CBSE X Science Natural Phenomenon - EducareHut (Next Gen Knowledge Point)

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#1. A concave mirror of focal length 20 cm forms an image having twice the size of object. For the virtual position of object, the position of object will be at

40 cm

At infinity

25 cm

10 cm

Explanation: (c) For virtual image,

M=-v/u = 2 => v=-2u

As 1/u=1/v 1/f

So 1/u 1/(-2u)=1/(-20)

1/2u=1/(-20)
U=-10cm

#2. You are given three media A, B and C of refractive index 1.33, 1.65 and 1.46. The medium in which the light will travel fastest is

equal in all three media

The lower the refractive index, the faster the velocity of light.light will travel faster in a medium having lower refractive index.

#3. In which of the following, the image of an object placed at infinity will be highly diminished and point sized?

Concave mirror only

Convex mirror only

Concave mirror, convex mirror, concave lens and convex lens.

Convex lens only

Explanation: (d) The incident ray coming from the object placed at infinity will be parallel to the principal axis. When the parallel beam of light incident on a mirror or lens, irrespective of their nature, after reflection/refraction, will pass or appear to pass through their principal focus. Hence highly diminished and point size image will be formed at their focus.

#4. In torches, search lights and headlights of vehicles the bulb is placed

between the focus and centre of curvature of the reflector

at the centre of curvature of the reflector

very near to the focus of the reflector

between the pole and the focus of the reflector

Explanation: (b) When source of light is placed very near to the focus of the reflector, which are concave in shape, the reflected light becomes parallel to the principal axis.

#5. Large number of thin stripes of black paint are made on the surface of a convex lens of focal length 20 cm to catch the image of a white horse. The image will be

a zebra of black stripes

a horse of less brightness

a horse of black stripes

a zebra of less brightness

Explanation: (c) Complete image of the white horse is formed but of less intensity, the light falling on the curved portion will not react at the image position.

#6. Magnifying power of a concave lens is

always = 1

always > 1

can have any value

always < 1

#7. If the image is formed in front of the mirror, then the image distance will be

positive

negative

positive or negative depending on the size of the object

neither positive nor negative

#8. The nature of image formed by a convex mirror when the object distance from the mirror is less than the distance between pole and focal point (F) of the mirror would be

real, inverted and diminished in size

virtual, upright and enlarged in size

real, inverted and enlarged in size

virtual, upright and diminished in size

Explanation: (c) Convex mirror always forms a virtual, erect diminished image irrespective of the position of object in front of it.

#9. The refractive index of transparent medium is greater than one because

Speed flight in vacuum = speed of light in transparent medium

Speed of light in vacuum > speed of light in transparent medium

Frequency of light wave changes when it moves from rarer to denser medium

Speed of light in vacuum < speed of light in transparent medium

µ=speed of light in Vacuum (c)/speed of light in medium(v)

As c> v so µ>1

#10. If the power of a lens is – 2 D, what is its focal length?

-50 cm

-100 cm

+100 cm

+50 cm

#11. When light falls on a smooth polished surface, most of it

is reflected in the same direction

is reflected in different directions

is refracted into the second medium

is scattered

#12. When the object is placed between f and 2f of a convex lens, the image formed is

at f

at 2f

beyond 2f

between O and f

#13. An object at a distance of 30 cm from a concave mirror gets its image at the same point. The focal length of the mirror is

30 cm

– 30 cm

+15 cm

– 15 cm

Explanation: (c) When object is placed at 2F, the image formed by concave mirror is also at 2F.
So 2F = -30 or F = -15 cm.

#14. An object at a distance of + 15 cm is slowly moved towards the pole of a convex mirror. The image will get

enlarge and virtual

shortened and real

diminished and virtual

enlarged and real

Explanation: (d) Convex mirror always formed virtual and diminished image.

#15. When object moves closer to convex lens, the image formed by it shift

towards the lens

first towards and then away from the lens

away from the lens

first away and then towards the lens

Explanation: (a) As object moves from infinity towards optical centre of the convex lens image is shifted away from its focal point and towards infinity.

#16. If a man’s face is 25 cm in front of concave shaving mirror producing erect image 1.5 times the size of face, focal length of the mirror would be

75 cm

25 cm

60 cm

15 cm

Explanation: (a) In concave shaving mirror, virtual erect and large size image, behind the mirror is obtained, using

m=-v/u=> 1.5=-v/(-25)

V=75/2 Cm

Now for Mirror Formula

1/f=1/v 1/u = 1/(75/2) 1/(-25)=-1/75

So f= -75 Cm

Hence, focal length of concave mirror is 75 cm.

#17. Which of the following statements is/are true?

A concave lens has – 4 dioptre having a focal 0.25 m

A convex 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

A convex lens has 4 dioptre power having a focal length 0.25 m

Explanation: (a) Positive sign with power and focal length indicates that the given lens is convex.
Also f = 1P=14 = 0.25m

#18. The distance between the optical centre and point of convergence is called focal length in which of the following cases?

