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InterviewSolution
This section includes InterviewSolutions, each offering curated multiple-choice questions to sharpen your knowledge and support exam preparation. Choose a topic below to get started.
| 1. |
A concave mirror of focal length 20 cm is to be used as a shaving mirror. Which of the following is the suitable object distance from the mirror ? (i)10 cm (ii)20 cm (iii) 30 cm. |
| Answer» For using a concave mirror as a shaving mirror, u lt f. Therefore, object distance can only be 10 cm. | |
| 2. |
A concave mirror is used as a head mirror by ENT specialists. The same mirror can also be used as a shaving mirror. Why? |
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Answer» A concave mirror converges the rays of light falling on it in a direction parallel to the principal axis, on to its focus. That is why ENT specialists use it as a head mirror. When an object is held between the pole and the principal focus of a concave mirror, the mirror forms a virtual, erect and magnified image of the object. That is why it can be used as a shaving mirror. |
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| 3. |
(a) State the relation between object distance, image at a distance of 30 cm from the mirror when the object is Give the New Cartesian Sign Conventions used. (b) A concave mirror forms a virtual and erect image at a distance of 30 cm from the mirror when the object is held at 10 cm in front of the mirror. Calculate focal length of the mirror. |
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Answer» (a) N/A (b) N/A |
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| 4. |
Can a concave mirror form a virtual image of same size as the object? |
| Answer» No, virtual image formed by a concave mirror is always enlarged. | |
| 5. |
What is the basic difference between a concave mirror and a convex mirror? |
| Answer» In a concave mirror, the reflecting surface is towards the centre of the sphere of which the mirror is a part. In a convex mirror, the reflecting surface is away from the centre of the sphere, of which the mirror is a part. | |
| 6. |
For what position of an object, a concave mirror forms a real image equal in size to the object? |
| Answer» The object must be at the centre of curvature of the mirror. | |
| 7. |
A student has to project a three times magnified image of a candle flame on a wall. Name the type of the lens (converging/diversing) required for the purpose. If the candle flame is at a distance of 6 m from the wall, find the focal length of the lens. |
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Answer» Here, `m = -3` As image is formed on the wall, the lens must be converging. As `m = v/u = 3, v = 3u`, in magnitude. As candle flame is at a distance of 6 m from the wall, `:. u + v = 6 or u + 3u = 6, u = 6/4 = 1.5 m` `v = 3u = 3 xx 1.5 = 4.5 m` With proper sign convention, `u = -1.5 m, v = +4.5 m` `1/f = 1/v - 1/u = 1/(4.5) - 1/(-1.5) = (1+3)/(4.5)` `f = (4.5)/4 = 1.125 m` |
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| 8. |
A converging mirror forms a real image of height 4 cm, of an object of height 1 cm placed 20 cm away from the mirror. Calculate the image distance. What is the focal length of the mirror ? |
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Answer» Here, height of image, `h_(2) = -4 cm`, minus sign is for real image, which is inverted i.e. below the principal axis. Height of object, `h_(1) = + 1 cm`. Object distance, `u = -20 cm`, minus sign for the object distance is measured from pole in a direction opposite to the direction of incidence of light. We have to calculate the image distance, `v = ?` As `m = h_(2)/h_(1) = -v/u`, putting the values given above, we get `(-4)/1 = (-v)/(-20)` or `v = -80 cm` Negative sign of v indicates that the image is on the side of the object. It must be real and inverted. To calculate focal length of the mirror, we use mirror formula :`(1)/(f) = (1)/(u) + (1)/(v)` putting `u = -20 cm`, and `v = - 80 cm`, `(1)/(f)=(1)/(-20)+(1)/(-80) = (-4-1)/(80) = (-5)/(80)` `f = (-80)/5 = -16cm` Negative sign of f indicates that mirror is concave. |
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| 9. |
A thin lens has a focal length of `- 25 cm`. What is the power of the lens? It is convex or concave ? |
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Answer» Correct Answer - `- 4 D` ; concave Here, `f = - 25 cm, P = ?` From `P = 100/f = 100/(-25) = -4 D`. Negative sign shows that the lens is concave. |
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| 10. |
