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ScienceClass 10CBSE

A convex lens of focal length 20 cm can produce a magnified virtual as well as real image. Is this a correct statement? If yes, where shall the object be placed in each case for obtaining these images?

Soln: When an object is placed in F and F2 of a convex lens, we get inverted, enlarged and real image is formed beyond 2F2 which is on the other side of the lens. Hence we need to place the object between 20 and 40 cm of the lens. When an object is placed between F and 0 of a convex lens, its enlarged, erect and virtual image is formed beyond FL i.e. on the same side of lens. So for this we need to place the object at a distance less than 20 cm from the lens.

ScienceClass 10CBSE

Refractive index of diamond with respect to glass is 1.6 and absolute refractive index of glass is 1.5. Find out the absolute refractive index of diamond.

Soln: Absolute RI of diamond= 1.6 Absolute RI of glass= 1.5 Multiplying them we get 2.4

ScienceClass 10CBSE

How is the refractive index of a medium related to the speed of light? Obtain an expression for refractive index of a medium with respect to another in terms of speed of light in these two media?

Refractive Index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values. w=civ (where n:refractive index,c=speed of light,v:velocity of light in that medium) Refractive index of one medium in relation to a second medium is given by ratio of speed of light In second medium to speed of light in first medium.

ScienceClass 10CBSE

A pencil when dipped in water in a glass tumbler appears to be bent at the interface of air and water. Will the pencil appear to be bent to the same extent, if instead of water we use liquids like, kerosene or turpentine. Support your answer with reason.

Bending of light here is a function of refraction. Refraction is dependent on refractive indices. Refractive indices of kerosene or turpentine would not be same as water. Hence degree of bend would be different in different mediums.

ScienceClass 10CBSE

Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself? Explain using a diagram.

When light ray enters denser medium from rarer medium it bends towards the normal In this case extent of bending of ray at opposite parallel is same. Hence emergent ray is parallel to incident ray.

ScienceClass 10CBSE

Identify the device used as a spherical mirror or lens in following cases, when the image formed is virtual and erect in each case. (a) Object is placed between device and its focus, image formed is enlarged and behind it. (b) Object is placed between the focus and device, image formed is enlarged and on the same side as that of the object. (c) Object is placed between infinity and device, image formed is diminished and between focus and optical centre on the same side as that of the object. (d) Object is placed between infinity and device, image formed is diminished and between pole and focus, behind it.

Soln: Concave mirror Convex lens Concave lens Convex mirror

ScienceClass 10CBSE

In which of the following, the image of an object placed at infinity will be highly diminished and point sized? (a) Concave mirror only (b) Convex mirror only (c) Convex lens only Concave mirror, convex mirror, concave lens and convex lens

Soln: Answer is (d) Concave mirror, convex mirror, concave lens and convex lens

ScienceClass 10CBSE

A child is standing in front of a magic 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 concave (b) Convex, concave and plane (c) Concave, plane and convex Convex, plane and concave

Soln: Answer is (c) Concave, plane and convex Explanation: When the object is between F and P of concave mirror enlarged image is formed behind the mirror. Hence child can see her head bigger in a concave mirror. She can see her body size of the same size because plane mirror gives image of original size. Convex mirror gives diminished images and babies legs appear smaller.

ScienceClass 10CBSE

You are given water, mustard oil, glycerine and kerosene. In which of these media a ray of light incident obliquely at same angle would bend the most? (a) Kerosene (b) Water (c) Mustard oil Glycerine

Soln: Answer is (d) Glycerine Explanation: Refractive indices Water-1.33 Kerosene-1.44 Mustard oil-1.46 Glycerine-1.47

ScienceClass 10CBSE

The laws of reflection hold good for (a) plane mirror only (b) concave mirror only (c) convex mirror only all mirrors irrespective of their shape

Soln: Answer is (d) all mirrors irrespective of their shape

ScienceClass 10CBSE

In torches, search lights and headlights of vehicles the bulb is placed (a) between the pole and the focus of the reflector (b) very near to the focus of the reflector (c) between the focus and centre of curvature of the reflector at the centre of curvature of the reflector

Soln: Answer is (b) very near to the focus of the reflector Explanation: Headlight reflectors and search lights are in the shape of concave mirror. When source of light is placed at the focus, reflected light appears like a beam.

ScienceClass 10CBSE

A full length image of a distant tall building can definitely be seen by using (a) a concave mirror (b) a convex mirror (c) a plane mirror both concave as well as plane mirror

Soln: Answer is (b) a convex mirror Explanation: Field of convex mirror is more than any type of mirror. Hence full length size of building can be seen by using a convex mirror.

ScienceClass 10CBSE

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 mirror (b) 30 cm in front of the mirror (c) between 15 cm and 30 cm in front of the mirror more than 30 cm in front of the mirror. 

