Twinkling of stars is primarily due to the atmospheric
- A.refraction
- B.reflection
- C.polarization
- D.dispersion
Correct answer
Explanation
The correct answer is A, refraction. Starlight is bent again and again as it passes through layers of air whose temperature and density keep changing, so the star seems to flicker in brightness and position.
Stars are so far away that they act as point sources of light. As their light enters the Earth’s atmosphere, it passes through moving pockets of warmer and cooler air with slightly different refractive indices. The light bends by changing amounts from moment to moment, so the amount of light reaching the eye keeps varying and the star appears to twinkle. Planets do not twinkle because they are much closer and look like small discs; the changes from different points of the disc cancel out. The same refraction makes stars appear a little higher than their true position and gives us about two extra minutes of daylight.
B is wrong because reflection sends light back from a surface; it does not cause flickering in the air. C is wrong because polarization restricts the vibration of light waves to one plane. D is wrong because dispersion splits white light into colours, as in a rainbow.
Exam tip: atmospheric refraction = twinkling stars, early sunrise and late sunset; planets do not twinkle.
Practice Questions
View allOsmosis is the process of movement of water molecules from its
- A.higher concentration to its lower concentration through a cell wall.
- B.lower concentration to its higher concentration through a fully permeable membrane.
- C.higher concentration to its lower concentration through a fully permeable membrane.
- D.higher concentration to its lower concentration through a semi-permeable membrane.
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Osmosis is the passage of water from where water is more concentrated (a dilute solution) to where it is less concentrated, across a membrane that lets water through but holds back solutes.
Read the full explanation →Liquids and gases never show
- A.diamagnetic property
- B.paramagnetic property
- C.ferromagnetic property
- D.electromagnetic property
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Ferromagnetism needs the atomic magnets of a substance to line up together in fixed regions called domains, and this happens only in the ordered structure of a solid.
Read the full explanation →The chemical properties of an element depend upon
- A.the number of isotopes of the element
- B.the mass number of the element
- C.the total number of neutrons in the element
- D.the number of electrons in the outermost shell of the element
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Chemical reactions involve only the outer, or valence, electrons, so these decide how an element behaves. Atoms combine by losing, gaining or sharing their valence electrons to reach a stable arrangement, usually eight electrons in the outer shell.
Read the full explanation →Which one of the following is not found in animal cells ?
- A.Free ribosomes
- B.Mitochondria
- C.Nucleolus
- D.Cell wall
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Animal cells are bounded only by a flexible plasma membrane and have no rigid cell wall outside it. A cell wall is found in plants, where it is made mainly of cellulose, in fungi, where it is made of chitin, and in bacteria, where it is made of peptidoglycan.
Read the full explanation →A student was doing an experiment on increasing the cell division among plants. She asked her supervisor to suggest the specific plant hormone for the same. Had you been her supervisor, which plant hormone would you suggest?
- A.Abscisic acid
- B.Gibberellins
- C.Cytokinin
- D.Auxin
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Cytokinins are the plant hormones whose main job is to promote cell division, or cytokinesis, which gives them their name.
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