Physics

11. The image formed by a plane mirror is :

The image formed by a plane mirror is :

always virtual and erect.
always virtual but erect or inverted upon the size of the object.
never virtual but always erect.
always real and erect.
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A plane mirror is a flat reflective surface. The image formed by a plane mirror has specific characteristics:
1. **Virtual:** The image is formed where the light rays appear to diverge from, but do not actually intersect. It cannot be projected onto a screen.
2. **Erect:** The image is upright, meaning it is oriented the same way as the object (top is top, bottom is bottom).
3. **Laterally Inverted:** The image is flipped left-to-right.
4. **Same size:** The size of the image is equal to the size of the object.
5. **Same distance:** The image is located behind the mirror at the same distance as the object is in front of the mirror.

Based on these properties, the image formed by a plane mirror is always virtual and always erect.
Option A states “always virtual and erect,” which matches the known properties.
Option B is incorrect because the image is always erect, not dependent on object size.
Option C is incorrect because the image is always virtual.
Option D is incorrect because the image is always virtual, not real.

– A plane mirror produces a virtual image.
– A plane mirror produces an erect image.
– The image is also laterally inverted and the same size as the object.
Real images are formed when light rays actually converge at a point and can be projected onto a screen (e.g., image formed by a projector lens). Virtual images cannot be projected (e.g., image in a plane mirror or through a magnifying glass). Erect means the image orientation is the same as the object; inverted means it is upside down.

12. Dioptre is the SI unit of :

Dioptre is the SI unit of :

size of the lens.
curvature of the lens.
power of the lens.
magnification produced by a lens.
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Dioptre (symbol D) is the unit of measurement for the optical power of a lens or curved mirror.
Optical power (P) of a lens is defined as the reciprocal of its focal length (f) when the focal length is measured in metres.
P = 1 / f (where f is in metres)
If the focal length f is in metres, the unit of power P is m⁻¹, which is defined as the dioptre (D).
For example, a lens with a focal length of +0.5 metres has a power of P = 1 / 0.5 m = +2 D.
A lens with a focal length of -0.25 metres has a power of P = 1 / -0.25 m = -4 D.
– Dioptre is the unit used to measure the optical power of lenses.
– Optical power is the reciprocal of the focal length measured in metres.
Size of the lens is measured in units of length (e.g., diameter in cm). Curvature of the lens is related to the radii of curvature of its surfaces, usually measured in reciprocal metres (m⁻¹), and contributes to the power of the lens along with the refractive index of the material. Magnification is a ratio of image size to object size, which is a dimensionless quantity or expressed as ‘x’. Dioptre specifically quantifies the power of the lens to converge or diverge light.

13. Which one of the following materials cannot be used to make a lens ?

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

Water
Glass
Plastic
Clay
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A lens is an optical device that transmits and refracts light, causing the convergence or divergence of light rays. For a material to be used to make a lens, it must be transparent or at least translucent enough to allow light to pass through it and must have a refractive index different from the surrounding medium (usually air) so that refraction occurs.
Let’s examine the given options:
A) Water: Water is transparent and has a refractive index. Lenses can be made from water (e.g., water-filled lenses) or exist naturally (e.g., a droplet of water can act as a lens).
B) Glass: Glass is a common, transparent material with a suitable refractive index used extensively for making lenses.
C) Plastic: Many types of plastic are transparent and are widely used for making lenses, especially in eyeglasses, contact lenses, and camera lenses.
D) Clay: Clay is typically opaque (does not allow light to pass through) and therefore cannot be used to make a functional lens that refracts light in the required manner. While it can be molded, its opaqueness makes it unsuitable for transmitting images.
– A lens works by refracting light, which requires the material to be transparent or translucent.
– The material must have a different refractive index from the surrounding medium.
– Opaque materials cannot be used to make standard lenses.
Materials like glass and plastic are commonly used because they are transparent, can be precisely shaped, and have stable optical properties. Water’s shape changes easily, making it less practical for stable lenses unless contained. Clay’s opaqueness fundamentally prevents it from being used for light transmission and refraction in the way a lens operates.

14. The blue colour of the sky and the reddening of the Sun at sunrise and

The blue colour of the sky and the reddening of the Sun at sunrise and sunset are caused due to the phenomenon of :

dispersion of light.
reflection and refraction of light.
aberration of light.
scattering of light.
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The blue colour of the sky and the reddening of the Sun at sunrise and sunset are classic examples of the phenomenon of scattering of light by particles in the atmosphere.
When sunlight enters the Earth’s atmosphere, it interacts with the gas molecules (mainly Nitrogen and Oxygen) and tiny particles present. This interaction causes the light to be scattered in various directions. This process is called scattering.
Rayleigh scattering explains why the sky is blue. It states that the intensity of scattered light is inversely proportional to the fourth power of the wavelength. Shorter wavelengths, like blue and violet light, are scattered much more effectively than longer wavelengths, like red and orange light. When we look at the sky during the day, we see the scattered blue light from all directions.
At sunrise and sunset, the sunlight travels a much longer path through the atmosphere to reach our eyes. During this long journey, most of the shorter wavelength blue and green light is scattered away by the atmospheric particles. The longer wavelength light, such as red and orange, which is scattered less, is left to reach our eyes, making the Sun and the sky around it appear reddish or orange.
– The colour of the sky and the Sun’s colour at sunrise/sunset are due to the interaction of sunlight with atmospheric particles.
– This interaction involves scattering of light.
– Shorter wavelengths (blue) are scattered more effectively than longer wavelengths (red) by atmospheric gases (Rayleigh scattering).
– The path length of sunlight through the atmosphere affects which colours are predominantly seen.
Dispersion of light is the splitting of white light into its constituent colours (spectrum) due to the dependence of refractive index on wavelength (e.g., prism). Reflection is the bouncing of light off a surface. Refraction is the bending of light as it passes from one medium to another. Aberration refers to defects in image formation by optical systems. None of these fully explain the blue sky or red sunrise/sunset phenomenon, which is primarily a scattering effect.

