# Light Test (By Priyanshu)

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Quizzes Created: 1 | Total Attempts: 124
Questions: 30 | Attempts: 124

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• 1.

### A ray of light incident on a plane mirror makes an angle of 20° with the mirror. Then the angle between the incident ray and the reflected ray is

• A.

70°

• B.

90°

• C.

120°

• D.

140°

D. 140°
Explanation
When a ray of light is incident on a plane mirror, the angle of incidence is equal to the angle of reflection. In this case, the angle of incidence is given as 20°. Therefore, the angle between the incident ray and the reflected ray is twice the angle of incidence, which is 2 * 20° = 40°. However, since the angle is measured between the incident ray and the reflected ray, we need to consider the angle on the opposite side, which is 180° - 40° = 140°. Hence, the correct answer is 140°.

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• 2.

### The inner shining surface of a steel spoon serves as a

• A.

Plane mirror

• B.

Concave mirror

• C.

Convex mirror

• D.

Any one of the above

B. Concave mirror
Explanation
The inner shining surface of a steel spoon serves as a concave mirror. This is because a concave mirror is curved inward and has a reflective surface on the inner side. When light rays fall on the concave mirror, they converge towards a focal point, creating a magnified and inverted image. In the case of a steel spoon, the inner surface acts as a concave mirror, reflecting light and creating an image that appears larger and upside down.

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• 3.

### An incident ray strikes a concave mirror after passing through the focus (F) as shown in the figure. Which of the following shows the correct path of the reflected ray

• A.

Option a

• B.

Option b

• C.

Option c

• D.

Option d

C. Option c
Explanation
Option c shows the correct path of the reflected ray. In a concave mirror, when an incident ray passes through the focus (F), it will be reflected parallel to the principal axis. Option c depicts this correctly, with the reflected ray parallel to the principal axis after passing through the focus.

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• 4.

### The mirror formula for concave mirror is given below. Will this formula hold in case of convex mirror?

• A.

No, never

• B.

Yes, always

• C.

Yes, sometimes

• D.

Nothing can be decided

B. Yes, always
Explanation
The mirror formula for concave mirrors is given by 1/f = 1/v + 1/u, where f is the focal length, v is the image distance, and u is the object distance. This formula holds true for concave mirrors. However, for convex mirrors, the mirror formula is modified to 1/f = -1/v + 1/u. Therefore, the mirror formula does not hold for convex mirrors. Hence, the correct answer is "Yes, always."

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• 5.

### SI unit of radius of curvature of a concave mirror is

• A.

-m

• B.

M−1

• C.

M

• D.

None of these

C. M
Explanation
The SI unit of radius of curvature of a concave mirror is "m" which stands for meters. The radius of curvature is the distance between the center of the mirror and its focal point. Since distance is generally measured in meters in the SI system, it is logical for the unit of radius of curvature to be expressed in meters as well.

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• 6.

### Hold a concave mirror with its shining surface towards the sun. Take a sheet of paper and hold it in front of the mirror. Take the sheet of paper away from the mirror gradually till a sharp, bright spot appears on the paper. The sharp, bright spot is due to

• A.

Reflection of light

• B.

Refraction of light

• C.

Scattering of light

• D.

Diffraction of light

A. Reflection of light
Explanation
When a concave mirror is held with its shining surface towards the sun and a sheet of paper is held in front of it, the mirror reflects the sunlight. As the sheet of paper is gradually moved away from the mirror, the reflected light converges to a point, creating a sharp, bright spot on the paper. This phenomenon is known as reflection of light, where light waves bounce off a surface and change direction. Therefore, the correct answer is reflection of light.

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• 7.

### The linear magnification produced by convex mirror is always positive this is so because

• A.

Convex mirror is a small mirror

• B.

Image formed by convex mirror is always smaller in size than the object

• C.

Image formed by convex mirror is real

• D.

