A concave lens has a focal length of 50 cm. Its power is

The correct answer is C. 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. In such an eye the rays from a distant object converge too early and the image falls in front of the retina, not beyond it, so statement C is the incorrect one and is therefore the answer. Option A is a correct statement, because a concave or diverging lens of suitable negative power moves the image back on to the retina. Option B is correct, since near sightedness is simply the common English name for myopia. Option D is also correct, because the far point of a myopic eye shifts in from infinity to a few metres.

The correct answer is B, cornea. 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. Because the change of medium from air to cornea is the sharpest in the whole eye, most of the refraction of incoming light also happens here. Option A is wrong because the retina is the light-sensitive screen at the back of the eye on which the image is finally formed, so light reaches it last and not first. Option C is wrong because the pupil is not a membrane at all but an opening, the central hole through which light passes after crossing the cornea. Option D is wrong because the iris is the coloured muscular curtain that widens or narrows the pupil to control the quantity of light entering the eye.

The correct answer is B, interference. A soap film is extremely thin, and light reflected from its front surface meets light reflected from its back surface. The two waves have travelled slightly different distances, so for each wavelength they either reinforce or cancel one another. This thin-film interference strengthens one colour and removes another at every point of the film, which is why a bubble shows shifting bands of colour. Option A, scattering, is wrong because scattering redirects light by particles and explains the blue sky and the red sunset, not the colours of a film. Option C, dispersion, is wrong because dispersion separates colours by refraction through a medium such as a prism or a raindrop, and a soap film is too thin to disperse light this way. Option D, diffraction, is wrong because that is the bending of light around obstacles and through narrow slits, as in a CD track or a grating.

The correct answer is B, the iris. 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. Option A is wrong because the cornea is the fixed transparent front membrane; it refracts light strongly but cannot change the size of any opening. Option C is wrong because the retina is the screen of rod and cone cells at the back of the eye where the image is formed and converted into nerve signals. Option D is wrong because the ciliary muscles act on the crystalline lens, changing its curvature and focal length to focus objects at different distances, which is accommodation and not light control.

The correct answer is C, 25 cm. 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. For a normal adult eye this distance is taken as about 25 centimetres, and it is the standard value used in every numerical problem on magnifying glasses and microscopes. Option A, 10 cm, is closer than an adult eye can comfortably focus, although a young child can manage something near it. Option B, 15 cm, is likewise too small for the standard adult value. Option D, 50 cm, is far beyond the near point of a healthy eye and is nearer to the receded near point of a person with hypermetropia or presbyopia, whose ciliary muscles can no longer thicken the lens enough.