Study the given ray diagrams and select the correct statement from the following : A. Device X is a concave mirror and device Y is a convex lens, whose focal lengths are `20 cm` and `25 cm` respectively.B. Device X is a convex lens and device Y is a concave mirror, whose focal lengths are `10 cm` and `25 cm` respectively.C. Device X is a concave lens and device Y is a convex mirror, whose focal lengths are `20 cm` and `25 cm` respectively.D. Device X is a convex lens and device Y is a concave mirror, whose focal lengths are `20 cm` and `25 cm` respectively. |
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Answer» Correct Answer - D (d) The device X is a convex lens of focal length 20 cm. Parallel rays falling on convex lens are focussed on the screen at 20 cm from the lens. The device Y is a concave mirror of focal length 25 cm. Parallel rays falling on the concave mirror are focussed on the screen at 25 cm from the concave mirror. |
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| 11. |
Find the focal length and nature of lens which should be placed in contact with a lens of focal length `10 cm` so that the power of the combination becomes 5 dioptre. |
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Answer» Correct Answer - `- 20 cm`, concave Here, `f_(1) = ?, f_(2) = 10 cm, P_(2) = 100/(f_(2)) = 100/10 = 10 D, P = 5 D`. As `P_(1) + P_(2) = P, P_(1) + 10 = 5` or `P_(1) = = 5- 10 = - 5 D`. and `f_(1) = 100/(P_(1)) = 100/(-5) = -20 cm`. Lens must be be concave. |
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| 12. |
The power of lens is `2.5 D`. What is its focal length ? |
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Answer» Correct Answer - 40 cm Here, `P = 2.5 D, f = ? As f = 100/P, f = 100/(2.5) = 40 cm`. |
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| 13. |
A convergent lens of power 8 D is combined with a divergent lens of power `- 10 D`. Calculate focal length of the combination. |
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Answer» Correct Answer - ` - 0.5 m` Here, `P_(1) = 8 D, P_(2) = - 10 D, F = ?` As `P = P_(1) + P_(2) = 8 - 10 = - 2 D, F = 1/P = 1/(-2) = - 0.5 m` |
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| 14. |
(a) A ray of light in air enters glass. Does it bend towards normal ? (b) If the same ray enters water, which way will it bend? |
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Answer» (a) Yes, it bends towards normal. (b) The ray entering water from air will also bend towards normal. |
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| 15. |
Arrange air, glass and water in terms of decreasing refractive index. |
| Answer» Glass `.^(a)n_(g)=1.5` Water : `.^(a)n_(w)=1.33` Air : `n_(a)=1.0003` | |
| 16. |
A parallel beam of light passes through a lens held at O as shown in Fig. What is the nature of the lens? |
| Answer» The lens at O must be concave as the beam is diverging on passing through the lens. | |
| 17. |
Three students A, B and C focussed a distant building on a screen with help of a concave mirror. To determine focal length of the concave mirror they measured the distances as given below: Student A : From mirror to the screen , Student B : From building to the screen Student C : From building to the mirror Who measured the focal length correctly :A. Only AB. Only BC. A and BD. B and C |
| Answer» Focal length of concave mirror is distance from mirror to the screen. Therefore, student-A is correct. | |
| 18. |
In torches, search lights and headlights of vehicles the bulb is placedA. between the pole and the focus of the reflectorB. very near to the focus of the reflectorC. between the focus and centre of curvature of the reflectorD. at the centre of curvature of the reflector |
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Answer» Correct Answer - B In torches, search lights and head lights of vehicles, the bulb is placed very close to the focus of the reflector, so that a parallel beam of light is obtained. |
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| 19. |
(a) How will you distinguish between convex lens without touching them? (b) Linear magnification of a concave lens is always positive, whereas that of a convex lens can be both, positive or negative . Why? |
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Answer» (a) N/A (b) N/A |
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| 20. |
Between which two points of a concave mirror should an object be placed to obtain a magnification of -3? |
| Answer» For m = - 3, the image is real, inverted and enlarged. The object must be placed between F and C of the concave mirror. | |
| 21. |
