alphabet or any another language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.
Ans. Letters of English Alphabet
A, H, O, I, M, N, T, U, V, W, X
Letter in Hindi and Punjabi
ठ. | ठ.
Q. 6. What is a virtual image ? Give one situation where a virtual image is formed.
Ans. Virtual image When light rays starting from an object after reflection from a mirror or after refraction through a lens do not actually meet but appear to meet then the image formed is called virtual image which cannot be obtained on a screen.
Images formed in a plane mirror are always virtual. Image in a concave mirror is virtual when object is placed very close to the mirror.
Image formed in a convex mirror is always virtual.
Q. 7. State two differences between a convex and a concave lens:
Ans. Differences between a Convex and a Concave lens.
Convex lens use
1.It is thick at the centre
and thin at the edges.
2.The image is generally
formed on the other side of
the lens (except when the
object is between F and
optical centre).
3. It converges a beam of
Light to a point
Concave lens
1. It is thin at the centre
and thick at the edges.
The image is formed on
the same side of the object.
2.It diverges a beam of light
light to a point. 2MOIT as
coming from a point.
Q. 8. Give one use each of a concave and a convex mirror.
Ans. Use of concave mirror. It is used as reflector in a search-light or car head light.
Use of convex mirror. It is used in cars and other vehicles as a rear view mirror.
Q. 9. Which type of mirror can form a real image ?
Ans. A concave mirror forms a real image.
Q. 10. Which type of lens always forms a virtual image?
Ans. Concave lens.
Choose the correct option in questions 11-13.
Q. 11. A virtual image larger than the object can be produced by a :
(i) Concave lens
(iii) Convex mirror
(ii) Concave 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 1m towards the mirror, then the distance between David and his image will be.
(i) 3 m (ii) 5 m (iii) 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 truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be.
(i) 1 m/s (ii) 2 m/s (iii) 4 m/s (iv) 8 m/s. 10
Ans. (iii) 4 m/s.
SOME ADDITIONAL IMPORTANT QUESTIONS
Q. 1. Write a note on convex lens. thick in the centre and thin at the edges.
Ans. Convex lens. A convex lens is h
This lens has the capability to converge
a beam of light. So this lens is also called
a converging lens.
Q. 2 . What is a concave lens ?
Ans. Concave lens. A concave lens
is thin at the centre and thick from
outside. This lens has a capacity to
diverge a beam of light. So this lens is
also called a diverging lens.
Q. 3 . Name two objects which split white light into many colours.
Ans. Water bubbles, surface of CD, prism.
Q. 4 . Name the seven colours of light.
Ans. Violet, Indigo, Blue, Green, Yellow, Orange, Red.
Q. 5 . What is natural example depicting seven colours of light.
Ans. A Rainbow.
Q. 6 . Why should not we see towards sun through a convex lens ?
Ans. A convex lens is a converging lens. It converges a beam of parallel rays coming from the Sun at the focus. If a piece of paper or cloth is placed at the focus of a convex lens held in front of the sun, these things catch fire. So, if we see towards à a convex lens, it is very harmful for the eyes. Hence We should not see the sun through a convex lens.
Q. 7. Give an experiment to show that light travel light travels in a straight line, we should do the following
Ans. Rectilinear Propagation of light. To prove that light travels in a street line we should do the following experiment .
Experiment.
Q. 8 . Give the conditions necessary for seeing an object.
Ans. Conditions for seeing an object. To see an object the following three conditions are required to be satisfied:
(i) Source of light to make the object visible.
(ii) The object.
(iii) Eye sight.
Q. 9 . What are the uses of mirrors ?
Ans. Uses of Mirrors. We use mirrors in our daily life for one or the other purpose.
1. Plane Mirror. The plane mirrors are used in the barber's shops and in homes as looking glass. It is also used in periscope and other optical instruments.
2. Concave mirror.
(i) It is used as reflector in a search light or a car head light.
(ii) It is used in the solar cookers to converge the radiations coming from the sun.
3. Convex mirror.
(1) It is used in the cars and other vehicles as a rear view mirror to have look at the vehicles behind.
(ii) It is used in amusement parks for showing strange looking images of people who stand in front of them.
Q. 10 . While passing through a prism, why does the white light split into seven colours.
Ans. The rays of different colours pass smoothly through air but when passing through a prism they have to change the speed according to the angle of prism. Different colours have different wave lengths and thus they choose different paths. So the spectrum is seen.
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