Which one of the following statements is not correct for light rays?
- A.Light travels at different speeds in different media.
- B.Light travels at almost 300 million metres per second in air.
- C.Light speeds down as it leaves a water surface and enters the air.
- D.Light speeds up as it leaves a glass surface and enters the air.
Correct answer
Explanation
The correct answer is C. Air is optically rarer than water, so light speeds up, not slows down, when it passes from water into air.
The speed of light depends on the medium. It is about 3 × 10⁸ m/s in vacuum and almost the same in air, about 2.25 × 10⁸ m/s in water (refractive index 1.33) and about 2 × 10⁸ m/s in glass (refractive index about 1.5). The refractive index of a medium is the speed of light in vacuum divided by its speed in that medium. Light going from a denser to a rarer medium speeds up and bends away from the normal; this change of speed is the cause of refraction.
A is wrong because light really does travel at different speeds in different media, so the statement is correct.
B is wrong because 3 × 10⁸ m/s is 300 million metres per second, close to its speed in air.
D is wrong because glass is denser than air, so light does speed up on entering air.
Exam tip: denser to rarer – light speeds up and bends away from the normal; n = c / v.
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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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
Show answer
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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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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