In a Telescope, compared to the eyepiece lens the objective lens has
- A.larger focal length and larger aperture
- B.larger focal length and smaller aperture
- C.smaller focal length and larger aperture
- D.smaller focal length and smaller aperture
Correct answer
Explanation
The correct answer is A, larger focal length and larger aperture. A telescope's objective must gather as much light as possible from a distant object and form a large image, so it is both wide and of long focal length.
In an astronomical refracting telescope the objective forms a real, inverted image of a far object at its focus, and the eyepiece magnifies that image like a simple microscope. The magnifying power in normal adjustment is the focal length of the objective divided by the focal length of the eyepiece, so a long-focus objective and a short-focus eyepiece give high magnification. A wide aperture collects more light, which shows faint stars, and also improves resolving power. A compound microscope reverses this: its objective has the smaller focal length and aperture.
B is wrong: a small aperture would collect too little light from faint objects.
C is wrong: a short-focus objective would lower the magnifying power.
D is wrong: those are the features of a microscope objective, not a telescope objective.
Exam tip: telescope objective is long and wide; magnifying power = fo / fe.
Practice Questions
View allWhich one of the following frequency ranges is sensitive to human ears ?
- A.0-200 Hz
- B.20-20,000 Hz
- C.200-20,000 Hz only
- D.2,000-20,000 Hz only
Show answer
The normal human ear hears sound from about 20 hertz to 20,000 hertz, the audible range. Frequency, measured in hertz, is the number of vibrations per second, and it decides the pitch of a sound: the higher the frequency, the shriller the note.
Read the full explanation →The frequency of ultrasound waves is
- A.less than 20 Hz
- B.between 20 Hz and 2 kHz
- C.between 2 kHz and 20 kHz
- D.greater than 20 kHz
Show answer
Ultrasound is sound with a frequency above 20,000 Hz, the upper limit of human hearing. Humans hear sound between about 20 Hz and 20 kHz.
Read the full explanation →Which one of the following is an example of a clean fuel ?
- A.Coke
- B.Propane
- C.Petrol
- D.Wax
Show answer
Propane is a light gaseous hydrocarbon that burns completely with a blue flame, leaving no smoke, soot or ash. A clean fuel burns completely, gives high heat and releases few pollutants.
Read the full explanation →A sample of 'soft soap' contains
- A.Caesium
- B.Potassium
- C.Calcium
- D.Magnesium
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Soft soaps are potassium salts of fatty acids, while ordinary hard bar soaps are sodium salts. Soap is made by saponification, heating an oil or fat with an alkali.
Read the full explanation →Which of the following sets of elements has the same valency?
- A.Na, Mg, Ca
- B.Na, Mg, Al
- C.Mg, Ca, K
- D.Mg, Ca, Ba
Show answer
Magnesium, calcium and barium are all alkaline earth metals of group 2, and each has a valency of 2. Elements in the same group of the periodic table have the same number of electrons in their outermost shell, and so the same valency.
Read the full explanation →