Beams of light are incident through the holes A and B and emerge out of box through the holes C and D respectively as shown in Figure. Which of the following could be inside the box ? A. A rectangular glass slabB. A convex lensC. A concave lensD. A prism |
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Answer» Correct Answer - A The emergent rays are parallel and displaced laterally. Therefore, a rectangular glass slab might be inside the box. |
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| 22. |
The image formed by a convex lens is always real. Is it true? |
| Answer» No. A convex lens may also form a virtual and erect image. | |
| 23. |
What is meant by number of lens? |
| Answer» Number of a lens represents power of the lens (in dioptres). This number may be positive or negative. | |
| 24. |
In which of the following cases, linear magnification is positive? |
| Answer» Linear magnification is positive in case (b) as image is virtual and erect. | |
| 25. |
Look at . What is the radius of curvature of the spherical mirror? Given PA=10 cm. |
| Answer» `R = 2f =2 PA=2(10) =20 cm` (numerically) | |
| 26. |
Can a real image be taken on a screen ? |
| Answer» Correct Answer - yes | |
| 27. |
Do lights rays actually pass through a virtual image ? |
| Answer» No, they only appear to pass through a virtual image? | |
| 28. |
(a) The linear magnification of a concave lens is always positive. Why? (b) The linear magnification of a convex lens may be positive or negative. Why ? |
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Answer» (a) This is because a concave lens forms a virtual and erect image for any position of the object. (b) This is because image formed by a convex lens may be real and inverted for some positions of the object and image formed may also be virtual and erect for some other positions to the object. |
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| 29. |
An object 2 cm high is placed at a distance 2 f from a convex lens. What is the height of the image formed ? |
| Answer» The height of image =the height of the object =2 cm. | |
| 30. |
What is the largest value of refractive index? Name the medium. |
| Answer» The largest value of refractive index is 2.42 for diamond. | |
| 31. |
Power of a lens is a measure of the ability of the lens to converge the rays of light falling on it. Quantitatively, power of a lens = reciprocal of focal length of lens, i.e., `P=(1)/(f)`. If a lens happens to diverge the ray of light falling on it, its power is said to be negative. Thus, power of a convex lens is positive and power of a concave lens is negative. If `P_1,P_2` are powers of two lenses held in contact with each other, the power of the combination is `P=P_1+P_2`. Note that `P_1,P_2` are to be added with proper sign. Read the above passage and answer the following question: (i) What is the SI unit of power? (ii) Focal length of a concave lens is `20 cm`. What is its power ? (iii) What lessons of life do you learn from the relation `P=P_1+P_2`? |
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Answer» (i) SI unit of power is diopter (D). One diopter is the power of lens of focal length one meter. (ii) Here, `f =- 20 cm = -20/100 m= -1/5m. P=(1)/(f)=(1)/(-1//5) = -5D` (iii) If `P_1=3D` and `P_2 = - 2D` then `P=P_1+P_2=3D-2D=1 D` Again, if `P_1=3 D` and `P_2 = 2 D`, then `P = P_1 + P_2 = 3D +2D = 5D` i.e., power of combination is much larger when the two lenses are of same nature. The same is true in real life. To make a powerful combination in business, partners should preferably be of same nature. Same is true for life partners, i.e., husband and wife. If their nature is identical, they will make an adorable couple. |
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| 32. |
Rays from Sun converge at a point `15 cm` in front of a concave mirror. Where should an object be placed so that size of its image is equal to the size of the object ?A. ` 15 cm` in front of the mirrorB. `30 cm` in front of the mirrorC. between `15 cm and 30 cm` in front of the mirrorD. more than `30 cm` in front of the mirror |
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Answer» Correct Answer - B As rays from the sun converge at `15 cm, :. f = 15 cm`. Size of image = size of object, when object is placed to `R = 2f = 30 cm` in front of the mirror. |
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| 33. |
A child is standing in front of a magic of a mirror, She finds the image of her head bigger, the middle portion of her body of the same size and that of the legs smaller. The following is the order of combinations for the magic mirror from the top.A. Plane, convex and concaveB. Convex, concave and planeC. Concave, plane and convexD. Convex, plane and concave |
