Light of uniform intensity impinges perpendicularly on a totally reflecting surface. If the area of the surface is halved, the radiation force on it will become
- A.double
- B.half
- C.four times
- D.one fourth
Correct answer
Explanation
The correct answer is B, half. The force exerted by light depends on how much light falls on the surface, so with the same intensity, half the area receives half the light and feels half the force.
Light carries momentum, and when it is absorbed or reflected it pushes on the surface; this push per unit area is called radiation pressure. For light of intensity I falling straight on a perfectly reflecting surface, the pressure is 2I/c, where c is the speed of light, because the light’s momentum is reversed on reflection. For a perfect absorber it is I/c. The total force is pressure multiplied by area, F = 2IA/c. With I fixed, F is directly proportional to A, so halving the area halves the force. The same pressure drives solar sails and makes comet tails point away from the Sun.
A is wrong because doubling would need the area, or the intensity, to be doubled. C is wrong because force varies directly with area, not as its inverse square. D is wrong because a one-fourth result would need the force to depend on the square of the area.
Exam tip: radiation force F = 2IA/c on a perfect reflector, IA/c on a perfect absorber; F is directly proportional to area.
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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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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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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