Which one of the following statements about X-rays is not true ?
- A.They have wavelengths of about 1 Å.
- B.These can be generated by bombarding a metal target by high energy electrons.
- C.Due to their wavelengths being shorter, these can be used for radar systems.
- D.These are also used for the treatment of certain forms of cancer.
Correct answer
Explanation
The correct answer is C. X-rays are not used in radar; radar works with microwaves, which have wavelengths of a few centimetres.
X-rays lie between ultraviolet rays and gamma rays in the electromagnetic spectrum, with wavelengths from about 10 nm down to 0.01 nm, so about 1 Å (0.1 nm) is typical. They are produced in an X-ray tube when fast electrons strike a heavy metal target such as tungsten and are suddenly stopped. Radar needs waves that travel far through air and reflect from aircraft and ships, which is why microwaves are chosen.
A is true because 1 Å lies well inside the X-ray band.
B is true because this is exactly how an X-ray tube works.
D is true because controlled doses of high-energy X-rays destroy cancer cells in radiotherapy.
Exam tip: radar and microwave ovens use microwaves; X-rays are for imaging, crystal study and radiotherapy.
Practice Questions
View allMarginal placentation, in which the ovules are borne along the fused margin of a single carpel, is found in
- A.Tomato
- B.Pea
- C.Mustard
- D.Sunflower
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The pea's ovary is made of one carpel whose two margins fuse to form a ridge, and the ovules are arranged in two rows along that ridge; this is marginal placentation, and it is why the pod opens to show a single row of seeds on each side.
Read the full explanation →Mango and coconut are examples of which type of fruit?
- A.Berry
- B.Drupe
- C.Pome
- D.Legume
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A drupe is a fleshy fruit with a thin epicarp, a mesocarp that may be pulpy as in the mango or fibrous as in the coconut, and a hard, stony endocarp enclosing a single seed; the coconut's shell is that endocarp and the white kernel is the seed.
Read the full explanation →The apple is regarded as a false fruit because
- A.It has no seeds
- B.It develops from the thalamus along with the ovary
- C.It develops without fertilisation
- D.Its pericarp is stony
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A true fruit forms from the ovary alone, but in the apple and the pear the thalamus grows, becomes fleshy and forms the greater part of the edible tissue, while the ovary makes only the core; such a fruit is called a false or spurious fruit, and the specific type…
Read the full explanation →Vexillary aestivation of the corolla is found in the flower of
- A.Pea
- B.Cotton
- C.Calotropis
- D.Gulmohar
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In vexillary or papilionaceous aestivation the corolla has five petals of unequal size: one large posterior petal, the standard, which covers two lateral wing petals, which in turn cover the two smallest anterior petals that are fused into a keel.
Read the full explanation →In a racemose inflorescence, the flowers are borne in
- A.Basipetal succession
- B.Acropetal succession
- C.A single terminal cluster
- D.No definite order
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In a racemose inflorescence the main axis does not end in a flower but keeps growing, so the lowest flower is the oldest and the youngest buds are at the tip; this upward order is called acropetal, as in radish, mustard and Larkspur.
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