Electron emission from a metallic surface by application of light is known as
- A.Thermionic emission
- B.Photoelectric emission
- C.High field emission
- D.Autoelectronic emission
Correct answer
Explanation
The correct answer is B, Photoelectric emission. When light of a suitable frequency falls on a metal and knocks electrons out of it, the process is called photoelectric emission.
Heinrich Hertz first observed the effect in 1887, and Albert Einstein explained it in 1905 by treating light as packets of energy called photons; he received the Nobel Prize in Physics in 1921 for this work. An electron is emitted only if the frequency of the light is above a threshold value for that metal. Brighter light ejects more electrons but does not make them faster. Photocells, solar cells and light meters work on this principle.
A is wrong because thermionic emission releases electrons by heating the metal, as from the filament of an old vacuum tube.
C is wrong because high field emission pulls electrons out with a very strong electric field, not light.
D is wrong because autoelectronic emission is another name for field emission, which also uses an electric field.
Exam tip: emission by light = photoelectric; by heat = thermionic; by strong electric field = field (cold) emission; Einstein's Nobel was for the photoelectric effect.
Practice Questions
View allWhich one of the following is not true for anodizing process?
- A.It makes aluminium corrosion resistant.
- B.Metals like aluminium, titanium and magnesium can be anodized.
- C.Clean aluminium article is the anode and oxygen gas is evolved at the cathode.
- D.It is used in aircraft industry.
Show answer
In anodizing the clean aluminium article is indeed the anode, but oxygen is released at the anode, not at the cathode; hydrogen is released at the cathode.
Read the full explanation →Which one of the following statements is not correct?
- A.The response of the ear to sound of increasing intensity is approximately logarithmic.
- B.The sensitivity of the human ear does not vary with the frequency of the sound.
- C.When two or more waves traverse the same medium, the displacement of any element of the medium is the algebraic sum of the displacements due to each wave.
- D.Longitudinal waves can travel in all media—solids, liquids and gases.
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The statement that the sensitivity of the human ear does not vary with frequency is false, because the ear is far more sensitive to some frequencies than to others.
Read the full explanation →Kwashiorkor disease in children is caused by
- A.sufficient carbohydrates but less fats in diet
- B.sufficient carbohydrates and fats but deficient proteins in diet
- C.sufficient vitamins but deficient fats in diet
- D.sufficient fats but deficient vitamins in diet
Show answer
Kwashiorkor is a disease of protein deficiency in a child who still gets enough calories. It usually appears when a child is weaned from breast milk onto a starchy diet with little protein.
Read the full explanation →Which one of the following is the correct order of pH for the given substances?
- A.Coffee < Lemon juice < Milk of magnesia < Blood
- B.Milk of magnesia < Blood < Coffee < Lemon juice
- C.Lemon juice < Blood < Coffee < Milk of magnesia
- D.Lemon juice < Coffee < Blood < Milk of magnesia
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Arranged from most acidic to most basic, the four substances fall in exactly this order. Lemon juice, rich in citric acid, has a pH of about 2 to 2.5.
Read the full explanation →Which one of the following diseases may be caused by the deficiency of vitamin C?
- A.Rickets
- B.Rabies
- C.Hepatitis
- D.Scurvy
Show answer
Scurvy is caused by a lack of vitamin C, or ascorbic acid, which the body needs to make collagen, the protein that holds skin, gums and blood vessels together.
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