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Answer» Correct Answer - C As image of head is bigger, the top portion of magic mirror must be concave. The middle portion of her body appears to be of same size, therefore, middle portion of magic mirror must be plane. As image of her legs appears smaller, the bottom of magic mirror must be convex. |
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| 34. |
In which of the following, the image of an object placed at infinity will be highly diminished and point sized ?A. Concave mirror onlyB. Convex mirror onlyC. Convex lens onlyD. Concave mirror, convex mirror, concave lens and convex lens |
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Answer» Correct Answer - D Image of an object placed at infinity will be highly diminished and point sized in case of all the four i.e. concave mirror, convex mirror, concave lens and convex lens. |
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| 35. |
A convex lens forms a real and inverted image of a needle at a distance of `50 cm` from it. Where is the needle placed in front of the convex lens if the image is equal to size of the object ? Also, find the power of the lens. |
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Answer» Here, the image distance, `v = 50 cm`, object distance, `u = ?`, power of lens, `P = ?` As the image is of the same size as the object, therefore, object must be at a distance equal to twice the focal length of the lens. In this case, `-u = v = 2f = 50 cm`. Thus, `u = -50 cm` and `f = 25 cm`. As `P = 100/(f(cm)), P = 100/25 = 4 dioptre`. |
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| 36. |
The image of a small electric bulb fixed on the wall of a room is to be obtained on the opposite wall `3 m` away by means of a large convex lens. What is the maximum possible focal length of the lens required for the purpose ? |
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Answer» Correct Answer - `0.75 m` Here, distance between the object and image, `u + v = 3 m` focal length of lens, `f = ?` As maximum distance between an object and image in case of convex lens `= 4 f`, `4 f = 3 m, f = 3/4 = 0.75 m`. |
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| 37. |
How do you measure quickly the approximately focal length of a concave mirror? |
| Answer» Hold the concave mirror facing some distant object like a tree. Hold a cardboard screen in front of the mirror. Adjust the position of the screen so that a sharp image of distant object is formed on the screen. The distance of screen from the mirror gives us the approximate focal length of the concave mirrror. | |
| 38. |
A spherical mirror in which reflecting surface is towards the centre of the sphere of which the mirror is a part, is called concave mirror, i.e., reflection of light for this mirror occurs at concave surface or bent-in surface. The rays of light incident on concave mirror in a direction parallel to the principal axis, actually meet at a single point F on the principal axis of the mirror. This point is called principal focus of the mirror. Read the above passage and answer the following questions : (i) Is principal focus of a concave mirror, a real point or a virtual point? (ii) What happens in case of a convex mirror ? (iii) Our teachers and parents advise us to stay focussed. What does it imply? |
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Answer» (i) The principal focus F of a concave mirror is a real point. This is because the rays of light,on reflection from the mirror meet actually at F. (ii) In case of a convex mirror, reflection of light occurs at convex surface or the bulged out surface. Principal focus F, in this case is a virtual point, as reflected rays only appear to come from F. (iii) Our parents and teachers advise us to stay focussed. It implies that we concentrate all our energies at a single `"point"//"problem"` so that we can resolve the same easily. Staying focussed means we do not divert our energies and attention to several things at a time. This would lead us nowhere. Hence, the secret of success is to stay focussed. |
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| 39. |
Two concave spherical mirrors are parts of two spheres of diameters 1:3. What is the ratio of their focal lengths? |
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Answer» Here, `(R_(1))/(R_(2)) = (1//2)/(3//2)=1/3` `f_1=f_2 = (R_(1)//2)/(R_(2)//2) = (R_(1))/(R_(2)) =1/3`. |
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| 40. |