Explanation :(c) Parallel rays after refiracting meet at the focus of a convex lens.

#19. Magnification produced by a rear view mirror fitted in vehicles

is more than one

can be more than or less than one depending upon the position of the object in front of it.

is less than one

is equal to one

Explanation: (a) Convex mirror is used as rear view mirror and always forms virtual, erect and diminished image. So magnification produced by a rear view mirror is less than one.

#20. Which of the following mirror is used by a dentist to examine a small cavity?

Concave mirror

Convex mirror

Combination of convex and concave mirror

Plane mirror

Explanation: (c) Concave mirror forms erect and enlarged image when held close to the cavity.

#21. The path of a ray of light coming from air passing through a rectangular glass slab traced by four students are shown as A, B, C and D in figure. Which one of them is correct?

Explanation: (b) When a light ray is incident obliquely on one face of rectangular glass slab, the emergent ray will be parallel to the incident ray and shifted sideward slightly.

#22. An object is placed at a distance of 0.25 m in front of a plane mirror. The distance between the object and image will be

0.5 m

0.125 m

0.25 m

1.0 m

Explanation: (c) Distance between object and image = 0.25 0.25 = 0.5 m

#23. The image formed by concave mirror is real, inverted and of the same size as that of the object. The position of object should be

beyond centre of curvature

at the focus

between focus and centre of curvature

at the centre of curvature

Explanation:(c) When object lies at C of a concave mirror, image is also formed at ‘C’ and having same size real and inverted.

#24. A beam of light is incident through the holes on side A and emerges out of the hole on the other face of the box as shown in the figure. Which of the following could be inside the box?

Concave lens

Prism

Rectangular glass slab

Convex lens

Explanation: (d) From figure, it is clear that parallel rays converge at a point and emerges from face B. So convex lens would be possible inside the box.

#25. Focal length of a concave mirror is

positive

negative

depends on the position of image

depends on the position of object

#26. For a real object, which of the following can produce a real image?

Plane mirror

Convex mirror

Concave lens

Concave mirror

Explanation: (b) Only concave mirror can produces a real image for the any position of object between its focus and infinity.

#27. A divergent lens will produce

always virtual image

none of these

both real and virtual image

always real image

Explanation:(b) Diverging (concave) lens will always forms a virtual, erect and diminished image.

#28. Figure shows a ray of light as it travels from medium A to medium B. Refractive index of the medium B relative to medium A is

#29. The refractive index of water is 1.33. The speed of light in water will be

3 × 108 m/s

2.66 × 108 m/s

1.33 × 108 m/s

2.26 × 108 m/s

µ=c/v

V=c/ µ
V=(3×10⁸)/1.33=2.26×10⁸ m/s

#30. A concave mirror of radius 30 cm is placed in water. It’s focal length in air and water differ by

Explanation: (d) The focal length of spherical mirror does not depends on the surrounding medium.

#31. 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 the figure. Which of the following could be inside the box?

A convex lens

A prism

A concave lens

A rectangular glass slab

Explanation: (a) Figure shows that emergent ray are parallel to the incident ray and shifted side ward slightly. This can be done by the rectangular glass slab only.

#32. A magnified real image is formed by a convex lens when the object is at

only (1) and (2) both

between F and 2F

Explanation: (d) When object is at 2F, real inverted and same size image is formed by convex lens. So, according to question option (1) and (2) both are correct.

#33. If an incident ray passes through the focus, the reflected ray will

be parallel to the principal axis

pass through the pole

retrace its path

pass through the centre of curvature

#34. Light from the Sun falling on a convex lens will converge at a point called

radius of curvature

focus

centre of curvature

optical centre

Explanation: (b) The parallel ray coming from the sun, after refraction through the convex lens converge at its focus.

#35. A ray of light is travelling from a rarer medium to a denser medium. While entering the denser medium at the point of incidence, it

bends towards the normal

goes straight into the second medium

bends away from the normal

does not enter at all

#36. The angle of incidence for a ray of light having zero reflection angle is

90°

30°

45°

Explanation: (a) For reflecting surface ∠i = ∠r

#37. A point object is placed at a distance of 20 cm from a convex mirror of focal length 20 cm. The image will form at:

at infinity

behind the mirror

at the pole

at focus

#38. If the magnification produced by a lens has a negative value, the image will be

real and inverted

virtual and erect

real and erect

virtual and inverted

#39. The angle of incidence i and refraction r are equal in a transparent slab when the value of i is

0°

depend on the material of the slab

45°

90°

Explanation: (a) When the incident ray falls normally on the glass slab, it will refracted without deviation, i.e. along the normal in the glass slab. So, ∠i = ∠r = 0

#40. A light ray enters from medium A to medium B as shown in figure. The refractive index of medium B relative to A will be

less than unity

greater than unity

zero

equal to unity

(a) Here, ∠i = 60°, ∠r = 45°

Using Snell’s law of refraction, refractive index of medium B with respect to medium A.

n=sin i / sin r

=> sin 60 / sin 45

=> √3/√2

finish