Light rays move in straight lines. But through an optical fibre, they can move in any type of zigzag path because
- A.the holes through the fibre are extremely fine.
- B.light rays are absorbed at the entry end and relieved at the exit end of the fibre.
- C.scattering of light occurs inside the fibre.
- D.successive total internal reflections occur as a ray moves through the fibre.
Correct answer
Explanation
The correct answer is D. Light inside an optical fibre strikes the wall at more than the critical angle, is totally reflected back inside, and repeats this again and again along the fibre.
An optical fibre has a glass core of higher refractive index surrounded by a cladding of lower refractive index. Total internal reflection happens only when light goes from a denser to a rarer medium at an angle greater than the critical angle, so almost no light escapes even when the fibre bends. This is why fibres carry telephone and internet signals over long distances and are used in endoscopes to see inside the body.
A is wrong because a fibre is a solid glass thread, not a hollow tube with holes.
B is wrong because light is not absorbed and released again; absorption would only cause loss.
C is wrong because scattering spreads light out and wastes it rather than guiding it.
Exam tip: optical fibre = total internal reflection, core denser than cladding; same effect makes diamonds sparkle.
Practice Questions
View allThe fact that the planets move around the Sun, not in circles but in ellipses, was first demonstrated by
- A.Galileo
- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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In 1609 Kepler showed that the planets move around the Sun in elliptical orbits, with the Sun at one focus of the ellipse.
Read the full explanation →Which one of the following statements is correct?
- A.Urea is produced in liver.
- B.Urea is produced in blood.
- C.Urea is produced from digestion of starch.
- D.Urea is produced in lung and kidney.
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Liver cells turn the toxic ammonia released from the breakdown of amino acids into urea through the ornithine cycle, also called the urea cycle.
Read the full explanation →Which one of the following is the correct sequence of passage of light in a compound microscope?
- A.Condenser-Objective lens-Eyepiece-Body tube
- B.Objective lens-Condenser-Body tube-Eyepiece
- C.Condenser-Objective lens-Body tube-Eyepiece
- D.Eyepiece-Objective lens-Body tube-Mirror
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Light passes through the condenser to the specimen, is picked up by the objective lens, travels up the body tube and reaches the eye through the eyepiece.
Read the full explanation →Which one of the following statements regarding haemoglobin is correct?
- A.Haemoglobin present in RBC can carry only oxygen but not carbon dioxide.
- B.Haemoglobin of RBC can carry both oxygen and carbon dioxide.
- C.Haemoglobin of RBC can carry only carbon dioxide.
- D.Haemoglobin is only used for blood clotting and not for carrying gases.
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It binds oxygen at its iron-containing haem groups and carries some carbon dioxide on its protein chains. Each haemoglobin molecule has four polypeptide chains and four haem groups, and each haem iron can hold one oxygen molecule, forming oxyhaemoglobin in the lungs.
Read the full explanation →Which one of the following statements regarding histone proteins is correct?
- A.Histones are proteins that are present in mitochondrial membrane.
- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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They are the proteins around which DNA winds to form chromatin in the nucleus of eukaryotic cells. Histones are small basic proteins, rich in the positively charged amino acids lysine and arginine, which attract the negatively charged phosphate groups of DNA.
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