1. The first theory of light was put forward by Newton. This theory treats light as a particle called corpuscle.
2. We can say that light propagates as a wave and is emitted and absorbed as a particle.
3. One may regard light as having dual nature.
4. Maxwell proved that light has wave nature; it is an electromagnetic transverse wave, which does not require medium to propagate.
5. The speed of light in air is a universal constant. It has a value of 3x108m/s.
6. Light gets reflected when it falls on polished surface; like mirror.
7. Light suffers refraction when it travels from one medium to another.
8. There is a change in the wavelength of light when it moves from one medium to another.
9. The image is unmagnified, virtual and erect in a plane mirror.
10 Focal length of a plane mirror is infinity.
11. Power of a plane mirror is zero.'
12. It should be noted that, if the spherical lenses are surrounded by a material with a refractive index greater than that of the lens, the convex lens gets converted into a concave lens or vice-versa.
f = R/2
1 / v + 1 / u = 1 / f = 2 /R
M = h2 /h1 = -v/u
n = c/v
1n2 = 1/2n1
1n2 = n2/n1= sin i/sin r
a n w = x/y
1/v –1/u = 1/f
m =I/O = h2 /h1 = v/u
P= 1/f metre
P = p1 + p2 + p3 + …….
VERY SHORT QUESTIONS CARRYING 1 MARK EACH.
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?
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.
11. If an object is placed at a distance of 10cm in front of a plane mirror, how far would it be from its image?
Ans: The object would be at a distance of 20cm from its image.
12. Name the physical quantity which remains the same when light goes from one medium to another?
Ans: Frequency of light.
13. Define power of a lens.
Ans: The reciprocal of the focal length of the lens is called power of a lens. P = 1/f
14. Calculate the power of a convex lens of focal length 25cm?
Ans: Focal length f = 25cm.
Power = 1/.25 = 100/25 = +4D.
15. Name the phenomenon responsible for the following affect.
Ans: When we sit in front of a plane mirror and write with our right hand, it appears in the mirror that we are writing with the left hand.Ans: Lateral Inversion
16. Define Refractive Index
Ans: It is the ratio of speed of light in vacuum to the speed of light in medium is called Refractive index.
17. Why is the convex lens also known as a converging lens?
Ans: A convex lens is also known as a converging lens because it converges a parallel beam of light raying passing through it.
18. Give the ratio of velocities of two light waves travelling is vacuum and having wave lengths 4000A and 8000A.
Ans: In Vacuum, light of all the wave lengths travel with the same velocity i.e. 3 X 108 m/sec.
19. For what angle of incidence, the lateral shift produced by parallel sided glass plate is zero?
Ans: For i = 0.
20. What are the factors on which the lateral shift depends?
Ans: Thickness of the refracting medium, angle of incidence and its refractive index.
21. What is the power of a plane glass plate?
22. An object is placed at the focus of a concave lens. Where will its image be formed?
Ans: The rays will appear to come from infinity. Therefore, a virtual image will be formed at infinity.
23. What is the unit of power of a lens?
24. Why the stars appear to twinkle?
Ans: The twinkling of a star is due to the atmospheric refraction of star’s light.
25. What is the value of refractive index of air?
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