Light: Reflection and Refraction Notes with PYQs

The correct answer is B, it slows down and bends towards the normal. A denser medium has a higher refractive index, and the refractive index is the speed of light in vacuum divided by its speed in the medium, so light must travel more slowly inside it. When the ray meets the surface at an angle, the part of the wavefront that enters first is slowed first, which swings the ray towards the normal. Air to glass and air to water are the standard examples.

Option A takes the correct change of speed with the wrong bending. Option C describes what happens in the opposite case, when light passes from a denser medium to a rarer one. Option D combines two errors at once. A ray that falls along the normal is a special case and passes straight through without bending, though its speed still changes.

The correct answer is B, minus 10 centimetres. By the sign convention the focal length of a concave lens is negative, so f is minus 15 centimetres, and the object distance measured against the direction of the incident light is minus 30 centimetres. Putting these in the lens formula 1/v minus 1/u equals 1/f gives 1/v equal to 1/f plus 1/u, that is minus 1/15 plus minus 1/30, which is minus 3/30 or minus 1/10. Hence v is minus 10 centimetres.

The negative sign shows that the image lies on the same side as the object, so it is virtual and erect, and being nearer the lens than the object it is also diminished. That is true of a concave lens for every position of the object. Options A, C and D come from dropping a sign or from adding the reciprocals wrongly.

The correct answer is C, Isaac Newton. Newton passed a narrow beam of sunlight through a glass prism in a darkened room and obtained a band of seven colours on the wall, which he named the spectrum. He then showed with a second prism that the colours could be recombined into white light, proving that the prism does not colour the light but only separates the colours already present in it. The order of the colours from the base of the prism is remembered as VIBGYOR.

Option A, C. V. Raman, discovered the scattering effect named after him and won the Nobel Prize in 1930. Option B, Lord Rayleigh, explained the scattering that makes the sky blue. Option D, S. Chandrasekhar, worked on the structure and evolution of stars.

The correct answer is B, virtual, erect and of the same size. Rays reflected from a plane mirror diverge and only appear to come from a point behind the mirror, so the image cannot be caught on a screen and is virtual. It stands the same way up as the object, is exactly as large as the object and lies as far behind the mirror as the object is in front of it. It is also laterally inverted, which is why the left hand appears as the right hand and why the word AMBULANCE is painted in reverse on the front of the van so that a driver ahead reads it correctly in the mirror.

Options A and D describe real images, which a plane mirror can never form. Option C describes the image in a convex mirror, which is smaller than the object.

The correct answer is C, a convex mirror. A convex mirror curves outward and spreads the light that falls on it, so it always forms a virtual, erect and diminished image, whatever the distance of the object. Because the image is smaller, a small mirror can show a very wide field of view, and the driver sees a long stretch of the road behind. The same reason puts convex mirrors at blind turnings on hill roads and in shops.

Option A, a plane mirror, would give a true sized image but a narrow field of view. Option B, a concave mirror, converges light and can form a large inverted real image of a distant object, which would be useless and dangerous in a vehicle; it is used instead for shaving, in dentistry and in headlights. Option D is not a mirror of this kind at all.

The correct answer is B, plus 2 dioptres. The power of a lens is the reciprocal of its focal length expressed in metres, that is P equals 1 divided by f. Fifty centimetres is 0.5 metre, and the reciprocal of 0.5 is 2, so the power is two dioptres. The lens is convex and therefore converging, so the power carries a positive sign. Power measures how strongly a lens bends light, so a short focal length means a powerful lens.

Option A takes the focal length in metres as the answer instead of its reciprocal. Option C gives the right number with the wrong sign, which would describe a concave lens. Option D uses the focal length in centimetres in the formula, which is the commonest mistake in this calculation.

The correct answer is C, total internal reflection after refraction in layers of air. On a hot day the air touching the road is heated and becomes rarer than the cooler air above it. Light coming down from the sky passes from denser layers to rarer ones, bends away from the normal at each step, and when the angle of incidence exceeds the critical angle it is totally internally reflected upward. The eye traces it back in a straight line and sees a shimmering patch that looks like water.

Option A is wrong because the road is rough and not a mirror. Option B, dispersion, splits white light into colours and explains a rainbow, not a mirage. Option D, scattering, explains the blue of the sky and the red of the setting sun.

The correct answer is B, total internal reflection. An optical fibre is a thin thread of glass or plastic with a core of higher refractive index surrounded by a cladding of lower refractive index. Light entering one end strikes the boundary at an angle greater than the critical angle, so none of it escapes and all of it is reflected back into the core. Repeating this thousands of times, the signal travels along the fibre even when it is bent, with very little loss, which is why fibres carry telephone, internet and cable television signals and are also used in endoscopes.

Option A, dispersion, is the splitting of white light by a prism. Option C, diffraction, is the bending of waves around obstacles. Option D, polarisation, restricts the vibration of light to one plane and is used in sunglasses.

The correct answer is B, a concave lens. In myopia the eyeball is too long or the lens of the eye too strongly converging, so rays from a distant object meet in front of the retina and the person cannot see far objects clearly. A concave lens diverges the incoming rays before they enter the eye, so the image is pushed back and falls exactly on the retina.

Option A, a convex lens, converges light and is used for hypermetropia or long sight, where the image forms behind the retina and near objects look blurred. Option C, a cylindrical lens, corrects astigmatism, which is caused by an uneven curvature of the cornea. Option D, a bifocal lens, has a concave upper part and a convex lower part and is used for presbyopia, the loss of the power of accommodation with age.

The correct answer is D, violet. White light is a mixture of colours, and the refractive index of the glass of a prism is slightly different for each of them, being greatest for violet and least for red. A larger refractive index means a greater bending, so violet is deviated most and red least, and the colours spread out into the band called the spectrum, remembered as VIBGYOR from violet at the bottom to red at the top. This splitting is called dispersion.

Option A, red, has the longest wavelength of the visible colours and is bent the least, which is why it appears at the other end of the band. Options B and C, green and yellow, lie in the middle of the spectrum and are deviated by amounts between those of violet and red.