Soln:Answer is (d) more than 30 cm in front of the mirrorExplanation:Here f= 15,Radius of curvature is double the focal length hence c=30When object is placed on c, its image is of the same size, inverted and is formed on c.Let’s go step by step and make the concept very clear.1. What does “rays from the Sun” mean?The Sun is very far away from the Earth, so the light rays from it are parallel when they reach a mirror.For a concave mirror, parallel rays always:converge (meet) at the focus (F)You are told that these rays meet 15 cm in front of the mirror, so:Focal length (f) = 15 cm2. Important mirror pointsFor a concave mirror:Focus (F) = ( f = 15 cm} )Centre of curvature (C) = ( 2f )C = 2fSo, C = 2 \times 15 = 30 cm}3. When is image equal to object?In a concave mirror:If the object is placed at centre of curvature (C):Image is formed at CImage is same size as the objectImage is real and invertedThis is a standard result in ray diagrams.4. Why does this happen?At ( C = 2f ):Rays reflect symmetricallyThe image distance = object distanceMagnification ( m = 1 )So,Image size = Object size5. Final AnswerThe object must be placed at 30 cm in front of the mirror✔️ Correct option: (b) 30 cm in front of the mirror

ScienceClass 10CBSE

Which of the following statements is true? (a) A convex lens has 4 dioptre power having a focal length 0.25 m (b) A convex lens has –4 dioptre power having a focal length 0.25 m (c) A concave lens has 4 dioptre power having a focal length 0.25 m A concave lens has –4 dioptre power having a focal length 0.25 m

Soln: Answer is (a) A convex lens has 4 dioptre power having a focal length 0.25 m Explanation: Positive value for focal length indicates convex lens.

ScienceClass 10CBSE

Magnification produced by a rear view mirror fitted in vehicles (a) is less than one (b) is more than one (c) is equal to one can be more than or less than one depending upon the position of the object in front of it

Soln: Answer is (a) is less than one Explanation: Convex mirror is used in rear view mirror. Convex mirror always give smaller image. Hence, magnification produced by rear view mirror is always less than 1.

ScienceClass 10CBSE

1. Under which of the following conditions a concave mirror can form an image larger than the actual object? (a) When the object is kept at a distance equal to its radius of curvature (b) When object is kept at a distance less than its focal length (c) When object is placed between the focus and centre of curvature When object is kept at a distance greater than its radius of curvature

Soln: Answer is (c) When object is placed between the focus and centre of curvature Explanation: When object is placed between F and C an enlarged image is formed beyond C.

ScienceClass 10CBSE

1. A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this mirror is (a) – 30 cm (b) – 20 cm (c) – 40 cm – 60 cm

Soln: Answer is (b) – 20 cm Explanation: Here, size of object = O = + 10.0 mm = + 1.0 cm (as, 1 cm = 10 mm) Size of Image size = I = 5.0 mm = 0.5 cm Image distance, v = − 30 cm (as image is real) Let, object distance = u Focal length, f =? Magnification m= I (Size of image) O(Size of image) Magnification is given by m=−𝑣 𝑢 𝐼 = −𝑣 𝑂 𝑢 0.5=−30 1 𝑢 U= -60cm Focal length is given by 1= 1+1 𝑓 𝑣 𝑢 1= 1 + 1 𝑓 −30 −60 =−2−1 60 =−3 60 F= -20cm

ScienceClass 10CBSE

1. Which of the following can make a parallel beam of light when light from a point source is incident on it? (a) Concave mirror as well as convex lens (b) Convex mirror as well as concave lens (c) Two plane mirrors placed at 90° to each other Concave mirror as well as concave lens

Soln: Answer is (a) Concave mirror as well as convex lens Explanation: Emergent beams are parallel and the image is produced at infinity when light rays from a point source are incident on a concave mirror and convex lens.

ScienceClass 10CBSE

1.      How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?

Sun appears red at sunrise and sunset : At sunrise and sunset, sun is closer to the horizon. The sunlight passes through denser layer of air and covers larger distance before reaching our eyes. Most of the blue light gets scattered. And red color reaches us which make the sun appears red at sunrise. During noon sunlight has to travel less distance to reach us. Because most of the colors get scattered we see sun white in color during noon.

ScienceClass 10CBSE

1.      How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?

When ray of light enters a prism, it bends because of refraction of light. When the ray of light finally emerges out of the prism. it deviates drastically from its original path. This happens because of unique shape of prism. Different colours in the visible spectrum have different speeds. Due to this, different colours bend at different angles of deviation. As a result, the emergent light appears as a band of seven colours: the colours which are the components of white light. These colours are Violet. Indigo, Blue, Green, Yellow, Orange and Red. Segregation of white light into its different components is called dispersion of light.

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