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Thursday, December 29, 2011

Class VII Science Chapter Light Mirror and Lens Solved questions and Notes

Objective type Questions:
1. Out of convex mirror and concave mirror, whose focus is situated behind the mirror?
Ans: The focus of convex mirror is situated behind it.
2. For what position of an object, a concave mirror forms an enlarged virtual image?
Ans: Object between pole (P) and focus (F) of the concave mirror.
3. If the focal length of a concave mirror is 25cm. What is its radius of curvature?
Ans: Focal length = Radius of curvature / 2
25 = R / 2
R = 25 X 2 = 50cm.
4. A ray of light falls on a mirror normally. What are the values of angle of incidence and the angle of reflection?
Ans: Both angle of incidence and the angle of reflection are zero.
5. What is the focal length of a plane mirror?
Ans: Infinite.
6. Which spherical mirror is called a divergent mirror?
Ans: A convex mirror is called a divergent mirror.
7. What is the angle of incidence, when a ray of light falls on the spherical mirror from its centre of curvature?
Ans: The angle of incidence is zero, when a ray of light falls on the spherical mirror from its centre of curvature.
8. Name the type of mirror that always forms a virtual image for a real object?
Ans: Convex Mirror.
9. State the relation between radius of curvature and focal length of spherical mirrors.
Ans: Radius of curvature = 2 X focal length.
R = 2f.
10. Which of the two is a diverging lens? Convex lens (or) Concave lens.
Ans: Concave lens.
Short types Questions
Q1. What do you mean by reflection of light?
Ans. Bouncing back of light when it falls on a shiny surface is called reflection of light.
Q2. How does a path of light can be changed?
Ans. By reflecting it from a Polished Surface.
Q3. What are the laws of reflection?
Ans. Angle of incidence is equal to angle of reflection.Incedent, reflected ray & the normal lie on the same plane.
Q4. What is the nature of image formed in a plane mirror?
Ans. Virtual, erect, same image, laterally Inverted (left appears right & vice versa), at same distance from the mirror as the object.
Q5. Difference between real & virtual image?
Ans Real Image
Image which can be obtained on a screen Image e.g.- Plane Mirror
Virtual Image
Image which cannot be obtained on a screen e.g. – Pinhole Camera, Photograph Camera.
Q6. What type of mirror – the inner surfact of the Spoon acts as and the outer surface of the spoon acts as ?
Ans. Inner surface of the spoon acts as Concave mirror and the outer surface of the spoon acts as concave mirror.
Q7. Which concave mirror called a converging mirror and a convex mirror called a diverging mirror?
Ans. Concave mirror is called a converging mirror because parallel rays of light fall on the mirror they Converge at a point called focus.
Convex mirror is called a diverging mirror because parallel rays of light fall on it they diverge after reflection.
Q8. What is the nature of image formed in a convex mirror?
Ans. It is always virtual, erect and diminished.
Q9. What is the nature of image formed in a concave mirror?
Ans. It depends on the portion of the object in front of the mirror. If the object is very close to the mirror the image is virtual, erect & magnified. As the distance increases image becomes real, inverted & its size keeps changing.
Q10. What are the uses of concave mirror?
Ans. 1) Used by the ENT Specialists, dentists.
2.) Used as Shaving mirror.
3) Used by makeup artists.
4) Used in torches & Car headlights to get a parallel beam of light.
Q11. What are the uses of Convex mirror?
Ans. 1) Used as a rear view mirror in vehicles because it has a wide field of view as images are smaller.
2) Used at metro Stations and big departmental stores to keep a check.
Q12. What are uses of Plane Mirror?
Ans. Plane Mirrors are used in optical testing, blind corners instrumental scales, Periscopes, telescope etc.
Q13. What is a lens?
Ans. A piece of transparent material bound by curved surfaces.
Q14. Difference between a convex lens & a Concave lens?
CONVEX LENS
1. It is Thick at the Centre and thinner at edges
2. It is called converging lens
CONCAVE LENS
1. It is thin at the center and thicker at edges
2. It is called diverging lens.
Q15. Why a convex lens is called a converging lens and a concave lens a diverging lens?
Ans. If parallel rays of light fall on a convex lens they converge at a point.
If parallel rays of light fall on a concave lens they diverge as they pass through the lens.
Q16. What is the nature of image formed in a concave lens?
Ans. Virtual, erect & diminished.
Q17. What is the nature of image formed in a convex lens?
Ans. Same as in Concave mirror.
Q18. What are the uses of lenses?
Ans. 1.Magnifying glass – Convex lens.
2. To correct eye defects –people who can not see far off objects clearly wear Concave lens.
People who cant not see nearby object clearly wear Convex lens.
3. lenses are also used in Microscope, Telescope etc.
Q19. How is the rainbow formed?
Ans. Rainbow is formed after rainfall when Sun Shines. When sunlight passes through suspended raindrops it is split into seven colours “VIBGYOR’.
Q20. Where else you can see seven colours of sunlight?
Ans. You can see Seven colours of Sunlight when it falls over Soap bubbles, oil films, Shiny surfaces of CD etc.
Q 21. State the characteristics of the image formed by a plane mirror.
Ans:
1. Image is Virtual
2. It is behind the mirror
3. Image is erect (means not inverted)
4. Image is of same size as the object
5. Image is at same distance from the mirror as the object is from the mirror.
Q22. 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 the same size as that of the object.
d. An image which can be obtained on a screen is called a ----------- image.
e. An image formed by a concave ------------ cannot be obtained on a screen.
Answers:  a) virtual image  b) convex mirror c) planed) real e) concave lens
For full chapter
CBSE Class 7 Science Chapters