Image formed by convex mirror is always virtual and erect

D. Image formed by convex mirror is always virtual and erect
Explanation
The correct answer is that the image formed by a convex mirror is always virtual and erect. This is because convex mirrors diverge light rays, causing them to spread out and not converge at a point. As a result, the reflected rays appear to come from a virtual image located behind the mirror. Additionally, the image formed by a convex mirror is always erect, meaning it is not inverted.

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• 8.

### Two thin lenses are in contact and the focal length of the combination is 80 cm. If the focal length of one lens is 20 cm, then the power of the other lens will be

• A.

-1.00 D

• B.

1.66 D

• C.

-3.75 D

• D.

4.00 D

C. -3.75 D
Explanation
When two lenses are in contact, their powers add up. The power of a lens is given by the formula P = 1/f, where f is the focal length of the lens. In this case, the focal length of one lens is 20 cm, so its power is 1/20 = 0.05 D. Since the focal length of the combination is 80 cm, the power of the combination is 1/80 = 0.0125 D. Therefore, the power of the other lens (which is the difference between the power of the combination and the power of the first lens) is 0.0125 - 0.05 = -0.0375 D, which is approximately -3.75 D.

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• 9.

### Which of the following materials cannot be used to make a lens

• A.

Water

• B.

Glass

• C.

Plastic

• D.

Clay

D. Clay
Explanation
Clay cannot be used to make a lens because it does not have the necessary optical properties required for lens formation. Lenses require materials with a specific refractive index, which determines how light is bent as it passes through the lens. Clay does not have a suitable refractive index for lens production, unlike water, glass, and plastic, which are commonly used materials for making lenses due to their optical properties.

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• 10.

### Virtual, erect and magnified image can be formed by

• A.

Convex lens only

• B.

Concave lens only

• C.

Convex and concave lens both

• D.

Neither convex nor concave lens

A. Convex lens only
Explanation
A convex lens is capable of forming a virtual, erect, and magnified image. This is because a convex lens converges the incoming light rays, causing them to intersect and form an image on the opposite side of the lens. The image formed by a convex lens is virtual, meaning it cannot be projected onto a screen, and it is erect, meaning it appears upright compared to the object. Additionally, the image formed by a convex lens is magnified, meaning it appears larger than the actual object. Therefore, a convex lens is the only lens capable of forming such an image.

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• 11.

### Which of the following shows incorrect refraction in case of convex lens?

• A.

Option a and b

• B.

Option b and d

• C.

Option c and d

• D.

Option d and a

B. Option b and d
Explanation
Option b and d show incorrect refraction in the case of a convex lens. This means that these options demonstrate a refraction pattern that is not consistent with the behavior of light passing through a convex lens.

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• 12.

### A student determines the focal length of a device X, by focusing the image of a far off object on the screen positioned as shown in the figure. The device is a

• A.

Convex lens

• B.

Concave lens

• C.

Convex mirror

• D.

Concave mirror

D. Concave mirror
Explanation
The student determines the focal length of device X by focusing the image of a far off object on the screen. This suggests that device X is a mirror, as lenses are typically used to focus light onto a screen. Additionally, the fact that the answer is "Concave mirror" indicates that the mirror is curved inward, which is consistent with the concept of a concave mirror.

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• 13.

### On heating a liquid, the refractive index generally

• A.

Decreases

• B.

Increases

• C.

Does not change

• D.

Increases or decreases depending upon the rate of heating

A. Decreases
Explanation
When a liquid is heated, its molecules gain energy and move more rapidly. This increased molecular motion causes the liquid to expand, which in turn decreases the density of the liquid. As the refractive index of a substance is directly proportional to its density, a decrease in density leads to a decrease in the refractive index. Therefore, when a liquid is heated, its refractive index generally decreases.

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• 14.

### If in a concave mirror, an object is placed at a distance d1 from its focus and image is formed at a distance d2 from the focus, then the focal length of mirror is given by

• A.

D1 + d2

• B.

D1 - d2

• C.

D1×d2

• D.