Focal length of a convex lens in air is 25 cm. It is cut into two equal halves along its principal axis. What will be the focal length of each half ? |
| Answer» When the given convex lens is cut into two equal halves along its principal axis, focal length of each half remains the same as focal length of original lens. | |
| 41. |
Find out from Table `1.3`, the medium having highest optical density. Also, find the medium with lowest optical density. |
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Answer» The medium with highest optical density is diamond, and its refractive index is maximum `(= 2.42)`. Also, the medium with lowest optical density is air, and its refractive index is minimum `(= 1.0003)`. |
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| 42. |
A ray of light is refracted. Which medium a or b is optically denser than the other ? |
| Answer» As the ray bends away from normal, medium a is optically denser than medium b. | |
| 43. |
Which is denser optically out of alcohol with `n=1*36` and carbon disulphide with `n=1*63`? |
| Answer» Carbon disulphide with greater value of refractive index is optically denser compared to alcohol with lower refractive index. | |
| 44. |
A concave lens has focal length of 15 cm. At what distance should an object from the lens be placed so that it forms an image at 10 cm from the lens ? Also, find the magnification of the lens. |
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Answer» Here, focal length of concave lens, `f = -15 cm`. object distance, `u = ?`, image distance, `v = -10 cm`, magnification of lens, `m = ?` As `(1)/(f) = (1)/(v) - 1/u, 1/u = (1)/(v) -(1)/(f) = 1/(-10) + 1/15 = (-1)/30 or u = -30 cm` Thus the object should be placed at a distance of 30 cm on the left side of the concave lens. Linear magnification, `m = v/u = (-10)/(-30) = 1/3`. The positive sign of m shows that the image is virtual and erect, and its size is `(1//3)` of the size of the object. |
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| 45. |
A concave lens has focal length of `20 cm`. At what distance from the lens a 5 cm tall object be placed so that it forms an image at `15 cm` from the lens? Also, calculate the size of the image formed. |
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Answer» Here, `f = -20 cm` `u =?, h_(1)=5cm` `v = -15cm`, `h_(2)=?` As `(1)/(v) - 1/u = (1)/(f) :. 1/u = (1)/(v) - (1)/(f) = 1/-15 + 1/20 =(-1)/(60)` `u = -60cm` From `m=(h_(2))/(h_(1))=v/u, (h_(2))/(5) = (-15)/(-16)=1/4, h_(2) = 5/4cm =1.25cm` This is the height of virtual image. |
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| 46. |
An object 2 cm high is placed at a distance of 16 cm from a concave mirror, which produces 3 cm high inverted image. What is the focal length of the mirror? Also, find the position of the image. |
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Answer» Correct Answer - `-9.6cm, -24cm` Here, `h_(1) = 2 cm, u = -16 cm, h_(2) = -3 cm` `f = ? v = ?` As `m = (h_(2))/(h_(1)) = -v/u, (-3)/2 = (-v)/(-16)` `v = -24 cm` As `(1)/(f) = (1)/(v) + 1/u, (1)/(f) = 1/(-24) - 1/16 = (-2-3)/28 = (-5)/48` or `f = (-48)/5 = -9.6 cm`. |
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| 47. |
A convex lens of power 3 D is held in contact with a concave lens of power -1D.A parallel beam of light is made to fall on the combination. At what distance from the combination will the beam get focussed? |
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Answer» Here, `P_1=3D,P_2= -1D, F=?` `P=P_1 +P_2=(3-1)=2D` `F=100/P = 100/2= 50cm` `:.` The beam will get focussed at a distance of 50cm from the combination of lenses. |
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| 48. |
Do the laws of reflection change, when we use a spherical mirror instead of a plane mirror ? |
| Answer» No, the laws of reflection do not change. | |
| 49. |
A concave mirror produces a real image 10 mm tall, of an object 2.5 mm tall placed at 5 cm from the mirror. Calculate focal length of the mirror and the position of the image? |
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Answer» Correct Answer - `-4cm ; -20cm` Here, `h_(2) = - 10 mm, h_(1) = 2.5 mm, u = - 5 cm, f = ? , v = ?` As `m = (h_(2))/(h_(1)) = -v/u, (-10)/(2.5) = (-v)/(-5) or v = -20 cm` Negative sign of v shows that image is real. `(1)/(f) = (1)/(v) + 1/u = 1/(-20) - 1/5 = (-1-4)/20 or f = (-20)/5 = -4 cm`. |
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| 50. |
What is meant by linear magnification of a concave mirror? |
| Answer» Linear magnifiaction (m) of a concave mirror is the ratio size of the image `(h_(2))` to the size of the object `(h_(1))` i.e., `m=(h_(2))/(h_(1))` | |