General Science GK Questions with Answers – Page 91

An alpha particle takes away two protons and two neutrons, so the parent loses four units of mass number and two units of atomic number and becomes an element two places earlier in the periodic table; uranium-238, for instance, becomes thorium-234.

An alpha particle consists of two protons and two neutrons bound together, which is exactly the nucleus of a helium-4 atom, so it carries a charge of plus two and a mass number of four.

In 1896 Becquerel found that a uranium salt kept in the dark fogged a wrapped photographic plate, which showed that the salt emitted its own penetrating radiation without being exposed to light.

A hydrogen bomb, also called a thermonuclear bomb, works in two steps. A fission explosion of uranium or plutonium produces the temperature of millions of degrees needed to force light nuclei together, and the fusion of those nuclei then releases far more energy than the fission stage alone.

Carbon-14 is the radioactive isotope of carbon used for radiocarbon dating, and its half-life is close to 5,700 years, which is the figure quoted in the question.

The molecules of the atmosphere are much smaller than the wavelength of visible light, and such particles scatter short wavelengths far more strongly than long ones.

In hypermetropia the eye lens is too weak or the eyeball is too short, so rays from a nearby object are not converged enough and the image falls behind the retina.

The retina carries two kinds of photoreceptor. Cone cells need reasonably bright light to work and respond selectively to different wavelengths, so they give us the sensation of colour; when the cones are defective a person is colour blind.

The refractive index of a transparent medium is not the same for every colour; it rises as the wavelength falls.

Two steps are needed. First, the focal length must be converted to metres: 50 centimetres is 0.5 metre.

The power of a lens measures how strongly it converges or diverges light, and it is defined as one divided by the focal length expressed in metres.

The least distance of distinct vision, also called the near point, is the smallest distance at which an object can be held and still be seen clearly without strain.

The iris is the coloured ring of muscle lying just behind the cornea. By contracting and relaxing it makes the central opening, the pupil, smaller in bright light and larger in dim light, and in this way it regulates how much light falls on the retina.

A soap film is extremely thin, and light reflected from its front surface meets light reflected from its back surface.

The cornea is the thin, transparent, slightly bulging membrane that forms the front surface of the eyeball, and light must pass through it before reaching anything else.

Myopia, or short-sightedness, is the defect in which nearby objects are seen clearly but distant objects look blurred. It arises either because the eye lens has become too converging, that is its focal length is too short, or because the eyeball has grown too long from front to back.

While the ice floats it displaces a volume of water whose weight equals the weight of the ice, by the law of flotation.

In a liquid, viscosity comes chiefly from cohesion between molecules; heating weakens that cohesion, so the layers slide past one another more easily and the viscosity falls, which is why engine oil becomes thinner as the engine warms and why honey flows freely when it is warmed.

Cohesive forces pull the molecules at the surface of a liquid inward, so the surface behaves like a stretched membrane and takes the smallest area that can enclose the given volume; for a fixed volume the shape of least surface area is a sphere, which is why a free drop…

The correct answer is B, pascal second, equal to a newton second per square metre, which follows from Newton's law of viscosity, where the shearing stress in pascals is divided by the velocity gradient in reciprocal seconds.