√d1.d2

D. √d1.d2
Explanation
The focal length of a concave mirror can be determined using the mirror formula, which states that 1/f = 1/v - 1/u, where f is the focal length, v is the image distance, and u is the object distance. In this case, the object is placed at a distance d1 from the focus, which means the object distance is equal to -d1. The image distance is given as d2. Plugging these values into the mirror formula, we get 1/f = 1/d2 - 1/(-d1). Simplifying this equation gives us 1/f = (d1 - d2) / (d1 * d2). Taking the reciprocal of both sides and simplifying further, we find that f = √(d1 * d2).

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• 15.

### one can see a rainbow on the moon?

• A.

True

• B.

False

B. False
Explanation
It is not possible to see a rainbow on the moon because a rainbow is formed when sunlight is refracted, reflected, and dispersed by water droplets in the atmosphere, creating a spectrum of colors. The moon does not have an atmosphere or water droplets, so the conditions necessary for a rainbow to form are not present. Therefore, it is false to say that one can see a rainbow on the moon.

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• 16.

• A.

Metre

• B.

Watt

• C.

Kilowatt

• D.

Dioptre

D. Dioptre
• 17.

### Does the focal length of a lens change on changing the object distance

• A.

Yes, always

• B.

Yes, sometimes

• C.

No, never

• D.

Cannot say

C. No, never
Explanation
The focal length of a lens does not change on changing the object distance. The focal length is a characteristic property of the lens and remains constant regardless of the object distance. This is a fundamental principle in optics and is used in various applications like photography and microscopy.

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• 18.

### How many refraction of light occur on passing through a glass slab?

• A.

One

• B.

Two

• C.

Three

• D.

Four

B. Two
Explanation
When light passes through a glass slab, it undergoes two refractions. The first refraction occurs as the light enters the glass slab, causing it to change direction and speed. The second refraction occurs as the light exits the glass slab and enters the air again, causing another change in direction and speed. Therefore, there are two refractions of light when it passes through a glass slab.

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• 19.

### If x, y, z, t denote object distance, image distance, focal length and radius of curvature in case of mirror respectively then the only correct relation connect in this parameter is

• A.

Option a

• B.

Option b

• C.

Option c

• D.

Option d

A. Option a
Explanation
The correct relation connecting the object distance (x), image distance (y), focal length (z), and radius of curvature (t) in the case of a mirror is given by Option a. Without knowing the specific options provided, it is not possible to provide further details or explanations.

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• 20.

### Where should an object be placed in front of a convex lens to get real image of size of the object?

• A.

At the principal focus of the lens

• B.

At twice the focal length

• C.

At infinity

• D.

Between the optical centre of the lens and its principal focus

B. At twice the focal length
Explanation
To obtain a real image of the same size as the object, the object should be placed at twice the focal length of the convex lens. This is because when an object is placed at twice the focal length, the image formed by the lens will also be located at twice the focal length on the opposite side. The image will be real, inverted, and of the same size as the object. This is a result of the lens' focal properties and the relationship between object distance and image distance in lens systems.

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• 21.

### The refractive index of glass is 1.5 then the velocity of light in glass is

• A.

1.5x108 m/s

• B.

2x108 m/s

• C.

3x108 m/s

• D.

4.5x108 m/s

B. 2x108 m/s
Explanation
The refractive index of a material is defined as the ratio of the speed of light in vacuum to the speed of light in that material. In this case, the refractive index of glass is given as 1.5. Therefore, the velocity of light in glass can be calculated by dividing the speed of light in vacuum (which is approximately 3x10^8 m/s) by the refractive index of glass. This gives us a velocity of 2x10^8 m/s.

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• 22.

### Sky appears blue and red (during sunrise and sunset ).It is due to scattering of light by the

• A.

Dust particle in the atmosphere

• B.

Water droplets in the atmosphere

• C.

CO2 in the atmosphere

• D.

None of these

A. Dust particle in the atmospHere
Explanation
The sky appears blue and red during sunrise and sunset due to the scattering of light by dust particles in the atmosphere. When the sun is lower in the sky, its light has to pass through a larger portion of the Earth's atmosphere, which causes shorter blue wavelengths to scatter more than longer red wavelengths. This scattering effect is more pronounced when there are more dust particles in the atmosphere, leading to the blue and red colors observed during these times of the day.

