Optics Questions and Answers

Answer: A

LASIK stands for Laser-Assisted In Situ Keratomileusis.

In LASIK a thin flap is cut in the cornea and an excimer laser reshapes the underlying corneal tissue to correct myopia, hypermetropia or astigmatism; the flap is then replaced. 'Keratomileusis' means reshaping the cornea, and 'in situ' means it is done in place.

The procedure uses a laser, not just light, so B is wrong, and 'keranomaly' and 'laser-affected' are not the terms in the name.

The word keratomileusis comes from the Greek for cornea (kerat-) and carving (mileusis).

Answer: A

The correct statements are (a), (b) and (c) only.

For a converging (convex) lens:

• (a) Object at 2F: image at 2F on the other side, real, inverted and the same size. True.
• (b) Object between 2F and F: image beyond 2F, real, inverted and magnified. True.
• (c) Object beyond 2F: image between F and 2F, real, inverted and diminished. True.
• (d) Object nearer than F: the image is virtual, magnified and erect, on the same side as the object; this is the magnifying glass. The statement says inverted, so it is false.

Every option that includes (d) is therefore wrong.

Answer: C

The power needed is about 0.67 D.

The eye cannot see clearly nearer than 30 cm, a case of long-sightedness (hypermetropia), so it needs a converging lens that forms a virtual image at 30 cm of an object placed at 25 cm. With u = -25 cm and v = -30 cm:

\[ \begin{aligned} \frac{1}{f} &= \frac{1}{v} - \frac{1}{u} = -\frac{1}{30} + \frac{1}{25} \\ &= \frac{1}{150}\ \text{cm}^{-1} \\ P &= \frac{100}{150} = 0.67\ \text{D} \end{aligned} \]

The 0.25 option is in gauss, a unit of magnetic field, not of lens power. 0.50 D and 1.0 D do not satisfy the lens formula for these distances.

Answer: A

Astigmatism is corrected with a cylindrical lens.

In astigmatism the cornea or lens is curved more in one direction than another, so the eye focuses lines in one direction differently from lines at right angles to them. A cylindrical lens has power in one direction only, so it can correct the error in that particular meridian.

A converging (convex) lens corrects long-sightedness and a diverging (concave) lens corrects short-sightedness; both have the same power in all directions. Mirrors are not used to correct vision.

Answer: A

The splitting of light into its component colours is dispersion.

When white light passes through a prism, each colour is refracted by a different amount because the refractive index of glass depends on wavelength. Violet bends most and red least, so the light spreads into a spectrum, as Newton showed.

Reflection is the bouncing of light from a surface, refraction is its bending on passing between media (the cause of dispersion, but not the splitting itself), and scattering is its redirection by small particles, which makes the sky blue.

Answer: C

For violet and red light, f_V \(\lt\) f_R.

The refractive index of glass is greater for violet light than for red light (dispersion). By the lens maker's formula, 1/f = (μ − 1)(1/R₁ − 1/R₂), so a larger μ gives a larger 1/f, that is a shorter focal length. Violet light therefore focuses closer to the lens than red light.

Equal focal lengths would mean no dispersion, and f_V \(\gt\) f_R reverses the effect. No simple factor of two relates them.

Answer: A

The defect is chromatic aberration.

Because the refractive index of glass depends on wavelength, a lens brings different colours to focus at different points, violet nearest and red farthest. The resulting coloured fringes are chromatic aberration, corrected with an achromatic doublet of crown and flint glass.

Spherical aberration is the failure of rays at different distances from the axis to meet at one focus, even for a single colour. Distortion and coma are other monochromatic aberrations affecting image shape and off-axis points.

Answer: A

Light bends towards the normal line.

Glass is optically denser than air, with a refractive index of about 1.5. When light passes from a rarer to a denser medium it slows down, and by Snell's law

\[ n_1 \sin i = n_2 \sin r \]

with \(n_2 > n_1\), the angle of refraction is smaller than the angle of incidence, so the ray bends towards the normal.

It bends away from the normal only when going from glass to air. There is no change in direction only when the ray strikes along the normal itself, and a refracted ray never becomes parallel to the normal at oblique incidence.

Answer: C

The procedure is Endoscopy.

An endoscope carries light into the body and the image back out through bundles of thin optical fibres. Light entering a fibre strikes the boundary between its glass core and the cladding at an angle greater than the critical angle, so it is totally internally reflected again and again and travels along the fibre even round bends, with little loss.

X-ray imaging relies on X-rays passing through soft tissue but being absorbed by bone, and a C.T. scan builds cross-sectional images from many X-ray readings by computer. Neither uses total internal reflection, and since endoscopy does, 'None of the above' is wrong.

Answer: C

We need to wear Polaroid glass.

Many 3D films are projected as two overlapping images, one for each eye, in light polarised in two different directions. The viewer's glasses have polarising (Polaroid) filters set at matching angles, so each eye sees only its own image, and the brain combines the two slightly different views into a picture with depth.

A magnifying glass or convex lens only enlarges an image and cannot send different pictures to the two eyes, so it gives no 3D effect. Since Polaroid glasses are the answer, 'None of the above' is wrong.