Question 1. Define the principal focus of a concave mirror. Question 2. Which mirror is used by dentists? Question 3. What are the advantages and disadvantages of using convex mirror for seeing traffic at the rear? Question 4. What are the advantages and disadvantages of using a convex mirror for seeing traffic at the rear? Question 5. The refractive index of diamond is 2.42. What is the meaning of this statement? Question 6. A ray is incident along ACB on a spherical mirror, where C is the centre of curvature. Question 7. Find the focal length of a convex mirror whose radius of curvature is 32 cm. Question 8. Why do we prefer a convex mirror as a rear- view mirror in vehicles? Question 9. What happens when the rays of sun are focused at a point on the paper by using concave mirror? Question 10. How is the refractive index of an optical medium related to the speed of light in that medium? Question 11. Define the term absolute refractive index of a medium.
Answers 1. The point on the principal axis where all the incident rays ,parallel to the principal axis actually meet after reflection from the concave mirror 2. Concave mirrors 5. It means ratio of speed of light in air to the speed of light in diamond is equal to 2.42 7.focal length =32/2 = 16 cm 8. Two reasons
i. It always produced erect and virtual image
ii. The size of the image is smaller then the object.
Both of the above reason allows the driver to have a wider view of the traffic behind the vehicle in small mirror 9. The paper burns due to converging of the light at that point. 10. Refractive index of an optical medium is ratio speed of the light in air to the speed of the light in the optical medium 11. Absolute refractive index of a medium is defined as the ratio of the velocity of light in vacuum to the velocity of light in the medium. If c is the velocity of light in vacuum and v in the medium, then n = c/v
QUESTIONS: (2 marks each)
Question 1. Name the type of mirror used in the following situations:
i.Headlights of a car.
ii.Side/rear view mirror of a vehicle.
iii.Solar furnace.
Support your answer with reason. Question 2. Show that the mirror formula for spherical mirrors also holds for a plane mirror too. Question 3. A concave mirror produces three times magnified real image of an object placed at 10 cm in front of it. Where is the image located? Question 4. Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is $3 \times 10^8 m/s$ Question 5. Name the type of mirror which is known as shaving mirror. Give relevant ray diagram to justify your answer. Question 6. Find the focal length in cm of a lens of power -2D. Identify the type of this lens in this case. Question 7. Find the refractive index of glass with respect to water where refractive index of glass with respect to air is $n_{ga} = \frac {3}{2}$ and that of water with respect to air is $n_{wa} = \frac {4}{3}$. Question 8. A converging lens has a focal length of 250 mm. Calculate its power and express it according to sign convention. Question 9. List four specific characteristics of the images of the object formed by convex mirrors. Question 10. State two positions in which a concave mirror produces a magnified image of a given object. List two differences between the two images. Question 11. State the meaning of linear magnification. How is it related to object distance and image distance? When is magnification positive or negative? click for answer
Answers 1. a. Concave mirror
b. convex mirror
c. concave mirror
2.
Spherical Mirror formula is given as
$ \frac {1}{f} = \frac {1}{v} + \frac {1}{u}$
For a plane mirror, $f = \infty$ , v = v , u = -v
Applying these to Mirror formula
$\frac {1}{\infty} = \frac {1}{v} + \frac {1}{-v}$
$0 = 0$
Hence mirror formula holds good for plane mirrors too.
3.
Here, linear magnification (m) = - 3 (Negative sign for real image, which is inverted)
Object distance(u)=-10 cm
Image distance =x
Now $ m= -\frac {v}{u}$
or v =3u =-30 cm
The image is located at 30 cm in front of the mirror.
4. Refractive index is given by
$n= \frac { speed \; light \; in \; air}{speed \; light \; in \; glass}$
$1.5 = \frac {3 \times 10^8 }{x}$
$x=2 \times 10^8 $
5. Concave mirror
6. $Power = \frac {1}{f(m)}$
$f(meter) = \frac {1}{power} = -.5 m $
f=-50 cm. It is a concave lens
8. f=250 mm = .25 m
$Power = \frac {1}{f(m)} = 4 D$
9.
i.The image is always virtual and erect.
ii.The image is highly diminished or point sized.
iii.It is always formed between F and P.
iv. The object and image are on opposite sides of mirror.
10.
A concave mirror produces a magnified image when the object is placed in front of the mirror:
i.between its pole and focus
ii.between the focus and centre of curvature
For Ist one the image is virtual and erect, whereas in second the image is real and inverted.
11.
linear magnification is the ratio of the size of the image to the size of the object . It is denoted by Letter m.
For a mirror ,formula is given
$m = - \frac { v }{ u}$
For a lens ,formula is given
$m = - \frac { v }{ u}$
If the image formed is virtual then the magnification has positive value and in case of real images, it is negative
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