The human eye has a point that lacks photoreceptor cells. This is identified as :
- A.Fovea
- B.Blind spot
- C.Dark spot
- D.Orbit
Correct answer
Explanation
The correct answer is B, Blind spot. The blind spot is the place where the optic nerve leaves the eye, and it has no rods or cones, so light falling there is not seen.
The retina carries two kinds of photoreceptor cells: rods, which work in dim light, and cones, which sense colour and fine detail. At the optic disc the nerve fibres from all over the retina gather and exit towards the brain, leaving no room for these cells. We do not notice the gap in daily life because the two eyes cover each other's blind spots and the brain fills in the missing patch.
A is wrong because the fovea is the opposite case, a small pit packed with cones that gives the sharpest vision. C is wrong because 'dark spot' is not a structure of the eye. D is wrong because the orbit is the bony socket of the skull that holds the eyeball.
Exam tip: blind spot means optic disc with no rods or cones; yellow spot or fovea means the sharpest vision.
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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