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• 23.

### Water drop or air bubble in glass slab behaves like

• A.

Converging lens

• B.

Diverging lens

• C.

Plain glass slab

• D.

Convex mirror

B. Diverging lens
Explanation
A water drop or air bubble in a glass slab behaves like a diverging lens. A diverging lens is thinner at the center and thicker at the edges, causing light rays passing through it to spread out or diverge. Similarly, a water drop or air bubble in a glass slab causes the light rays passing through it to diverge, resulting in a virtual image that is smaller and upright. This behavior is opposite to that of a converging lens, which causes light rays to converge and form a real image. A plain glass slab does not have any effect on the light rays, and a convex mirror also diverges light, but in a different way than a diverging lens.

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• 24.

### A man standing in a swimming pool looks at a stone laying on the bottom. The depth of the swimming pool is h .At what distance from the surface of water is the image of the stone formed? Take ū  as a refractive index of water

• A.

H/ū

• B.

• C.

ū/h

• D.

H

A. H/ū
Explanation
The image of the stone is formed at a distance h/ū from the surface of the water. This is because when light travels from a denser medium (water) to a rarer medium (air), it bends away from the normal. So, the apparent position of the stone is shifted upwards, making it appear closer to the surface than it actually is. The amount of shift is determined by the refractive index of water (ū), which is a measure of how much the light bends when passing through water. Thus, the correct answer is h/ū.

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• 25.

### What power of a lens will change the reading distance from 1 m to 0.25 m ? (Answer in Dioptre)

+5,5,+5 D,5 D
Explanation
The power of a lens is inversely proportional to the reading distance. As the reading distance decreases from 1 m to 0.25 m, the power of the lens needs to increase. The options +5, +5, +5 D, and 5 D all indicate an increase in power, which would result in a shorter reading distance. Therefore, these options are correct.

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• 26.

### What is the angle of incidence when a Ray falls normally?

0,O,0°
Explanation
When a ray falls normally, it means that it is incident perpendicular to the surface. In this case, the angle of incidence is 0 degrees, as there is no deviation or change in direction of the ray. Therefore, the correct answer is 0°.

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• 27.

### A boy runs towards a plane mirror with a velocity 3 m/s. With what speed will his image move towards him?(answer in m/s)

6,6m/s
Explanation
When a boy runs towards a plane mirror, his image appears to move towards him at the same speed. This is because the image in a plane mirror is a reflection of the actual object, and the distance between the object and its image remains constant. Therefore, if the boy is running towards the mirror with a velocity of 3 m/s, his image will also appear to move towards him with the same velocity of 3 m/s. Hence, the speed at which his image moves towards him is 3 m/s.

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• 28.

### An object is placed at a large distance in front of a convex mirror of radius of curvature 60 cm. How far is the image behind the mirror? (Answer in cm)

30
Explanation
The image formed by a convex mirror is always virtual, erect, and diminished. When an object is placed at a large distance in front of a convex mirror, the image formed is also located at a distance equal to the radius of curvature of the mirror. In this case, the radius of curvature is given as 60 cm, so the image is located 60 cm behind the mirror. Therefore, the correct answer is 60 cm, not 30 cm.

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• 29.

### It is required to throw an image, magnified two times, on a screen placed at a distance of 6 cm from the object. Find the focal length of the lens.(Answer in cm)

12,12 cm,+12,+12 cm
Explanation
The answer provided, 12,12 cm,+12,+12 cm, suggests that the focal length of the lens is 12 cm. This means that the lens will converge parallel rays of light to a focal point at a distance of 12 cm from the lens. The +12,+12 cm indicates that the image formed will be magnified two times and will be located 12 cm away from the lens on the opposite side.

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• 30.

### A real image, exactly 1/5th the size of the object, is formed at a distance of 18 cm from the lens. Find the focal length of the lens. (Answer in cm)

+15,15,15 cm,+15 cm

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