15. What is the time difference between the actual sunset and apparent sun

What is the time difference between the actual sunset and apparent sunset ?

About half a minute
About one minute
About two minutes
About twenty minutes
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The time difference between actual sunset and apparent sunset is caused by atmospheric refraction. Sunlight bends as it passes through the Earth’s atmosphere because the density of the atmosphere increases towards the Earth’s surface.
This refraction causes the sun’s rays to bend downwards, making the sun appear higher in the sky than its actual position. Near the horizon, this effect is most pronounced. As a result, the sun appears to rise earlier in the morning (apparent sunrise before actual sunrise) and set later in the evening (apparent sunset after actual sunset).
The apparent flattening of the sun’s disc near the horizon is also a result of atmospheric refraction. The time difference between actual and apparent sunrise or sunset is typically about 2 minutes. This phenomenon effectively increases the duration of daylight by about 4 minutes each day (2 minutes in the morning and 2 minutes in the evening).

16. Who among the following is known as the Father of Nuclear Physics ?

Who among the following is known as the Father of Nuclear Physics ?

E. Rutherford
J.J. Thomson
Niels Bohr
James Chadwick
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Ernest Rutherford (E. Rutherford) is widely regarded as the “Father of Nuclear Physics”. His experiments, particularly the gold foil experiment, led to the discovery of the atomic nucleus and the idea that the atom is mostly empty space. He also coined the terms “alpha” and “beta” rays, discovered the proton, and performed the first artificial nuclear transmutation.
Rutherford’s work revolutionized the understanding of atomic structure and radioactivity.
J.J. Thomson discovered the electron.
Niels Bohr developed an early model of the atom based on quantum mechanics.
James Chadwick discovered the neutron.
Rutherford received the Nobel Prize in Chemistry in 1908 for his investigations into the disintegration of the elements, and the chemistry of radioactive substances.

17. Which one among the following is a non-luminous object ?

Which one among the following is a non-luminous object ?

Sun
Candle
LED bulb
Moon
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A non-luminous object is an object that does not produce its own light. It is visible because it reflects light from a luminous source. The Moon does not produce its own light; it reflects sunlight.
Luminous objects emit light (e.g., Sun, candle flame, light bulb).
Non-luminous objects reflect light (e.g., Moon, planets, furniture, books).
The apparent brightness of non-luminous objects depends on the intensity of the incident light and their reflective properties.

18. Light waves are

Light waves are

progressive in nature.
mechanical in nature.
longitudinal in nature.
transverse in nature.
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Light waves are electromagnetic waves, which means they are fluctuations of electric and magnetic fields. Electromagnetic waves, including light, are characterized by their transverse nature, where the oscillations of the fields are perpendicular to the direction of wave propagation.
Transverse waves have oscillations perpendicular to the direction of energy transfer (propagation).
Longitudinal waves have oscillations parallel to the direction of energy transfer (propagation), like sound waves in air.
Light is non-mechanical; it does not require a medium to travel.
The electromagnetic spectrum includes various types of waves like radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays, all of which are transverse electromagnetic waves.

19. An electric generator primarily converts

An electric generator primarily converts

electrical energy to heat energy.
electrical energy to sound energy.
electrical energy to mechanical energy.
mechanical energy to electrical energy.
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An electric generator is a device that converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction, where relative motion between a conductor and a magnetic field induces an electric current.
Generators transform mechanical work (like rotation) into electrical output.
Electric motors perform the opposite function, converting electrical energy into mechanical energy.
Examples of mechanical energy sources for generators include turbines driven by steam, water, wind, or internal combustion engines. The induced voltage and current can then be used to power electrical devices.

20. The equipment used to measure the electrical current in a closed circu

The equipment used to measure the electrical current in a closed circuit is

Galvanometer.
Ammeter.
Voltmeter.
Compass needle.
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An ammeter is the standard instrument used to measure the electric current flowing through a circuit. It is connected in series with the component through which the current is to be measured.
– An ammeter measures electric current.
– It must be connected in series in a circuit.
– A voltmeter measures voltage (potential difference).
– A galvanometer detects small currents and their direction.
Ammeter is derived from “ampere meter,” as current is measured in amperes (A). Ideal ammeters have very low internal resistance so as not to significantly affect the current they are measuring. Galvanometers can be used as components to build ammeters and voltmeters with appropriate shunts or series resistances.
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