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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 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.

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. What is the difference in colours of the Sun observed during sunrise/sunset and noon? Give explanation for each.

Sun appears red at sunrise and sunset : At sinrise 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. At noon, the sun is overhead. The sunlight passes through layers of air and covers shorter distance before reaching our eye. So, almost all colours of light are scattered equally. Hence the sun appears white.

ScienceClass 10CBSE

1. Why is the colour of the clear sky blue?

Blue is the colour in visible spectrum which is having maximum scattering. This makes the blue colour to reach us and the sky appears blue.

ScienceClass 10CBSE

1. Why do we see a rainbow in the sky only after rainfall?

Rainbow is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere. A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop.

ScienceClass 10CBSE

1. Is the position of a star as seen by us its true position? Justify your answer.

Star light undergo continuous refraction on entering earth’s atmosphere. Refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position.

ScienceClass 10CBSE

1. How are we able to see nearby and also the distant objects clearly?

Our eyes can focus on the images from varied distance by changing its focal length of lens. Action of Ciliary muscle helps changing focal length of the lens.

ScienceClass 10CBSE

1. Which of the following statement is correct? (a) A person with myopia can see distant objects clearly (b) A person with hypermetropia can see nearby objects clearly (c) A person with myopia can see nearby objects clearly A person with hypermetropia cannot see distant objects clearly

Answer is (c) A person with myopia can see nearby objects clearly Explanation: Myopia is a condition where a person cannot see the distant objects clears and he can see nearer objects clearly. Hypermetropia is a condition in which a person cannot see the nearer object clearly but he can see the distant objects clearly.

ScienceClass 10CBSE

1. The focal length of the eye lens increases when eye muscles (a) are relaxed and lens becomes thinner (b) contract and lens becomes thicker (c) are relaxed and lens becomes thicker contract and lens becomes thinner

Answer is (a) are relaxed and lens becomes thinner

ScienceClass 10CBSE

1. When light rays enter the eye, most of the refraction occurs at the (a) crystalline lens (b) outer surface of the cornea (c) iris pupil

Answer is (b) outer surface of the cornea Explanation: Cornea is a thin membrane through which light is entered. The incident light rays are bent due to cornea and are converged which results in image formation at retina. So, most of the refraction occurs at the iris of the eye.

ScienceClass 10CBSE

1. The bluish colour of water in deep sea is due to (a) the presence of algae and other plants found in water (b) reflection of sky in water (c) scattering of light absorption of light by the sea

Answer is (b) reflection of sky in water Explanation: Water is colorless water attains the color it is reflected by. Hence sea appears blue.

ScienceClass 10CBSE

1. Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset? (a) Dispersion of light (b) Scattering of light (c) Total internal reflection of light Reflection of light from the earth

Answer is (b) Scattering of light Explanation: Because red color scatters the least it travels long distance. During sunset or sunrise light has to travel long distance to reach the earth. Hence light will be red when it reaches us.

ScienceClass 10CBSE

1. The danger signals installed at the top of tall buildings are red in colour. These can be easily seen from a distance because among all other colours, the red light (a) is scattered the most by smoke or fog (b) is scattered the least by smoke or fog (c) is absorbed the most by smoke or fog moves fastest in air

Answer is (b) is scattered the least by smoke or fog Explanation: Wavelength of red color is the largest thus it can easily be seen from a distance. It is the color which is least scattered by the smoke or smog.

ScienceClass 10CBSE

1. Which of the following statements is correct regarding the propagation of light of different colours of white light in air? (a) Red light moves fastest (b) Blue light moves faster than green light (c) All the colours of the white light move with the same speed Yellow light moves with the mean speed as that of the red and the violet light

Answer is (c) All the colours of the white light move with the same speed

ScienceClass 10CBSE

1. The clear sky appears blue because (a) blue light gets absorbed in the atmosphere (b) ultraviolet radiations are absorbed in the atmosphere (c) violet and blue lights get scattered more than lights of all other colours by the atmosphere light of all other colours is scattered more than the violet and blue colour lights by the atmosphere

Answer is (c) violet and blue lights get scattered more than lights of all other colors by the atmosphere .

ScienceClass 10CBSE

1. Twinkling of stars is due to atmospheric (a) dispersion of light by water droplets (b) refraction of light by different layers of varying refractive indices (c) scattering of light by dust particles internal reflection of light by clouds

Answer is (b) refraction of light by different layers of varying refractive indices Explanation: Refraction of light keeps the position of source of light change. This will make the stars sparkle.

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